regional groundwater quality and surface water …€¦ · 5. nitrogen inputs to groundwater and...

REGIONAL GROUNDWATER QUALITY AND SURFACE WATER QUALITY

MODEL OF THE RUATANIWHA PLAINS

White, P.A. Institute of Geological and Nuclear Sciences, Private Bag 2000, Wairakei, Taupo.

Daughney, C. Institute of Geological and Nuclear Sciences, P O Box 30368, Lower Hutt.

HBRC Publication No. 4836Report No. SD16-08

i

Contents

i List of Figures

ii List of Tables

1. Introduction

2. Review

3. Land use scenarios

4. Chemical conditions in groundwater

5. Nitrogen inputs to groundwater and landuse

6. Groundwater flow model

7. Groundwater quality and surface water quality model

7.1 Design

7.2 Model components

7.3 Nitrogen loading

7.4 Non-irrigated and irrigated models

7.5 Estimation of nitrogen concentrations – steady state

7.6 Checks on the mass balance equations

7.7 Transient calculations

7.8 Model calibration

7.8.1 Surface water

7.8.2 Groundwater

8. Nitrogen concentrations without irrigation

9. Nitrogen inputs and outputs

9.1 Current land use

9.2 Irrigated pasture

9.3 Irrigated crops and irrigated dairy

10. Nitrogen concentrations with irrigation



11. Nitrogen concentrations in rivers over 20 years

11.1 River and stream flow means

11.2 Flows used in mixing calculations

11.2.1 Waipawa @ RD5, site 23235

11.2.2 Tukituki @ Tapairu Rd, site 23207

11.2.3 Kahahakuri @ Ongaonga, site 23248

11.3 Calculated nitrogen fluxes

11.4 Calculated surface water flows

11.5 Calculated river and stream nitrogen concentrations

ii

11.5.1 92 kgN/ha/yr irrigation


12. Ruataniwha Plains monitoring network

12.1 Surface-water monitoring

12.2 Groundwater monitoring

13. Conclusions

14. References

Appendix 1. Copy of contract

Appendix 2. Hydrological data used in this study

Appendix 3. Mean flow and mean water chemistry values

Appendix 4. Contents of Excel spreadsheets and worksheets.

A4.1 Excel spreadsheet Ruasteadystate.xls

A4.2 Excel spreadsheet Ruagwflux.xls

A4.3 Excel spreadsheet Ruagwqualtrans.xls

A4.4 Excel spreadsheet Ruasurfflowdist.xls

A4.5 Excel spreadsheet Ruawurfqualtrans.xls

A4.6 Excel spreadsheet Ruagwfluxirri.xls

A4.7 Excel spreadsheet Ruagwqualtransirri.xls

A4.8 Excel spreadsheet Ruasurfflowdistirri.xls

A4.9 Excel spreadsheet Ruasurfqualtransirri.xls

Appendix 5. Operation of the Excel spreadsheets


1.1 Background nitrogen loading

1.2 „Irrigation‟ nitrogen

1.3 „Point source‟ nitrogen

2.0 Nitrogen balance

3.0 Surface water quality predictions

4.0 Groundwater quality predictions

5.0 Maintenance of spreadsheets

5.1 Changing groundwater flow velocities

5.2 Changing observed surface water and groundwater nitrogen concentrations

5.3 Adjusting the capture zones for surface water monitoring sites and groundwater

5.4 Adding new surface water or groundwater zones

5.5 Update seepage velocities in „transient‟ calculations

5.5.1 Groundwater travel times to monitoring wells

5.5.2 Travel times to surface water monitoring sites

5.5.3 Transient calculations

6.0 Assumptions

6.1 Surface water zones

iii

6.2 Groundwater capture

6.3 Mixing in surface water

6.4 Groundwater mixing ratio

6.5 Water balance

6.6 Chemistry balance

7.0 Spreadsheet security

iv

LIST OF FIGURES

Figure 1. Ruataniwha Plains and location of rivers, streams and roads

Figure 2. Ruataniwha Plains - 20 m contours

Figure 3. Regions of the Ruataniwha Plains where rivers and streams gain and lose flow

Figure 4. Rivers, streams and overall ranking (after Sarrazin, 2002)

Figure 5. Groundwater flow model grid of the Ruataniwha Plains with contours of predicted

groundwater level

Figure 6. Scatter plot of ammonium vs. nitrate concentrations in Ruataniwha groundwater

samples

Figure 7. Scatter plot of nitrate vs. total nitrogen concentration for Ruataniwha groundwater

samples with more than 0.1 mg/l nitrate

Figure 8. Scatter plot of ammonium vs. total nitrogen concentrations for Ruataniwha

groundwater samples with less than 0.1 mg/l nitrate

Figure 9. Scatter plot of nitrate concentration vs. well depth for Ruataniwha groundwater

samples

Figure 10. Location of surface-water zones

Figure 11. Location of surface-water monitoring sites

Figure 12. Location of groundwater capture zones

Figure 13. Location of groundwater monitoring sites

Figure 14. Estimated seepage velocity across the Ruataniwha Plains (m/day), non-irrigated

model

Figure 15. Estimated seepage velocity across the Ruataniwha Plains (m/day), irrigated model

Figure 16. Predicted weekly nitrogen concentration in the Waipawa River @ RD5 with a 92

kgN/ha/yr irrigation.

Figure 17. Predicted weekly nitrogen concentration in the Tukituki River @ Tapairu Rd with a

92 kgN/ha/yr irrigation.

Figure 18. Predicted weekly nitrogen concentration in the Kahahakuri @ Ongaonga with a 92

kgN/ha/yr irrigation.

Figure 19. Predicted weekly nitrogen concentration in the Waipawa River @ RDS with a


v

Figure 20. Predicted weekly nitrogen concentration in the Tukituki River @ Tapairu Rd with a


Figure 21. Predicted weekly nitrogen concentration in the Kahahakuri @ Ongaonga with a


vi

LIST OF TABLES

Table 1. Proxy variable to use for total nitrogen concentration.

Table 2. Zone loads and predicted surface water quality, steady-state.

Table 3. River N concentrations used in the model.

Table 4. Zone loads and predicted groundwater quality assuming full mixing.

Table 5. Zone loads and predicted groundwater quality assuming partial mixing.

Table 6. Nitrogen leaching for various land uses (HortResearch pers. comm.)

Table 7. Surface water quality with a nitrogen loading of 16 kgN/ha/yr to Ruataniwha Plains

irrigable area


irrigable area


irrigable area


irrigable area

Table 11. Groundwater quality with a nitrogen loading of 176 kgN/ha/yr to all irrigable cells

Table 12. Mean nitrogen concentrations in streams over 50 years due to N loading of

16 kgN/ha/yr to all irrigable cells







Table 16. Calculated nitrogen balance with existing land use

Table 17. Nitrogen balance with existing land use plus 17 kgN/ha/yr, equivalent to an irrigated

beef land use, over the irrigable area of the Ruataniwha Plains

Table 18. Nitrogen balance with existing land use plus 92 kgN/ha/yr, equivalent to irrigated

crops and irrigated dairy, over the irrigable area of the Ruataniwha Plains

vii

Table 19. Steady-state mean surface water nitrogen concentrations with a loading of

17 kgN/ha/yr and irrigation.

Table 20. Steady-state mean groundwater nitrogen concentrations with a loading of

17 kgN/ha/yr and irrigation and partial mixing.

Table 21. Nitrogen concentrations in surface water over 50 years, loading of 17 kgN/ha/yr and

irrigation.

Table 22. Nitrogen concentrations in groundwater over 50 years, loading of 17 kgN/ha/yr and

irrigation, assuming full mixing.

Table 23. Nitrogen concentrations in groundwater over 50 years, loading of 17 kgN/ha/yr and

irrigation, assuming partial mixing.

Table 24. Surface water nitrogen concentrations with a 92 kgN/ha/yr loading and irrigation.

Table 25. Groundwater nitrogen concentrations with a 92 kgN/ha/yr loading and irrigation and

partial mixing.

Table 26. Nitrogen concentrations in surface water over 50 years, 92 kgN/ha/yr loading and

irrigation.

Table 27. Nitrogen concentrations in groundwater over 50 years, 92 kgN/ha/yr loading and

irrigation and partial mixing.

Table 28. Nitrogen concentration in three rivers and streams over 20 years due to irrigation of

92 kgN/ha/yr.

Table 29. Nitrogen concentration in three rivers and streams over 20 years due to irrigation of

17 kg N/ha/yr.

Table 30. Surface water sub-zones and „unique‟ identification of land use effects.

Table 31. Surface water quality network that could allow the monitoring of land use in each

subzone.

Table 32. Monitoring network that allows measurement of nitrogen entering the Ruataniwha

Plains through rivers and streams.

viii

Appendix 2

Table A2.1 Surface water hydrological data in TIDEDA file Rpflow.mtd.

Table A2.2 Groundwater hydrological data in TIDEDA file RPGWMan.mtd.

Table A2.3 Groundwater hydrological data in TIDEDA file RPGWAuto.mtd.

Table A2.4 Water chemistry – groundwater sites.

Table A2.5 Water chemistry – surface water sites.

Appendix 3

Table A3.1 Mean flow from TIDEDA records.

Table A3.2 Mean flows estimated by Geoff Wood from available gaugings or correlations.

Table A3.3 Mean nitrogen concentrations in groundwater.

Table A3.4 Mean nitrogen concentrations in surface water.

Appendix 4

Table A4.1. Location of surface water zones.

Table A4.2. Hawkes Bay Regional Council surface water quality monitoring sites.

Table A4.3. Groundwater capture zones and monitoring wells.

Table A4.4. Summary of all nitrogen loadings to the Ruataniwha Plains.

Table A4.5. Nitrogen loadings to land in Zone 1.

Table A4.6. Nitrogen application from irrigation.

Table A4.7. Nitrogen applications used in Table 7.

Table A4.8. Site-by-site nitrogen summary on the N point worksheet.

Table A4.9. Stream number and stream name.

Table A5.1 Entering nitrogen loadings in surface water zones, Nbkg worksheet.

Table A5.2 Example of estimation of nitrogen concentration of surface water monitoring site 26,

„Nbkg‟ worksheet.

Table A5.3 Example of estimation of nitrogen concentration of groundwater at site 222, „Nbkg‟

worksheet.

Table A5.4 Example of land use loadings of zone 19, „Nirri‟ worksheet.

Table A5.5 Example of an error in entering the land use areas for zone 19, „Nirri‟ worksheet.

ix

Table A5.6 Example of nitrogen balance calculations, „Nsummary‟ worksheet.

Table A5.7 Example of stream nitrogen concentrations, „Nbkgrd‟ worksheet.

Table A5.8 Example of stream nitrogen concentrations, „Npoint‟ worksheet.

Table A5.9 Example of groundwater nitrogen concentrations „Nbkgrd‟ worksheet.

Y:\Science\Groundwater\Staff Files\Husam\old_work\Paul\Ruataniwha Plains - 09 Oct.doc 1

1.0 INTRODUCTION

The Ruataniwha Plains, Hawkes Bay, New Zealand is an area of approximately 31000 ha

farmland. The area regularly suffers from agricultural drought in summer, in common with

other regions on the east coast of the North Island and South Island.

Development pressures in the area have led to investigations into water demand for irrigation,

and investigations into water availability from surface water and groundwater. These

pressures have also led the Hawkes Bay Regional Council to investigate the potential risks to

the environment of future developments.

This reports assesses the effects of land use on surface water and groundwater quality, in

particular nitrogen applications to land. One common effect of development is an increase in

nitrogen concentrations in surface water and groundwater from infiltration of surface

application of fertilisers, animal wastes, etc. to groundwater and then to surface water.

Residence time in groundwater systems can be relatively long (decades) so this lag in the

system needs to be considered when predicting the effects of land use on water quality.

These effects are modelled with a set of Excel spreadsheets that combine existing data

including: surface water quality, surface water quantity, aquifer geometry, groundwater

quantity, groundwater quality and land use. The model uses groundwater quantity predictions

from a groundwater flow model, irrigation recharge from an irrigation model, and predictions

of nitrogen discharge from a land use model. All these data are combined to predict surface

and groundwater nitrate-nitrogen concentrations for a number of potential sub-regional

development options. This model has been developed for evaluation of the effects of existing

land use, irrigation, and point sources on nitrogen levels in surface water and groundwater.

This report summarises the input data to the model, the model datasets, and predicts nitrogen

concentrations Ruataniwha Plains surface water and groundwater over time. The report also

outlines operation of the Excel spreadsheets and discusses some of the assumptions used in

designing the model.


2.0 REVIEW

Hawkes Bay Regional Council (2000, page 21) reported 18 nitrate concentrations in the

Ruataniwha Plains groundwater. Fifteen of these had mean nitrate concentrations less than

1 mg L-1

. Two wells had concentrations between 1 and 3 mg L-1

; one well had a

concentration in the range 3 to 5 mg L-1

. Nitrate concentrations were tending to increase in 8

of the 18 wells, and tending to decrease in 9 of the 18 wells.

Hawkes Bay Regional Council (2001, page 39) report nitrate concentrations in 11 Ruataniwha

wells. Nine wells had groundwater with nitrate concentrations of 0-1.9 mg L-1

and two wells

had groundwater with nitrate concentrations of 3 to 6 mg L-1

. Four surface water quality

measurements are reported for the Ruataniwha Plains in 2000/2001 (Hawkes Bay Regional

Council 2001, page 25). One site had a median concentration in the range 0.1 to 0.15 mg L-1

,

one site had median concentrations of 0.1 to 0.5 mg L-1

and two sites had median

concentrations between 0.51 and 1.0 mg L-1

. Hawkes Bay Regional Council (1998, page

74ff) report nitrate concentrations at low flows. Mean nitrate-nitrogen concentrations were in

the range 0.54 to 1.72 mg L-1

in Mangaonuku Stream. In general, mean nitrate concentration

for Ruataniwha Plains rivers range from below detection limit (upper Waipawa River) to 3.6

mg/L (Porangahau Stream). Concentrations are typically higher in the south western area of

the plains, most likely due to a combination of land use and underlying geology.

Groundwater flow directions in the Ruataniwha Plains are broadly towards the south east

(Hawkes Bay Regional Council, 1999 and Luba, 2001) converging on the Waipawa River and

Tukituki River gorges. West of line running approximately from Tikokino to Takapau the

groundwater elevation tends to decline with increasing depth (Brookes pers. comm.)

indicating a vertical-downwards component of groundwater flow. Groundwater level

elevation tends to increase with depth east of a line approximately between Tikokino and

Takapau indicating a vertical-upwards component of groundwater flow (Hawkes Bay

Regional Council, 1999 and Luba, 2001). Low-flow gaugings in February 1973 (Hawkes Bay

Regional Council, 1999) and Wood (pers. comm. 2002) identify sections of the Mangaonuku,

Waipawa, Tukituki, Tukipo, and Makaretu rivers that gain and lose flow (Fig. 3). The

Waipawa and Tukituki rivers lose water for most of their riverbed across the Ruataniwha

Plains. These rivers both gain flow in the lower plains between each rivers‟ gorge and

approximately 3 km upstream. Other rivers also tend to gain water in the lower sections of

the Ruataniwha Plains. Loris (pers.comm.) identifies a section of the lower Kahahakuri


Stream that consistently gains water. Sarrazin (2002) identifies the Ongaonga Stream gaining

water blow Ongaonga township.

Tertiary geological units are interpreted by Hawkes Bay Regional Council (2001) to occur

under the Ruataniwha Plains in a synclinal structure. Quaternary gravels (Hawkes Bay

Regional Council, 2001) are interpreted as in-filling the synclinal structure.

Sarrazin (2002) summarises river and stream-habitats in the Ruataniwha Plains (e.g. Fig. 4).

Rivers and streams are also identified as: „groundwater recharged‟, „groundwater and

catchment recharged‟, and „catchment recharged‟. Streams that are classified with some

component of groundwater recharge include: streams on the Mangaonuku Stream above the

confluence with the Waipawa River, Kahakuri, Ongaonga. Waipawamate, Black Stream,

Maharakeke, and small streams on the Tukipo above the confluence with the Makaretu River.

The potentially-irrigable land area in the Ruataniwha Plains is estimated at 35,100 ha (Lincoln

Ventures, 2002). It is estimated that a maximum irrigation rate of 0.21 L s-1

ha-1

is required

for grape production and a maximum irrigation rate of 0.49 L s-1

ha-1

is required for intensive

pastoral farming and cropping. Seasonal requirements for irrigation are between 450 mm

year-1

in the west to 660 mm year-1

in the drier eastern areas.

Annual drainage is predicted for five landuses (Lincoln Ventures, 2002). Extensive non-

irrigated pasture, irrigated cropping, mixed cropping, grapes and mixed cropping, and grapes

and irrigated pasture are associated with drainage between 200 mm year-1

and 1600 mm

year-1

.

Groundwater generally travels in a NW to SE direction across the plains (Fig. 5 shows

contours of groundwater level). All groundwater in the plains would discharge through the

Waipawa and Tukituki gorges with the (reasonable) assumption of an impermeable geological

base to the plains.


4.0 CHEMICAL CONDITIONS IN GROUNDWATER

The form of nitrogen in groundwater is strongly dependent on microbial processes (Chapelle,

1993). Under aerobic conditions, nitrate (NO3) is the dominant form of nitrogen, typically

being produced by oxidation of more reduced forms of nitrogen. Under anaerobic conditions,

ammonium (NH4) is the dominant forms of nitrogen, being produced by reduction of nitrate.

In addition to nitrate and ammonium, groundwaters typically contain minor components of

organic nitrogen, which is present in biological materials such as proteins and nitrite which is

an intermediate in the oxidation of NH4 to NO3.

Because nitrogen can exist as nitrate, nitrite, ammonium or organic nitrogen, a model of

nitrogen transport in groundwater should consider the total concentration of nitrogen, rather

than just one of these species. Unfortunately, the total nitrogen concentration has not been

measured for most groundwater samples from the Ruataniwha Plains. Hence a proxy for total

nitrogen concentration is required for modelling purposes.

Analysis of groundwater samples from wells on the Ruataniwha Plains clearly show that the

majority of groundwaters are either strongly oxidising, and therefore contain almost

exclusively nitrate, or are strongly reducing, and thus contain almost exclusively ammonium

(Figure 6). This relationship can also be presented by comparing the concentrations of nitrate

and ammonium to the concentration of total nitrogen (note that total nitrogen has not been

analysed for every sample). For samples that contain more than 0.1 mg/l nitrate, there is a

very good correlation between nitrate and total nitrogen (r2 = 0.99, n = 75), indicating that

almost all nitrogen is present as nitrate (Figure 7). For this same set of samples (NO3 > 0.1

mg/l), there is a very poor correlation between ammonium and total nitrate (r2 = 0.1, n = 56),

implying that very little of the nitrogen exists as ammonium. Conversely, for all groundwater

samples with less than 0.1 mg/L nitrate, total nitrogen is well correlated to ammonium

concentration (r2 = 0.92, n = 35) (Figure 8), but poorly correlated to nitrate concentration (r

2 =

0.03, n = 36). The importance of organic nitrogen in the Ruataniwha groundwaters is not

clear, because for most samples it has not been analysed.

Thus for the purpose of water quality modelling, nitrate or ammonium concentration can be

used to proxy total nitrogen concentration. For samples with more than 0.1 mg/L nitrate, the

nitrate concentration is a good proxy for total nitrogen concentration. For samples with less

than 0.1 mg/L nitrate, ammonium is a better proxy for total nitrogen concentration. Table 1


shows which of the two proxy variables should be employed for nitrogen transport modelling,

in cases where a total nitrogen concentration is not available.

Table 1. Proxy variable to use for total nitrogen concentration.

SiteID Depth Proxy

133 NO3

134 NO3

135 NO3

136 NO3

137 30 NO3

138 NO3

145 NO3

146 12.4 NO3

147 NO3

220 45.7 NO3

221 57.3 NO3

222 21.8 NO3

223 55.5 NO3

224 75 NH4

225 52 NO3

226 25.2 NH4

227 45 NH4

229 24.4 NH4

230 65.9 NH4

231 22.6 NO3

233 46.3 NO3

234 53 NH4

235 88.2 NH4

236 65.8 NO3

237 110 NH4

239 142 NH4

243 NH4

1365 ~30 NH4

1377 7 NO3

1385 2 NO3

1497 NO3

2224 5.3 NH4

2227 NO3

2229 NO3

2387 30 NO3

2597 NO3

2598 NO3

2599 NO3

Because the nitrate-ammonium conversion is controlled by the oxidation state of the

groundwater, it is logical that there should be a relationship between nitrate concentration and

the depth from which the groundwater was extracted. There is a greater probability of finding


elevated nitrate concentrations in shallow wells than in deeper wells (Figure 9). Similarly,

there is a greater probability of elevated ammonium concentrations in deeper wells. Note that

these relationships are probabilistic, not deterministic, and so they have limited use in water

quality modelling. In other words, nitrate on its own is not a conservative indicator of total

nitrogen, even if well depth is taken into consideration.

Wells with the highest nitrate-nitrogen concentration (Table A3.3) have not been sampled for

a considerable period. For example:

well 133, mean 21.6 mg/L, last sampled 1992







It is recommended that these wells are re-sampled to identify current groundwater quality.

Well 1487, last sampled in 1987 could also be re-sampled.

6.0 GROUNDWATER FLOW MODEL

Murray (2002) describes the groundwater flow model (Fig. 5) of the Ruataniwha Plains:

“The model grid is a single layer with 80 rows and 100 columns. The grid interval is

500 m… Active cells cover all the Ruataniwha Plains underlain by gravels, and in the east,

the north and the south, extend to the watersheds of the Waipawa and Tukituki Rivers. In the

west the grid boundary extends beyond the limits of the gravels to exclude the hill and

mountain catchments of the Makaroro, Waipawa, Tukituki and Makaretu Rivers above Burnt

Bridge, Pendle Hill, Folgers, Rd, and Pagetts Rd stage recording sites respectively.”

“The active grid within these boundaries contains 3719 cells, equivalent to 92,975 ha” and

34,600 ha of this area is irrigated in the model simulating irrigation scenarios.


“Elevation of the base of the gravel aquifer in the model ranges from 77 m below sea level in

the vicinity of Linburn Road between the Waipawa and Mangaonuku Rivers, and rises to

more than 260 m above sea level in the hill country in the north and west of the basin. In the

irrigable area of the plains, gravel thickness ranges from 45 m to more than 200 m.”

The model layer is treated as unconfined, although there is some evidence that confined

aquifers occur on the Ruataniwha Plains. Water is input to the model through rainfall

recharge and rivers. Rainfall recharge is estimated from daily „percolation below the root

zone from specific soil types and in annual rainfall zones‟. Stream-groundwater interaction is

determined from the relative heights of the stream and the groundwater system. The model

simulates interaction of groundwater with the Mangaonuku Stream, and four short-length

tributaries, the Waipawa River, Makaroro River, Kahahakuri Stream (lower reaches),

Tukituki River, Tukipo River (and two of its tributaries), Makaretu River, Porangahau

Stream, and the Maharakeke Stream. Water also enters the model across the western

boundary from flow in the Makaroro River, Waipawa River, Tukituki River and Makaretu

River.

Water is lost from the model through pumping and river discharge. All the discharge from the

groundwater system, and rivers, is modelled as discharging through the Waipawa River gorge

and Tukituki River gorge.

The groundwater flow model is calibrated to river loss using a series of gaugings on the

Waipawa, Tukituki, and Makaretu Rivers. Calibration uses aquifer hydraulic conductivity,

stream bed conductance, specific yield and zonal hydraulic conductivity distributions to

match the observed stream depletion and the range in groundwater levels.

7.0 GROUNDWATER QUALITY AND SURFACE WATER QUALITY MODEL

7.1 Design

The model uses datasets that represent the Ruataniwha Plains area, stream flow, groundwater

flow, stream quality, groundwater quality, and landuse. Microsoft Excel is used to link these

datasets to allow users flexibility in the assessment of land use effects on surface water and

groundwater.


The datasets and spreadsheets that form the model are described in detail in Appendix 4.

Follows an outline of the key components of the model.

7.2 Model components

The Ruataniwha Plains and surrounds are represented in this model by a 40 km-by-50 km

model with a grid of 500 m-by-500 m cells. This model has the same extents, and cell size, as

the Ruataniwha Plains groundwater flow model (Fig. 5).

Three major surface-hydrology zones are defined on the Ruataniwha Plains based on the

pattern of river losses and gains (Fig. 3), water quality ranking of streams (Fig. 4) and

groundwater flow directions (Fig. 5). This aims to associate land-use areas in the plains with

surface water quality, and groundwater quality, monitoring sites.

Three „major‟ land-use zones are defined:

Zone 1: Land that drains through streams or groundwater, to the Waipawa River.

Zone 2: Land that drains, through streams or groundwater, to the Tukituki River,

excluding the Tukipo River.

Zone 3: Land that drains, through streams or groundwater, to the Tukipo River.

These zones are further sub-divided (Fig. 10) into zones where surface water monitoring sites

may reflect the land use in the zone.

Surface water monitoring sites are chosen from the HBRC monitoring network (Fig. 11).

Generally monitoring sites are chosen in sections of river that are gaining flow. This is

because sections of river that are gaining flow likely to represent, at least in part, the water

quality effects of land use in zone.

Groundwater capture zones (Fig. 12) are defined based on the groundwater flow directions

derived from groundwater levels predicted by the groundwater flow model (Fig. 5). The up-

gradient end of the capture zone is generally taken as a hydrogeological boundary, e.g.

impermeable boundary or river boundary.


Groundwater monitoring sites are chosen from the HBRC monitoring network (Fig. 13). One

capture zone is assigned to each well.

Groundwater seepage residence time is a significant control on the long-term response of

surface water quality, and groundwater quality, to land use. Estimates of groundwater

seepage velocity (e.g. Fig. 14 and Fig. 15) are used to calculate the area of land that will

contribute to surface water, and groundwater quality on estimates of transient water trends.


The model allows zones, or individual cells, to be loaded with nitrogen. Nitrogen application

rates to land are expressed as kgN/ha/yr. Nitrogen can also be loaded to rivers as a

„background‟ concentration (in mg/L) representing the mean concentration that each river

begins its crossing of the Ruataniwha Plains.

There are three methods to load nitrogen onto the cells: „background‟, „irrigation‟, or just

„point‟ source.

„Background‟ nitrogen concentrations aim to represent observed nitrogen concentrations in

surface water and groundwater. „Irrigated‟ nitrogen concentrations aim to predict the effects

of land use change in the Ruataniwha Plains on top of the „background‟ land use. Nitrogen is

„applied‟ to the sub zones (Fig. 10) in three land uses. The land area and nitrogen application

rates are selectable by the user. Nitrogen applications at „point‟ sources are through

individual cells in the model. The rates of nitrogen application and locations of cells are

selectable by the user in a worksheet representing the Ruataniwha Plains.

7.4 Non-irrigated and irrigated models

Groundwater flow model predictions of groundwater flow rates without irrigation (Fig. 14)

and with irrigation (Fig. 15) are used to identify the areas of land that are contributing

nitrogen to surface water and groundwater monitoring sites within defined time periods.

Estimates of groundwater flow volumes calculated by these models are used in the estimates

of groundwater nitrogen concentrations.


7.5 Estimation of nitrogen concentrations – steady state

Mass balance equations are used to calculate nitrogen concentrations in rivers. Equations for

rivers crossing the Ruataniwha Plains take the form:

MS MU + ML

MS nitrogen mass passing the monitoring point (mass/time)

MU nitrogen mass in the river as the river enters the Ruataniwha Plains (=0 for

streams that rise in the plains) (mass/time)

ML nitrogen mass from land use. This is the net nitrogen mass passing out of the

soil (mass/time)

Masses are calculated from observed nitrogen concentrations and observed, or estimated,

river/stream flow rates. An aim in designing the sub-zones (Fig. 10) is that one stream water

flow monitoring site and one stream water quality monitoring sites (gaining streams) would

represent the effects of land use in that sub-zone. Unfortunately this could not be achieved as:

a number of sub-zones, where there is a case for landuse in a sub-zone having an influence on

surface water quality, do not have monitoring sites; and a number of surface water quantity

monitoring sites have no measurements of water quality and vice-versa.

The concentrations of nitrogen in surface water at the monitoring site is:

CS = MS/MSW

CS concentration of N in surface water

MSW mass of water passing the monitoring point (mass/time)

It is assumed that the nitrogen is fully mixed in the surface water.

The concentrations of nitrogen in groundwater is:

CG = MG/MW

CG concentration of N in groundwater


MG mass of nitrogen loaded to groundwater within the capture zone (mass/time)

MW mass of water passing the cell representing the groundwater monitoring site

(mass/time)

Nitrogen in groundwater is commonly not fully mixed within the formation, for example

nitrogen concentrations generally decline with depth on the Ruataniwha Plains (Fig. 9). This

is investigated in Section 7.8.

7.6 Checks on the mass balance equations

Background, irrigated, and point source applications in the Ruataniwha Plains model are

summarised in a model worksheet:

NIn = NU + NB + NI + NP

NOut = NW + NT

All units are kgN/yr

NIn nitrogen inputs to the Ruataniwha Plains

NU nitrogen entering through upstream boundary

NB nitrogen from „background‟ land use

NI nitrogen from „irrigated‟ land use

NP nitrogen from point sources

NOut nitrogen output from the Ruataniwha Plains

NW nitrogen leaving the Ruataniwha Plains through the Waipawa River

NT nitrogen leaving the Ruataniwha Plains through the Tukituki River

The difference between NIn and NOut, calculated on the worksheet, should be less than 1%.

Differences between these two numbers should only arise due to rounding errors in the

spreadsheets.


7.7 Transient calculations

The model allows the prediction of long-term response of nitrogen concentrations in surface

water and groundwater. It does this by determining the land-use cells that are within a

specified time. This is calculated from the groundwater seepage velocities in the Ruataniwha

Plains (non-irrigated and irrigated). The user can investigate longer-term trends in

groundwater quality in weekly (or longer time interval) time-steps. The calculations are of

transient stream concentrations considering land use to be constant. The „transient‟ look of

the calculated nitrate concentrations is because the stream flow diluting nitrogen inputs is

transient.

7.8 Model calibration

7.8.1 Surface water

The model is calibrated to existing mean surface water quality measurements (Appendix 3) at

14 sites.

The nitrogen loadings (as kg N/ha/yr) to sub-zones are adjusted manually to produce the best

comparison between predicted surface water nitrogen concentrations and observed surface

water nitrogen concentrations.

Some subzones are uniquely associated with one monitoring point and therefore nitrogen

loadings are uniquely associated with surface water quality. For example, surface water zone

16 is associated with monitoring site 273 and a loading of 4 kgN/ha/yr to subzone 16 is

associated with a nitrogen concentration of 1.08 mg/L at site 273 (Table 2). Other monitoring

sites are associated with a number of zones; for example monitoring site 26 combines the land

use of zones 16, 17, 18 and 19. Therefore the nitrogen concentration at site 26 is a

combination of loadings from the four subzones and the loadings for each subzone are not

uniquely identifiable.

One aim of the design of sub-regional boundaries is to relate land use and water quality in the

Ruataniwha Plains by a „tree structure‟ of sub-zones and monitoring sites. However, nitrogen

loadings could not be estimated uniquely because a number of sub-zones are not uniquely

associated with a monitoring site. Subzones with non-unique estimates of nitrogen loading


are: 11, 12, 13, 14, 15, 17, 18, 19, 21, 23, 25, 31, 33, 36, 37, 38 and 39. A monitoring

network that could give unique estimates of nitrogen loadings is outlined in Section 12.

The process of model calibration is summarised for the catchment of the Waipawa River:

set nitrogen concentrations and river flows of rivers and streams (Mangamate,

Mangaonuku, Waipawa) based on observations

set nitrogen concentration and river flows at monitoring sites

set nitrogen loadings of „up-catchment‟, unmonitored sub-zones:

- sub zone 11 0.5 kg N/ha/yr



adjust loading on sub zone 14 and 15 to match observed water quality at monitoring site

287:

- sub zone 14 1 kg N/ha/yr


This gives a predicted nitrogen concentration at site 287 of 0.5 mg/L versus observed mean

nitrogen concentration of 0.53 mg/L (Table 2).

adjust loading on sub zone 16 to match observed water quality at monitoring site 273:



nitrogen concentration of 1.08 mg/L.

adjust loading on sub zone 17 to match observed water quality of monitoring site 286:

- sub zone 17 28 kg N ha/yr


concentration of 1.64 mg/L. Monitoring site 286 represents land use in sub-zones 11 + 12 +

13 + 14 + 15 + 16 + 17.





concentration of 1.66 mg/L. Monitoring site 284 represents land use in sub-zones 11 + 12 +

13 + 14 + 15 + 16 + 17 + 18.




concentration of 0.63 mg/L. Monitoring site 284 represents land uses in sub-zones 11 to 19.

This method of calibration represents the cumulative process where surface water

progressively intersect nitrogen as water moves down the catchment. In the case of Waipawa

River catchment, the land use in sub zone 19 is estimated as 130 kg N/ha/yr to match the

observed nitrogen mass exported out from the Ruataniwha Plains through the Waipawa River.

This loading is much greater than the loadings likely from actual land use (Ironside pers.

comm.).

Combinations of nitrogen loadings can produce similar surface water nitrogen concentrations.

For example, a land use of 10 kg N/ha/yr in sub zone 18 and a land use of 10 kg N/ha/yr in

sub zone 19 gives an estimated mean nitrogen concentration of 2.34 mg/L at site 284 (versus

observed of 1.66) and 0.6 mg/L at site 26 (versus observed of 0.63). The estimated loading of

10 kgN/ha/yr for subzone 19 is significantly different from the 130 kgN/ha/yr estimated in the

calibration process (Table 2). This implies that, for some subzones, surface water nitrogen

concentrations are relatively insensitive to sub zonal nitrogen loading.

Table 2 lists the sub-zones, and sub zone applications, with calculated and observed nitrogen

concentrations at surface water monitoring sites for the calibrated model of the Ruataniwha

Plains.


Table 2. Zone loads and predicted surface water quality, steady-state.

Surface water subzones

11 12 13 14 15 16 17 18 19 21 22

Load kgN/ha/yr 0.5 0.5 0.5 1 1 4 28 5 130 2 22


23 24 25 31 32 33 34 35 36 37 38 39

Load kgN/ha/yr 3 4 0 0 5 25 46 9 13 0 0 0

Monitoring

points:

Surface water

site

River/stream name Calculated N

at site

(mg/L)

Observed

N

(mg/L)

Difference

calc-obs

273 Mangamate@SH50 bridge 1.08 1.08 0

284 Mangaonuku @ Tikokino Rd 1.67 1.66 0.01

286 Mangaonuku @ Argyll Rd 1.64 1.64 0

287 Mangaonuku@SH50 0.5 0.53 -0.03

26 Waipawa@RDS 0.63 0.63 0

356 Tukituki @SH50 0.13 0.18 -0.05

20 Tukituki at Ongaonga Br 0.26 0.26 0

659 Kahahakuri@Plant. Rd Bridge 2.56 2.59 -0.03

410 Kahahakuri@Ongaonga Rd 2.91 2.91 0

144 Tukipo@SH50 0.94 0.85 0.09

279 Tukipo@Burnside 2.07 2.08 -0.01

21 Tukipo@Ashcott 1.6 1.1 0.5

398 Porangahau@Fraser 3.5 3.53 -0.03

397 Porangahau@Oruawharo 1.83 1.91 -0.08

405 Maharakeke@SH2 1.91 1.96 -0.05

23 Tukituki@Coughlin 1.1 0.99 0.11

The comparison between calculated nitrogen concentrations and observed mean nitrogen

concentrations is worst at sites 356 (Tukituki River) and 21 (Tukipo River).

Observed nitrogen concentrations in the Tukituki River and the Tukipo River decline across

the Ruataniwha Plains (Table 3). For example, a mean concentration of 0.8 at site 356

compares with a mean concentration of 0.26 at site 20. The Tukituki River loses flow

between these two sites (Wood, pers. comm.) and a significant decline in mean concentration

is difficult to explain. The increase in N concentration between Tukituki River @ Folgers, an

assumed 0.1 mg/L, and Tukituki River @ SH50 requires significant land use effects. Zone 24

is predicted to have a nitrogen output of 72 kgN/ha/yr to match the increase in nitrogen

concentrations, and this output makes it impossible to model the nitrogen concentrations in

the lower reaches. Also, the reach of the Tukituki River between Folgers and SH50 loses

flow - up to 3 m3/s (Wood, pers. comm.). The mean of all the nitrogen concentrations is

significantly biased by one measurement of 38.18 mg/L on the 18/11/94. Removing this


number from the data set calculates a mean of 0.18 mg/L nitrate-nitrogen. Such outliers do

not appear in the data of the Tukituki River sites, or Tukipo River sites. The Tukipo River

gains flow (Wood, pers. comm.) so dilution may cause the decline in concentration.

Table 3. River N concentrations used in model.

Location Site

Number

Observed N

(mg/L)

Comment

Tukituki @ Folgers - 0.1 Assumed

Tukituki @ SH50 356 0.18 Mean of observations

Tukituki @ Ongaonga Bridge 20 0.26 Mean of observations

Tukituki @ Coughlin 23 0.99 Mean of observations

Tukipo @ SH50 144 0.85 Mean of observations

Tukipo @ Burnside 279 2.08 Mean of observations

Tukipo @ Ashcott 21 1.1 Mean of observations

7.8.2 Groundwater

The nitrogen loadings that match best the surface water quality tend to predict groundwater

quality values that are too low (Table 4). This is possibly because the calculation of

groundwater nitrogen concentrations assumes mixing with the full thickness of aquifer.

Generally nitrogen concentrations decrease with increasing depth in the aquifer (Figure 7).

For example, the shallowest well (Appendix 3, Table A3.3), well 1377, has the highest

observed nitrogen concentration (Table 4). Wells with predicted nitrogen concentrations less

than observed are increased by adjusting the ratio of the saturated thickness. Ratios less than

0.1 indicate that input nitrogen is mixing in less than about 20 m of aquifer as the aquifer is up

to around 200 m thick in the Ruataniwha Plains.


Table 4. Zone loads and predicted groundwater quality assuming full mixing.

Surface water

subzones

11 12 13 14 15 16 17 18 19 21 22

Load

kgN/ha/yr

0.5 0.5 0.5 1 1 4 28 5 130 2 22

Surface water

subzones

23 24 25 31 32 33 34 35 36 37 38 39

Load

kgN/ha/yr

3 4 0 0 5 25 46 9 13 0 0 0

Monitoring points:

groundwater

site

Calculated N

at site

(mg/L)

Observed

N

(mg/L)

Difference

calc-obs

220 4.7 0.3 4.4

236 1 1 0

146 1.2 1 0.2

222 0.9 0.48 0.42

224 0.1 0.27 -0.17

223 0.2 6.26 -6.06

239 0.2 0.55 -0.35

2227 1.5 3.52 -2.02

233 0.1 0.42 -0.32

2229 0 0.05 -0.05

231 0 4.41 -4.41

229 3.44 0.55 2.89

1497 0.03 1.22 -1.19

1377 1.62 22.6 -20.98


Table 5. Zone loads and predicted groundwater quality assuming partial mixing.

Surface water

subzones 11 12 13 14 15 16 17 18 19 21 22

Load

kgN/ha/yr 0.5 0.5 0.5 1 1 4 28 5 130 2 22

Surface water

subzones 23 24 25 31 32 33 34 35 36 37 38 39

Load

kgN/ha/yr 3 4 0 0 5 25 46 9 13 0 0 0

Monitoring

points:

groundwater

site

Calculated N

at site

(mg/L)

Observed

N

(mg/L)

Difference

calc-obs

Mixing

thickness

% of full

thickness

220 4.7 0.3 4.4 1

236 1 1 0 1

146 1.2 1 0.2 1

222 0.9 0.48 0.42 1

224 0.25 0.27 -0.02 0.4

223 6.67 6.26 0.41 0.03

239 0.5 0.55 -0.05 0.4

2227 3.75 3.52 0.23 0.4

233 0.4 0.42 -0.02 0.25

2229 0 0.05 -0.05 1

231 0 4.41 -4.41 1

229 3.44 0.55 2.89 1

1497 1.5 1.22 0.28 0.02

1377 23.14 22.6 0.54 0.07

8.0 NITROGEN CONCENTRATIONS WITHOUT IRRIGATION

The effects of N applications on surface and groundwater quality can be estimated for land

use systems (Table 6). These models use the seepage velocity calculations for the non-

irrigated MODFLOW model (Fig. 14). The „irrigable area‟ of the Ruataniwha Plains is

defined by Hawkes Bay Regional Council to include most of the flat land in the Plains.

Land use with a 16 kgN/ha/yr loading, equivalent to irrigated beef, over the irrigable area of

the Ruataniwha Plains is predicted to result in mean surface water quality that is generally a

little better than present (Table 7). This is because sub-zonal nitrogen loadings in Table 7 are

generally less than current land use as estimated by the „calibrated‟ model (Table 2).


Table 6. Nitrogen leaching for various land uses (HortResearch pers. comm.).

Land use system Leaching (kg/N/ha/yr)

Takapau Tikokino

Apples 15 17

Grapes 41 42

Maize 4 4

Potatoes 94 102

Squash 173 179

Dairy 42 44

Dryland beef 7 7

Irrigated beef 16 17

Dryland sheep 6 6

Irrigated sheep 20 21

The effects of increasing nitrogen leaching in the irrigable area of the Ruataniwha Plains from

43 kgN/ha/yr (equivalent to dairy), Table 8, to 98 kg/N/ha/yr (equivalent to potatoes), Table

9, and to 176 kg/N/ha/yr (equivalent to squash), Table 10 causes predicted mean nitrogen

concentrations in surface water to increase. For example, the predicted mean nitrogen

concentration at site 410 (Kahahakuri Stream) increases from 2.91 mg/L currently observed to

6.68 mg/L (dairy on the irrigable area) to 27.35 mg/L (squash on the irrigable area).

These calculations assume that mean stream flow remains the same under irrigation. These

calculations represent a „worst case‟ prediction because irrigation may increase stream flows,

due to an increase in soil drainage recharging spring-fed streams, and would cause some

dilution.


Table 7. Surface water quality with a nitrogen loading of 16 kgN/ha/yr (equivalent to

irrigated beef) to Ruataniwha Plains irrigable area*.

Surface water

subzones 11 12 13 14 15 16 17 18 19 21 22

Load kgN/ha/yr 0.5 0.5 0.5 1 1 2.3 3.1 6.5 11.5 10.8 5.1

Surface water

subzones 23 24 25 31 32 33 34 35 36 37 38 39

Load kgN/ha/yr 7.9 7.7 14.2 13.6 0.9 5.8 7.8 4.2 11.2 15.6 6.9 16

Monitoring

points:

Surface

water

site


at site

(mg/L)

Observed

N

(mg/L)

Difference

calc-obs

273 Mangamate@SH50 bridge 0.7 1.08 -0.38

284 Mangaonuku @ Tikokino Rd 1.12 1.66 -0.54

286 Mangaonuku @ Argyll Rd 0.35 1.64 -1.29

287 Mangaonuku@SH50 0.5 0.53 -0.03

26 Waipawa@RDS 0.32 0.63 -0.31

356 Tukituki @SH50 0.17 0.18 -0.01

20 Tukituki at Ongaonga Br 0.5 0.26 0.24


410 Kahahakuri@Ongaonga Rd 2.49 2.91 -0.42

144 Tukipo@SH50 0.16 0.85 -0.69


21 Tukipo@Ashcott 0.85 1.1 -0.25


397 Porangahau@Oruawharo 0.85 1.91 -1.06

405 Maharakeke@SH2 1.3 1.96 -0.66

23 Tukituki@Coughlin 0.8 0.99 -0.19

* Land use in zones 11, 12 and 13 is set to 0.5 kg N/ha/yr; land use in zones 14 and 15 is set

to 1 kg N/ha/yr; other land uses are set to 16 kg N/ha/yr in the irrigable area of each zone. In

all zones, except zone 39, the number of irrigable cells is less than the number of cells in the

zone; therefore the loadings in Table 7 are less than 16 kg N/ha/yr.


Table 8. Surface water quality with nitrogen loading of 43 kgN/ha/yr (equivalent to

dairy) to Ruataniwha Plains irrigable area.

Surface water

subzones 11 12 13 14 15 16 17 18 19 21 22

Load

kgN/ha/yr 0.5 0.5 0.5 1 1 6 8.3 17.5 30.9 29.1 13.7

Surface water

subzones 23 24 25 31 32 33 34 35 36 37 38 39

Load

kgN/ha/yr 21.3 20.7 38 36.5 2.3 15.5 21.1 11.3 30.1 42 18.4 43

Monitoring

points:

Surface

water

site


at site

(mg/L)

Observed

N

(mg/L)

Difference

calc-obs

273 Mangamate@SH50 bridge 1.52 1.08 0.44


286 Mangaonuku @ Argyll Rd 0.79 1.64 -0.85

287 Mangaonuku@SH50 0.5 0.53 -0.03

26 Waipawa@RDS 0.74 0.63 0.11

356 Tukituki @SH50 0.3 0.18 0.12



410 Kahahakuri@Ongaonga Rd 6.68 2.91 3.77

144 Tukipo@SH50 0.43 0.85 -0.42




397 Porangahau@Oruawharo 2.29 1.91 0.38

405 Maharakeke@SH2 3.48 1.96 1.52



Table 9. Predictions of surface water quality with nitrogen loading of 98 kgN/ha/yr

(equivalent to potatoes) to Ruataniwha Plains irrigable area.

Surface water

subzones

11 12 13 14 15 16 17 18 19 21 22

Load

kgN/ha/yr

0.5 0.5 0.5 1 1 13.8 19 39.9 70.4 66.4 31.3

Surface water

subzones

23 24 25 31 32 33 34 35 36 37 38 39

Load

kgN/ha/yr

48.6 47.1 86.7 83.1 5.3 35.4 48 25.6 68.6 95.8 42 98

Monitoring

points:

Surface

water

site


at site

(mg/L)

Observed

N

(mg/L)

Difference

calc-obs



286 Mangaonuku @ Argyll Rd 1.68 1.64 0.04

287 Mangaonuku@SH50 0.5 0.53 -0.03

26 Waipawa@RDS 1.6 0.63 0.97

356 Tukituki @SH50 0.56 0.18 0.38


659 Kahahakuri@Plant. Rd Bridge 3.65 2.59 1.06


144 Tukipo@SH50 0.98 0.85 0.13

279 Tukipo@Burnside 2.75 2.08 0.67


398 Porangahau@Fraser 3.65 3.53 0.12


405 Maharakeke@SH2 7.94 1.96 5.98



Table 10. Surface water quality with nitrogen loading of 176 kgN/ha/yr (equivalent to

squash) to Ruataniwha Plains irrigable area.

Surface water

subzones 11 12 13 14 15 16 17 18 19 21 22

Load kgN/ha/yr

0.5 0.5 0.5 1 1 24.8 34.1 71.7 126.5 119.2 56.3

Surface water subzones 23 24 25 31 32 33 34 35 36 37 38 39

Load kgN/ha/yr

87.2 84.6 155.7 149.3 9.4 63.5 86.2 46 123.2 172.1 75.4 176

Monitoring

points:

Surface water

site


at site

(mg/L)

Observed

N

(mg/L)

Difference

calc-obs




287 Mangaonuku@SH50 0.5 0.53 -0.03

26 Waipawa@RDS 2.82 0.63 2.19

356 Tukituki @SH50 0.93 0.18 0.75




144 Tukipo@SH50 1.77 0.85 0.92





405 Maharakeke@SH2 14.25 1.96 12.29


Groundwater concentrations are predicted to increase with increased nitrogen applications.

This is most marked for the wells that are possibly mixed in a small cross-section of aquifer.

For example, the nitrogen concentration in well 223 is predicted to rise to 570 mg/L with a

176 kg/N/ha/yr (Table 11).


Table 11. Groundwater quality with a nitrogen loading of 176 kg/N/ha/yr (equivalent to

squash) to all irrigable cells.

Surface water subzones 11 12 13 14 15 16 17 18 19 21 22

Load kgN/ha/yr

0.5 0.5 0.5 1 1 24.8 34.1 71.7 126.5 119.2 56.3


Load kgN/ha/yr

87.2 84.6 155.7 149.3 9.4 63.5 86.2 46 123.2 172.1 75.4 176

Monitoring points:

groundwater

site

Calculated N at site

(mg/L)

Observed

N

(mg/L)

Difference

calc-obs

Mixing

thickness

% of full

thickness

220 6.3 0.3 6 1

236 6.8 1 5.8 1

146 17.2 1 16.2 1

222 5.8 0.48 5.32 1

224 19.3 0.27 18.98 0.4

223 570 6.26 563.74 0.03

239 40.5 0.55 39.95 0.4

2227 17.3 3.52 13.73 0.4

233 16.4 0.42 15.98 0.25

2229 4.2 0.05 4.15 1

231 14.5 4.41 10.09 1

229 2.8 0.55 2.25 1

1497 415 1.22 413.78 0.02

1377 244 22.6 221.7 0.07

The previous calculations assume that all nitrogen is transported to the monitoring sites

instantaneously. This is not the case because residence time in the groundwater system results

in a lag between application of chemicals to the groundwater system and the arrival of those

chemicals at monitoring sites.

Cell-by-cell estimates of residence time in the groundwater system are used to predict the

response of surface water quality to nitrogen applications of 16 (Table 12), 43 (Table 13), 98

(Table 14) and 176 kg/N/ha/yr (Table 15) application to all irrigation cells. These predictions

are made with the non-irrigated flow velocity predictions. As such the predictions represent a

worst case prediction for concentration as the dilution given by the non-irrigated model is less

than the dilution given by the irrigated model. The Kahahakuri @ Ongaonga site is predicted

as the most impacted by nitrogen application. For example, a 43 kg/N/ha/yr application

(Table 13) is predicted to raise mean nitrogen concentrations from the present mean

2.91 mg/L to 9.01 mg/L. The time scale of predicted change is generally decadal. For

example, nitrogen concentrations at Kahahakuri @ Ongaonga are predicted to increase in

response to loading throughout the 50 year period of simulation. In contrast, the nitrogen


concentration in the Porangahau @ Oruawharo appears to have „stabilised‟ after five years of

simulation.


16 kg/N/ha/yr to all irrigable cells.

Surface

site

Site name Observed

N

(mg/L)

Calculated N (mg/L) with 16 kg/h/yr loading

Year

1 2 5 10 20 30 50

273 Mangamate@SH50 bridge 1.08 1.08 1.08 1.08 1.08 1.18 1.18 1.18

284 Mangaonuku @ Tikokino Rd 1.66 1.66 1.76 1.76 1.96 2.06 2.06 2.16

26 Waipawa@RDS 0.63 0.63 0.63 0.73 0.83 0.93 0.93 0.93

410 Kahahakuri@Ongaonga Rd 2.91 2.91 3.01 3.31 3.71 4.51 4.91 5.21

659 Kahahakuri@Plant. Rd Bridge 2.59 2.59 2.69 2.79 2.99 3.13 3.19 3.19

20 Tukituki at Ongaonga Br 0.26 0.26 0.26 0.26 0.26 0.36 0.36 0.36

356 Tukituki @SH50 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8

23 Tukituki@Coughlin 0.99 1.09 1.19 1.29 1.39 1.59 1.69 1.69

21 Tukipo@Ashcott 1.1 1.3 1.4 1.6 1.7 1.8 1.9 1.9

144 Tukipo@SH50 0.85 0.85 0.85 0.85 0.95 1.05 1.15 1.15

279 Tukipo@Burnside 2.08 2.08 2.18 2.38 2.68 2.98 3.08 3.08

398 Porangahau@Fraser 3.53 3.93 3.93 4.03 4.03 4.13 4.13 4.13

397 Porangahau@Oruawharo 1.91 1.91 1.91 2.01 2.01 2.11 2.11 2.21




Surface

site

Site name Observed

N

(mg/L)


Year

1 2 5 10 20 30 50



26 Waipawa@RDS 0.63 0.73 0.73 0.83 1.13 1.33 1.33 1.43




356 Tukituki @SH50 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8


21 Tukipo@Ashcott 1.1 1.5 1.9 2.3 2.8 3.1 3.2 3.2

144 Tukipo@SH50 0.85 0.85 0.85 0.95 1.15 1.45 1.55 1.55

279 Tukipo@Burnside 2.08 2.08 2.38 2.98 3.68 4.48 4.78 4.78





Table 14. Mean nitrogen concentrations in streams over 50 years due to N application of


Surface

site

Site name Observed

N

(mg/L)


Year

1 2 5 10 20 30 50



26 Waipawa@RDS 0.63 0.73 0.83 1.13 1.73 2.23 2.33 2.43




356 Tukituki @SH50 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8


21 Tukipo@Ashcott 1.1 2 2.9 3.9 4.9 5.6 5.8 5.9

144 Tukipo@SH50 0.85 0.85 0.95 1.05 1.65 2.25 2.45 2.55

279 Tukipo@Burnside 2.08 2.18 2.78 4.18 5.68 7.48 8.18 8.28




Table 15. Mean nitrogen concentrations in streams over 50 years due to N application of


Surface

site

Site name Observed

N

(mg/L)


Year

1 2 5 10 20 30 50



26 Waipawa@RDS 0.63 0.83 1.03 1.53 2.53 3.43 3.63 3.83




356 Tukituki @SH50 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.9


21 Tukipo@Ashcott 1.1 2.8 4.3 6.2 7.9 9.3 9.5 9.7

144 Tukipo@SH50 0.85 0.85 0.95 1.25 2.25 3.35 3.65 3.85

279 Tukipo@Burnside 2.08 2.18 3.28 5.78 8.48 11.88 12.98 13.28





9.0 NITROGEN INPUTS AND OUTPUTS

9.1 Current land use

The model predicts, for current land use, that approximately 47 tonnes/year of nitrogen is

entering the Ruataniwha Plains through rivers crossing the northern and western boundary

(Table 16). This is based on average nitrogen concentrations in surface water. The model of

existing land use (Table 2) predicts that a further approximately 795 tonnes/year of nitrogen is

entering groundwater and streams due to land use on the Plains. Approximately 321

tonnes/year of nitrogen is predicted to be leaving the Ruataniwha Plains through the Waipawa

River, and approximately 516 tonnes/year is predicted as leaving through the Tukituki River.


With a scenario of an irrigated beef land use (Table 7) it is predicted that nitrogen output to

surface and groundwater will increase by 16-17 kgN/ha/yr. An extra 17 kgN/ha/yr leaching

through soils, in addition to the existing land use, for all irrigable cells on the Ruataniwha

Plains (equivalent to 31000 ha) predicts an additional nitrogen output of 527 tonnes/year for

the whole Ruataniwha Plains (Table 17). Approximately 456 tonnes/year of the total nitrogen

loading (existing land use plus irrigated beef) is predicted to be leaving the Ruataniwha Plains

through the Waipawa River. Approximately 908 tonnes/year of nitrogen (existing land use

plus irrigated beef) is predicted as leaving through the Tukituki River.


A mean nitrogen output for a mix of land uses potatoes (20% of Plains area), onion (20% of

Plains area), squash (20% of Plains area) and dairy (0.4 of Plains area) is estimated at 92

kgN/ha/yr. Nitrogen output from onions (not listed in Table 6) is assumed as the same as

potatoes.

The nitrogen application to all the irrigable cells by a mix of irrigated crops and irrigated

dairy in the Plains predicts a nitrogen loading of 2852 tonnes/year (Table 18). It is predicted

that 1050 tonnes N/yr will leave through the Waipawa River and 2639 tonnes N/yr will leave

through the Tukituki River.


Table 16. Calculated nitrogen balance with existing land use.

N inputs kgN/yr

Background land use 794763

Irrigation 0

Point sources 0

Sum land use 794763

Boundary streams/rivers: 46989

Total N inputs 841752

N outputs kgN/yr

Waipawa River

From river boundary 29644

From land use 291588

Sum 321232

Tukituki River


From Zone 2 113700

From Zone 3 389475

Sum 515789

Other river boundaries 4730

Sum of N outputs: 841751

Balance (Out -In) -1

Percent difference 0


Table 17. Nitrogen balance with existing land use plus 17 kgN/ha/yr, equivalent to an

irrigated beef land use, over the irrigable area of the Ruataniwha Plains.

N inputs kgN/yr


Irrigation 527000

Point sources 0

Sum land use 1321763



N outputs kgN/yr

Waipawa River



Sum 455957

Tukituki River


From Zone 2 283700

From Zone 3 611750

Sum 908064






Table 18. Nitrogen balance with existing land use plus 92 kgN/ha/yr, equivalent to

irrigated crops and irrigated dairy, over the irrigable area of the Ruataniwha

Plains.

N inputs kgN/yr


Irrigation 2852000

Point sources 0

Sum land use 3646763



N outputs kgN/yr

Waipawa River



Sum 1050332

Tukituki River


From Zone 2 1033700

From Zone 3 1592375

Sum 2638689





10.0 NITROGEN CONCENTRATIONS WITH IRRIGATION

Predictions of nitrogen concentrations are made for two land uses, assuming that irrigation is

occurring on all irrigable land the Ruataniwha Plains. Each of the two land uses, irrigated

pasture and irrigated crops with irrigated dairy, are applied on all irrigable land on the Plains.

The models use the groundwater seepage velocity estimates made by the MODFLOW model

that simulates irrigation (Fig. 15). These seepage velocities are higher than seepage velocities

estimated for the model without irrigation (Fig. 14). The two effects of higher seepage

velocities are:

greater dilution of nitrogen by excess irrigation water. Therefore, assuming the same land

use, nitrogen in groundwater with irrigation will be of lower concentration than nitrogen

in groundwater without irrigation.


nitrogen in streams will respond to land use changes more quickly when land is irrigated.


It is predicted that irrigated pasture, with an estimated nitrogen leaching rate of 17 kgN/ha/yr,

will result in increased nitrogen concentrations at most surface water sites (Table 19) and

groundwater sites (Table 20).

For example, Kahahakuri @ Ongaonga Road is predicted to increase from a mean of 2.91

mg/L with the current land use to 5.55 mg/L with irrigated pasture in addition to the existing

land use.

Transient nitrogen concentrations in surface water (Table 21) are predicted to increase the

most at Kahahakuri @ Ongaonga. Generally, the surface water sites „impacted‟ the most by

irrigation are those where the largest increases occur. The sites not „impacted‟ by irrigation

(e.g. Tukituki @ SH50) show no increase in nitrogen levels.

Table 19. Steady-state mean surface water nitrogen concentrations with a loading of

17 kgN/ha/yr and irrigation. This loading is in addition to existing land use.

Surfacewater sub-zones

11 12 13 14 15 16 17 18 19 21 22

Load kgN/ha/yr

0.5 0.5 0.5 1 1 6.4 31.3 11.9 142.2 13.5 27.4

Surfacewater sub-zones

23 24 25 31 32 33 34 35 36 37 38 39

Load kgN/ha/yr

11.4 12.2 15 14.4 5.9 31.1 54.3 13.4 24.9 16.6 7.3 17

Monitoring

points: Surface

water site


at site

(mg/L)

Observed

N

(mg/L)

Difference

calc-obs




287 Mangaonuku@SH50 0.5 0.53 -0.03

26 Waipawa@RDS 0.9 0.63 0.27

356 Tukituki @SH50 0.21 0.18 0.03




144 Tukipo@SH50 1.11 0.85 0.26





405 Maharakeke@SH2 3.28 1.96 1.32



Concentrations of nitrogen in groundwater is predicted to increase because of irrigation

(Table 22). This calculation assumes full mixing of nitrogen with the full water column and

is a conservative estimate. Some of the estimates of mixing fractions (Section 7.8.2) produce

quite large predictions of nitrate concentrations (Table 23). This is generally at sites where

the estimate of the mixing ratio is low. For example, site 223 has an estimated mixing ratio of

0.03 from the calibration estimates i.e. the nitrogen load is mixed in 3% of the groundwater

flow. It is estimated (Table 23) that the nitrogen concentration after 50 years of irrigation will

be approximately 61 mg/L. Some 50-year estimates of nitrogen concentrations in Table 23

are greater than the estimated steady-state concentrations in Table 20. For example, Well

1377 has a predicted nitrogen concentration of 42.4 mg/L after 50 years. The predicted

steady-state concentration (Table 20) is 41.4 mg/L. Differences in concentration are because

the increases in concentration due to irrigation (Table 23) are added to the observed mean

nitrogen value, unlike the values in Table 23 which consider total zone loads.

Table 20. Steady-state mean groundwater nitrogen concentrations with a loading of

17 kgN/ha/yr and irrigation. This loading is in addition to existing land use.

Surface water zone subzones 11 12 13 14 15 16 17 18 19 21 22

Load kgN/ha/yr 0.5 0.5 0.5 1 1 6.4 31.3 11.9 142.2 13.5 27.4

Surface water zone subzones 23 24 25 31 32 33 34 35 36 37 38 39

Load kgN/ha/yr 11.4 12.2 15 14.4 5.9 31.1 54.3 13.4 24.9 16.6 7.3 17

Monitoring points:

groundwater

site

Calculated N

at site

(mg/L)

Observed

N

(mg/L)

Difference

calc-obs

Mixing

thickness

% of full

thickness

220 4.5 0.3 4.2 1

236 1.3 1 0.3 1

146 2.2 1 1.2 1

222 1.2 0.48 0.72 1

224 1.75 0.27 1.48 0.4

223 50 6.26 43.74 0.03

239 3.5 0.55 2.95 0.4

2227 4.5 3.52 0.98 0.4

233 1.6 0.42 1.18 0.25

2229 0.3 0.05 0.25 1

231 1.1 4.41 -3.31 1

229 3.6 0.55 3.05 1

1497 2.63 1.22 1.41 0.38

1377 41.43 22.6 18.83 0.07


Table 21. Nitrogen concentrations in surface water over 50 years, 17 kgN/ha/yr loading and irrigation. This loading is in addition to existing

land use.

Surface

site

Site name Observed

N

(mg/L)

Predicted concentrations (mg/L) with 17 kg/h/yr irrigation application

Year

1 2 5 10 20 30 50



26 Waipawa@RDS 0.63 0.63 0.63 0.73 0.83 0.93 0.93 0.93




356 Tukituki @SH50 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8


21 Tukipo@Ashcott 1.1 1.3 1.4 1.6 1.8 1.9 1.9 1.9

144 Tukipo@SH50 0.85 0.85 0.85 0.95 1.05 1.15 1.15 1.15

279 Tukipo@Burnside 2.08 2.08 2.28 2.48 2.78 3.08 3.18 3.18





Table 22. Nitrogen concentrations in groundwater over 50 years, loading of 17 kgN/ha/yr, and irrigation, assuming full mixing. This

loading is in addition to existing land use.

Nitrogen application: 17 kgN/ha/yr irrigation application

Well Observed N

(mg/L) Year

1 2 5 10 20 30 50

220 0.3 0.4 0.5 0.6 0.8 0.9 0.9 0.9

236 1 1 1.1 1.1 1.4 1.7 1.7 1.7

146 1 1.1 1.2 1.4 1.8 2.4 2.5 2.7

222 0.48 0.58 0.68 0.78 0.88 1.08 1.08 1.08

224 0.27 0.27 0.37 0.57 0.87 0.97 0.97 0.97

223 6.26 6.36 6.46 6.56 7.06 7.66 7.96 7.96

239 0.55 0.65 0.65 0.75 1.05 1.75 2.15 2.15

2227 3.52 3.62 3.72 4.02 4.02 4.02 4.02 4.22

233 0.42 0.52 0.62 0.82 0.82 0.82 0.82 0.82

2229 0.05 0.05 0.05 0.45 0.45 0.45 0.45 0.45

231 4.41 4.51 4.61 4.91 5.31 5.81 5.81 5.81

229 0.55 0.65 0.65 0.75 0.85 0.85 0.85 0.85

1497 1.22 1.22 1.32 1.52 1.72 2.02 2.12 2.12

1377 22.6 22.7 22.8 23.1 24 24.2 24.3 24.3


Table 23. Nitrogen concentrations in groundwater over 50 years, loading of 17 kgN/ha/yr and irrigation, assuming partial mixing. This

loading is in addition to existing land use.

Well Observed N

(mg/L)

Nitrogen application: 17 kgN/ha/yr irrigation application

Year

1 2 5 10 20 30 50

220 0.3 0.4 0.4 0.5 0.8 0.8 0.8 0.8

236 1 1.1 1.1 1.1 1.4 1.5 1.5 1.5

146 1 1.1 1.1 1.3 1.7 2 2.1 2.3

222 0.48 0.58 0.58 0.78 0.78 0.88 0.88 0.88

224 0.27 0.27 0.47 0.97 1.57 1.67 1.67 1.67

223 6.26 7.76 9.36 13.96 30.96 46.46 49.56 49.56

239 0.55 0.65 0.95 1.15 2.45 3.65 3.75 3.75

2227 3.52 3.62 3.92 4.32 4.52 4.52 4.62 4.72

233 0.42 0.42 0.82 1.52 1.52 1.52 1.52 1.52

2229 0.05 0.05 0.05 0.35 0.35 0.35 0.35 0.35

231 4.41 4.51 4.61 4.81 5.31 5.51 5.51 5.51

229 0.55 0.55 0.65 0.65 0.75 0.75 0.75 0.75

1497 1.22 1.22 1.42 1.92 2.52 3.12 3.12 3.12

1377 22.6 22.6 23.6 32.5 39.4 41.4 42.4 42.4


Table 24. Surface water nitrogen concentrations with a 92 kgN/ha/yr loading and

irrigation. This loading is in addition to existing land use.

Surface water subzones 11 12 13 14 15 16 17 18 19 21 22

Load kgN/ha/yr 0.5 0.5 0.5 1 1 16.9 45.8 42.5 196.1 64.3 51.4


Load kgN/ha/yr 48.6 48.2 81.4 78 9.9 58.2 91 33.1 77.4 90 39.4 92

Monitoring

points:

Surface water

site


at site

mg/L

Observed

N

Difference

calc-obs




287 Mangaonuku@SH50 0.5 0.53 -0.03

26 Waipawa@RDS 2.07 0.63 1.44

356 Tukituki @SH50 0.57 0.18 0.39




144 Tukipo@SH50 1.86 0.85 1.01





405 Maharakeke@SH2 9.36 1.96 7.4



Table 25. Groundwater nitrogen concentrations with a 92 kgN/ha/yr loading and

irrigation and partial mixing. This loading is in addition to existing land use.

Surface water zone subzones

11 12 13 14 15 16 17 18 19 21 22

Load kgN/ha/yr 0.5 0.5 0.5 1 1 16.9 45.8 42.5 196.1 64.3 51.4

Surface water zone subzones

23 24 25 31 32 33 34 35 36 37 38 39

Load kgN/ha/yr 48.6 48.2 81.4 78 9.9 58.2 91 33.1 77.4 90 39.4 92

Monitoring points:

groundwater

site

Calculated N

at site

mg/L

Observed

N

Difference

calc-obs

Mixing

thickness

% of full

thickness

220 6.7 0.3 6.4 1

236 3.7 1 2.7 1

146 8.1 1 7.1 1

222 3.2 0.48 2.72 1

224 8.25 0.27 7.98 0.4

223 246.67 6.26 240.41 0.03

239 17.75 0.55 17.2 0.4

2227 10.5 3.52 6.98 0.4

233 7.6 0.42 7.18 0.25

2229 1.7 0.05 1.65 1

231 6.1 4.41 1.69 1

229 4.7 0.55 4.15 1

1497 10.26 1.22 9.04 0.38

1377 132.86 22.6 110.26 0.07


Table 26. Nitrogen concentrations in surface water over 50 years, 92 kgN/ha/yr loading and irrigation. This loading is in addition to existing

land use.

Surface

site

Site name Observed

N

(mg/L)

Calculated N (mg/L) with 92 kg/ha/yr irrigation application

Year

1 2 5 10 20 30 50



26 Waipawa@RDS 0.63 0.73 0.83 1.23 1.83 2.13 2.23 2.33




356 Tukituki @SH50 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8


21 Tukipo@Ashcott 1.1 2.2 3 3.9 5 5.4 5.5 5.6

144 Tukipo@SH50 0.85 0.85 0.85 1.15 1.85 2.25 2.45 2.45

279 Tukipo@Burnside 2.08 2.28 2.98 4.28 5.88 7.48 7.88 7.98





Table 27. Nitrogen concentrations in groundwater over 50 years, 92 kgN/ha/yr loading and irrigation and partial mixing. This loading is in

addition to existing land use.

Well Observed

N

(mg/L)

Calculated N (mg/L) with a 92 kgN/ha/yr irrigation application

Year

1 2 5 10 20 30 50

220 0.3 0.6 0.9 1.2 2.7 2.7 2.7 2.7

236 1 1.3 1.6 1.6 2.9 3.9 3.9 3.9

146 1 1.5 1.8 2.9 4.7 6.6 7.1 7.9

222 0.48 0.88 1.08 1.88 2.08 2.78 2.78 2.78

224 0.27 0.27 1.37 4.07 7.27 7.87 7.87 7.87

223 6.26 14.66 22.96 48.06 140.06 223.66 240.46 240.46

239 0.55 1.15 2.45 3.75 10.95 17.35 18.05 18.05

2227 3.52 4.22 5.72 7.92 8.62 8.62 9.42 10.12

233 0.42 0.42 2.72 6.32 6.32 6.32 6.32 6.32

2229 0.05 0.05 0.05 1.75 1.75 1.75 1.75 1.75

231 4.41 4.81 5.51 6.81 9.41 10.31 10.31 10.31

229 0.55 0.55 0.95 1.25 1.65 1.65 1.65 1.65

1497 1.22 1.22 2.42 4.92 8.02 11.72 11.72 11.72

1377 22.6 22.6 28 76.1 113.6 124.3 129.7 129.7



The mean nitrogen output with this land use is estimated as 92 kgN/ha/yr (Section 9.3).

Mean surface water quality estimates of nitrogen concentrations with a 92 kgN/ha/yr

application (Table 24) are higher than nitrogen concentrations with a 17 kgN/ha/yr

application. For example the Kahahakuri @ Ongaonga has a predicted mean nitrogen

concentration of 17.21 mg/L with a 92 kgN/ha/yr application and 5.5 mg/L concentration with

a 17 kgN/ha/yr application.

Mean groundwater nitrogen concentrations are predicted as a maximum of approximately

247 mg/L in well 223 with a 92 kgN/ha/yr application (Table 25). The two wells with a low

mixing ratio (well 223, 0.03 and well 1377, 0.07) both have an estimated nitrogen

concentration that is very high. This value is possible if the mixing zone at these wells is thin

as indicated by the mixing ratio.

Transient surface water mean concentrations (Table 26) at some sites show a levelling-off of

predicted concentrations in the medium term, e.g. concentrations at Porangahau @ Fraser (site

398) reach 6.73 mg/L by 10 years and do not increase further. Nitrogen concentrations at

other sites, e.g. Kahahakuri @ Ongaonga (site 410), continue to increase through the 50 year

period.

Transient groundwater concentrations (Table 27) are predicted to increase significantly in

some wells. For example, in well 223 it is predicted that the current mean concentration of

6.26 mg/L in well 223 is predicted to increase to around 240 mg/L after 30 years of irrigation

at 92 kgN/ha/yr. Well 223, and well 1377, are predicted to have the lowest mixing ratio of the

wells (Table 20) and so the effects of nitrogen injection on groundwater quality are amplified

in these wells.

11.0 NITROGEN CONCENTRATIONS IN RIVERS OVER 20 YEARS

River and stream nitrogen concentrations are estimated for 20 years after the commencement

of loading nitrogen to land. This model considers the travel-time of nitrogen in the

groundwater system. River and stream flow is considered as variable over time in this model.

Stream and river flow was assumed as constant over time in Section 10.


11.1 River and stream flow means

Transient nitrogen concentration predictions are made for the Waipawa @ RDS, Tukituki @

Tapairu Rd, and Kahahakuri @ Ongaonga sites (Appendix 2). Mean annual, seasonal, and

weekly flows are calculated from rated stage data. Gaps in the records occur. This results in

gaps in the calculation of mean flows. For example, only ten annual mean flows can be

calculated for Waipawa @ RDS in the period 22/4/88 to 22/1/2002. No annual mean flow

could be calculated for Kahahakuri @ Ongaonga.

11.2 Flows used in mixing calculations

11.2.1 Waipawa @ RD5, site 23235

The 10 available annual mean flows are used and this data is repeated for years 11 to 21. A

total of 48 seasonal flow averages (mean flow over three months starting in July 1988) are

calculated from the record and a total of 690 mean weekly flows are calculated.

11.2.2 Tukituki @ Tapairu Rd, site 23207

A total of 13 annual mean flows in the period July 1987 to 31/10/01 are calculated. A total of

56 mean seasonal flows are calculated and a total of 728 mean weekly flows are calculated.

11.2.3 Kahahakuri @ Ongaonga, site 23248

No mean annual flows can be calculated from observed data, with a starting month of July.

Five seasonal mean flows are calculated and 108 weekly flows are calculated.

11.3 Calculated nitrogen fluxes

Nitrogen loadings are calculated using the spreadsheet Ruasurfqualtransirri.xls (Appendix 4)

in the period 1 to 20 years after the commencement of irrigation in the three catchments of

each river/stream (Table 28). Background, i.e. current land use, and nitrogen inputs from the

western boundary of the Ruataniwha Plains are not considered. Therefore, estimations of

water quality are expressed as an increase due to irrigation loading. Seasonal and weekly

nitrogen fluxes are calculated from these annual figures by dividing the annual figure equally.


11.4 Calculated surface water flows

The surface water flows used in the mixing calculations are the mean of observed flows, with

the following modifications to generate 20-year flow mean annual records:

Waipawa @ RDS The pattern of mean annual flow in years 1 to 10 is

repeated in years 11 to 20.

Tukituki @ Tapairu Rd Annual mean flows are used for years 1 to 13. The

pattern of mean annual flow in years 1 to 7 is repeated in

years 14 to 20.

Kahahakuri @ Ongaonga The mean annual flow of over 20 years is generated from

a repeating pattern of 11%, 102%, 105%, 110%, 115%,

120%, 98%, 95%, 90%, 85% and 80% of the mean

seasonal flow.

Mean seasonal flow record are generated for a 12 year period as follows:

Waipawa @ RD5 48 seasonal mean flows, equivalent to 12 years of record

Tukituki @ Tapairu Rd 48 seasonal mean flows equivalent to 12 years of record

Kahahakuri @ Ongaonga Rd The five seasonal mean flows are repeated through 48

seasons.

Mean weekly flow records are generated for up to a 14 year period as follows:

Waipawa @ RD5 690 mean weekly flows

Tukituki @ Tapairu Rd 728 mean weekly flows

Kahahakuri @ Ongaonga The 108 mean weekly flow values from observations are

repeated to generate a 728 week synthetic record.


11.5 Calculated river and stream nitrogen concentrations


Mean annual nitrogen concentrations in the three rivers and streams generally increase with

time when considering annual average measurements (Table 28). This reflects increasing

nitrogen loads with time. The maximum nitrogen concentrations increases of 2.91 mg/L

(Waipawa, year 17), 6.37 mg/L (Tukituki, year 19) and 31.35 mg/L (Kahahakuri, year 20)

show the influence of flow volumes on the calculation of concentration. Higher

concentrations are predicted at times of lower flows because dilution of nitrogen is reduced.

Calculated weekly nitrogen concentrations in the Waipawa River reaches approximately 10

mg/L in week 505 (the week of the 9/4/88), Fig. 16. The maximum weekly nitrogen

concentration in the Tukituki River reaches around 25 mg/L in week 558 (week of the

24/3/98), Fig. 17. Minimum mean weekly nitrogen concentrations in the Kahahakuri Stream

reach over 25 mg/L towards the end of the period of weekly flow simulation (Fig. 18). These

predictions use observed flow measurements to generate mean flow values. Irrigation will

likely increase the mean flows so the estimated nitrogen concentrations represent a „worst

case‟ scenario.

Table 28. Nitrogen concentration in three rivers and streams over 20 years due to

irrigation of 92 kgN/ha/yr.

Year Waipawa@RDS

kgN/yr from land use

from irrigation

Average annual flow

(L/s)

Increase in

N concentration

(mg/L)

1 71300 14990 0.15

2 117300 14677 0.25

3 172500 22118 0.25

4 223100 17969 0.39

5 294400 12359 0.76

6 351900 26229 0.43

7 418600 8074 1.64

8 478400 15683 0.97

9 533600 14049 1.2

10 586500 13405 1.39

11 618700 14990 1.31

12 646300 14677 1.4

13 664700 22118 0.95

14 696900 17969 1.23

15 713000 12359 1.83

16 733700 26229 0.89

17 740600 8074 2.91


18 742900 15683 1.5

19 756700 14049 1.71

20 759000 13405 1.8

Year Tukituki @Tapairu Rd


from irrigation

Average annual flow

(L/s)

Increase in

N concentration

(mg/L)

1 299000 26726 0.35

2 506000 15933 1.01

3 646300 16111 1.27

4 770500 13147 1.86

5 897000 24053 1.18

6 1014300 8575 3.75

7 1127000 16895 2.12

8 1193700 14883 2.54

9 1274200 14592 2.77

10 1334000 11726 3.61

11 1354700 15220 2.82

12 1403000 10462 4.25

13 1442100 15074 3.03

14 1490400 26726 1.77

15 1541000 15933 3.07

16 1619200 16111 3.19

17 1676700 13147 4.04

18 1697400 24053 2.24

19 1722700 8575 6.37

20 1748000 16895 3.28

Year Kahahakuri@Ongaonga Rd


from irrigation

Average annual flow

(L/s)

Increase in

N concentration

(mg/L)

1 6900 388 0.56

2 23000 396 1.84

3 41400 407 3.23

4 66700 427 4.95

5 96600 446 6.87

6 121900 466 8.29

7 133400 380 11.13

8 149500 369 12.85

9 158700 349 14.42

10 170200 330 16.35

11 174800 310 17.88

12 181700 388 14.85

13 181700 396 14.55

14 197800 407 15.41

15 225400 427 16.74

16 273700 446 19.46

17 312800 466 21.29

18 324300 380 27.06

19 333500 369 28.66

20 345000 349 31.35



Annual (Table 29), seasonal, and weekly (Figs 19 to 21) nitrogen concentrations with a

17 kgN/ha/yr irrigation are predicted to be lower than with a 92 kgN/ha/yr irrigation. Annual

average concentrations with a 17 kgN/ha/yr irrigation are predicted to increase by at most

0.33 mg/L in the Waipawa River, 1.18 mg/L in the Tukituki River and around 5.8 mg/L in the

Kahahakuri Stream.

Maximum concentrations in the Waipawa River with a 17 kgN/ha/yr irrigation is predicted as

approximately 1.9 mg/L (Fig. 19), approximately 4.7 mg/L in the Tukituki River (Fig. 20) and

approximately 5 mg/L in the Kahahakuri Stream (Fig. 21).

Table 29. Nitrogen concentration in three rivers and streams over 20 years due to

irrigation of 17 kgN/ha/yr.

Year Waipawa@RDS


from irrigation

Average annual flow

(L/s)

Increase in

N concentration

(mg/L)

1 13175 14990 0.03

2 21675 14677 0.05

3 31875 22118 0.05

4 41225 17969 0.07

5 54400 12359 0.14

6 65025 26229 0.08

7 77350 8074 0.3

8 88400 15683 0.18

9 98600 14049 0.22

10 108375 13405 0.26

11 114325 14990 0.24

12 119425 14677 0.26

13 122825 22118 0.18

14 128775 17969 0.23

15 131750 12359 0.34

16 135575 26229 0.16

17 136850 8074 0.54

18 137275 15683 0.28

19 139825 14049 0.32

20 140250 13405 0.33

Year Tukituki @TapairuRd


from irrigation

Average annual flow

(L/s)

Increase in

N concentration

(mg/L)

1 55250 26726 0.07

2 93500 15933 0.19

3 119425 16111 0.24

4 142375 13147 0.34

5 165750 24053 0.22

6 187425 8575 0.69


7 208250 16895 0.39

8 220575 14883 0.47

9 235450 14592 0.51

10 246500 11726 0.67

11 250325 15220 0.52

12 259250 10462 0.79

13 266475 15074 0.56

14 275400 26726 0.33

15 284750 15933 0.57

16 299200 16111 0.59

17 309825 13147 0.75

18 313650 24053 0.41

19 318325 8575 1.18

20 323000 16895 0.61

Year Kahahakuri@Ongaonga Rd


from irrigation

Average annual flow

(L/s)

Increase in

N concentration

(mg/L)

1 1275 388 0.1

2 4250 396 0.34

3 7650 407 0.6

4 12325 427 0.92

5 17850 446 1.27

6 22525 466 1.53

7 24650 380 2.06

8 27625 369 2.37

9 29325 349 2.66

10 31450 330 3.02

11 32300 310 3.3

12 33575 388 2.74

13 33575 396 2.69

14 36550 407 2.85

15 41650 427 3.09

16 50575 446 3.6

17 57800 466 3.93

18 59925 380 5

19 61625 369 5.3

20 63750 349 5.79

12.0 RUATANIWHA PLAINS MONITORING NETWORK

Mass balance models, such as the nitrogen mass-balance model described in the report, rely

on estimates of chemical mass moving through the system. Estimates of nitrogen mass

moving through the Ruataniwha Plains system are provided from the following sources:

Nitrogen leaching from soil - HortResearch (pers. comm.) estimates in Table 6

Nitrogen concentration in rivers - HBRC monitoring network

Water flow in rivers - HBRC monitoring network

Nitrogen in groundwater - HBRC monitoring network


Water flow in groundwater - Ruataniwha Plains groundwater flow model

Ideally, the inputs and outputs of nitrogen mass would be known for each sub region.

However, environmental measurements provide only an approximation of mass transfers for

reasons including:

monitoring may not be in ideal locations to measure water quality and flow;

monitoring sites may only measure water quality or only measure water flow;

measurement of water quality or water flow may be sporadic leading to poor statistical

estimates;

surface water quality measurements may be biased by biological and chemical processes

taking place in the river/stream; and

the distribution of chemical species within groundwater is generally poorly known so

estimates of mean concentrations in groundwater probably have large errors.

The present monitoring network is assessed against an „ideal‟ monitoring network for using

mass-balance models in the Ruataniwha Plains. Locations of surface-water, and groundwater,

monitoring sites are discussed with the aim of designing a network that will be of use to

HBRC to assess the effects of future land use change.

12.1 Surface water monitoring

The present monitoring network is not ideal for monitoring the effects of land use in the sub

regions because most sub regions do not have „unique‟ monitoring sites. For example (Table

30), only six of the sub regions are associated with monitoring sites that may „uniquely‟

identify the effects of land use on water quality. In this case „unique‟ means that one surface

water quality at one site relates only to one sub zone.


Table 30. Surface water sub-zones and „unique‟ identification of land use effects.

Surface water sub

zone

Present monitoring site ‘unique’

identification of land use effects

11 -

12 -

13 -

14 -

15 -

16 273

17 -

18 -

19 -

21 -

22 659

23 -

24 356

25 -

31 -

32 144

33 -

34 398

35 397

36 -

37 -

38 -

39 -

A „unique‟ monitoring location for each sub zone is unrealistic because of: the flow direction

of drainage, pattern of stream flow (or loss) to groundwater, flow direction of groundwater,

the definition of sub zone boundaries and the cumulative effects of land use means. However,

a monitoring network design that represents the cumulative effects of land use across the

Ruataniwha Plains is practical.

A set of surface water monitoring locations (Table 31) is proposed that will allow an estimate

of the effects of land use on surface water quality in each sub zone. A combination of 13

existing sites and 11 new sites should allow monitoring of the effects of land use through the

„chain‟ of cumulative effects (e.g. Table A4.5, panel titled „calc.stream N concentration‟).

HBRC could aim to collect water quantity and water quality information on a regular basis at

these sites.


Table 31. Surface water quality network that could allow the monitoring of land use in

each subzone.

Surface

water sub

zone

River/stream Location of potential monitoring site Location of site,

co-ordinate,

approx

11 Mangatahi Stream d/s of Argyll East bridge 120 426

12 Te Heka Stream d/s of conf. with Karawa Stream 108 474

13 Unnamed Stream end of Wharetoka Rd 102 502

14 Mangaonuku Stream #287, existing existing

15 Mangaonuku Stream d/s of conf. with Mangamate (east of

Creek Rd)

087 513

16 Mangamate Stream #273, existing existing

17 Mangaonuku Stream d/s of Wharetoka Rd bridge by approx.

1 km

099 495

18 Mangaonuku Stream #284, existing existing

19 Unnamed stream/drain As stream/drain crosses Kade Rd, near

Waipawa River

109 366

All zone 1 Waipawa River #26, at RDS existing

21 Kahahakuri Stream # 410, existing existing

22 Kahahakuri Stream #659, existing existing

23 Tukituki River #20, existing existing

24 Tukituki River #356, existing existing

25 Kahahakuri Stream u/s of conf. with Tukituki River (near

Lindsay Rd)

094 310

31 Tukipo River d/s of conf. with unnamed stream 048 316

32 Tukipo River d/s of conf. with Mangatewai River 960 305

33 Tukipo River #279, existing existing

34 Makaretu River off gravel road 987 284

35 Porangahau Stream #397, existing existing

36 Maharakeke Stream #405, existing existing

37 Tukipo River d/s of conf. of unnamed stream 062 311

38 Tukipo River #21, existing existing

39 All Zone 2

and Zone 3

Tukituki River #22, existing existing

A better understanding of the effects of land use will be obtained through a better

understanding of the „baseline‟ nitrogen concentrations in rivers and streams. Table 32

summarises the locations of monitoring sites required to estimate nitrogen concentrations of

rivers and streams entering Ruataniwha Plains. Twenty-eight sites are required to do this, and

five of these sites exist at present. The highest priority for new sites identified in Table 32 is

for measurements on the larger rivers. This is because the larger rivers carry the greatest mass

of nitrogen. Therefore the priority for new sites follow the relative flows and is, from highest

priority to lowest priority:

Tukituki

Tukipo


Mangaonuku

Porangahau

other small streams

Table 32. Monitoring network that allows measurement of nitrogen entering the

Ruataniwha Plains through rivers and streams.

Name Proposed monitoring site Location of site,

co-ordinate approx.

Mangatahi Stream d/s of Argyll East bridge 120 426

Te Heka Stream d/s of conf. with Karawa Stream 108 474

Unnamed stream end of Wharetoka Rd 102 502

Mangaonuku Stream #287, existing existing

Mangamate Stream #273, existing existing

Mangamauku Stream d/s of conf. with Upokororo (near Hukawai) 029 514

Mangaoho Stream crossing of Holden Rd 023 492

Makaroro River #402, existing existing

Waipawa River #283, existing existing

Unnamed stream crossing of McLeod Rd, west of Springhill 988 443

Ongaonga Stream d/s of Ngaruru Rd crossing 993 414

Unnamed stream crossing Pettit Valley Rd 004 381

Tukituki River at Hylton Burn conf. (or „Folgers‟) 896 422

Tukipo River East of Deans Bush 926 364

Tukipo River #144, existing existing

Mangatewai River at SH50 942 300

Makaretu River at Ellison Rd bridge 815 283

Porangahau Stream at Ormondville Rd bridge 923 229

Awanui Stream at Aorangi Rd bridge 991 241

Unnamed stream at Hinerangi Rd bridge 997 207

Unnamed stream at Hinerangi Rd bridge 013 210

Seven westward draining

streams

crossing Maharakeke Rd and SH2 between

Maharakeke and San Hill

12.2 Groundwater monitoring

Groundwater monitoring wells allow the measurement of long-term effects of land use on

water. Groundwater in the Ruataniwha Plains travels north west to south easterly direction.

Therefore groundwater quality measurements in the north western plains will anticipate the

changes in groundwater quality in the east.

Groundwater monitoring sites in the area where rivers and streams are losing flow (Fig. 3) are

likely to record, broadly, the effects of downwards percolating groundwater. Monitoring sites

underlying thin soils in this area should give the earliest indication of the effects of land use

on water in the Ruataniwha Plains.


The present network would be improved with the addition of monitoring wells in the area of

losing stream flow (Fig. 3) between Ongaonga and the Tukipo River. Preferred localities

would be below areas where the soil water holding capacity is low. The area of gaining

streams (Fig. 3) is relatively poorly monitored for groundwater quality. Some wells close to

the Mangaonuku Stream and the area of gaining streams between the Waipawa River and

Tukipo River would be useful to future assessments of groundwater discharge to surface

water.

Existing wells give a profile of nitrate concentration versus depth (Fig. 9). This information

could be improved in future to understand sub-regional nitrate profiles in groundwater by

measuring nitrate with other chemical species as wells are completed. Groundwater quality

information could be brought up to date by measurement of groundwater chemistry in wells

133, 135, 136, 138, 223, 1377, 1385 and 1487 as outlined in Section 7.8.1.

13.0 CONCLUSIONS

A model developed to predict the effects of land use in the Ruataniwha Plains on surface

water quality and groundwater quality. The model is Excel-based and is designed for

operation by non-expert modellers and is designed for ease of updating data used in the

model. The model includes input data of observed surface water quantity, surface water

quality, and groundwater quality at monitoring locations in the Ruataniwha Plains.

The model also includes data from the Ruataniwha Plains groundwater flow model such as

aquifer geometry and predictions of groundwater flow directions and predictions of

groundwater flow velocities under natural and irrigated conditions.

Surface water and groundwater capture zones are used to predict water quality and in these

zones the estimated groundwater travel time is considered in nitrogen transport calculations.

Nitrogen outputs from existing land uses may be compared with observed surface water

quality and observed groundwater quality with the model. The effects of sub-regional land

uses on water quality may be estimated with the model. The model may also estimate the

effects of point sources on water quality. Steady-state (i.e. constant flow) surface water and

groundwater quality, as result of land-use change, may be predicted. Groundwater quality and

surface water changes, as a response to land-use change, may be estimated over a time period

of years and decades.


Model calibration required the matching of nitrogen generation from existing land use to

observed surface water and groundwater quality. A calibration of existing land use to

observed mean surface water quality indicates the most land uses in the Ruataniwha Plains are

generating 5 kgN/ha/yr or less. The area of largest nitrogen leaching is predicted to generate

130 kgN/ha/yr in an 800 ha sub-zone near the eastern Waipawa Gorge.

Nitrogen input from land use predicted by mean surface water quality measurements tends to

predict nitrogen concentrations in groundwater that are lower than observations, assuming

that groundwater is fully mixed. Groundwater quality measurements in the Ruataniwha

Plains indicates that groundwater is not fully mixed. Mixing ratios are calculated to and from

assumed land uses and capture zones. It is concluded that nitrogen is fully mixing in the top

10 m, or less, of aquifer in a number of examples.

The model is used to predict the effects of a nitrogen generation from a number of irrigated

land uses on surface water quality at 16 sites and groundwater quality at 14 sites. For

example, a land use of irrigated beef, generating an extra leaching of 17 kgN/ha/yr over the

Plains is predicted to cause mean surface water nitrogen concentration at the Kahahakuri @

Ongaonga Bridge to rise from 2.91 mg/L to 5.55 mg/L. A 92 kgN/ha/yr application over the

Plains, equivalent to a mixed cropping and dairy land use is predicted to raise mean nitrogen

concentrations at this site to 17.2 mg/L.

The time scale of the change in surface water quality is decadal - for example the mean

nitrogen concentration at Kahahakura @ Ongaonga Bridge is predicted to reach within 0.5

mg/L of the steady-state concentration for a 17 kgN/ha/yr application in 20-30 years after

commencement of irrigation.

Predictions of surface water quality based on annual, seasonal, and weekly mean flows are

made at three surface water sites. Nitrogen concentrations tend to increase with time because

of nitrogen input to streams increases with time. For example, a 17 kgN/ha/yr application is

predicted to increase mean Kahahakuri @ Ongaonga Bridge nitrogen concentrations by about

2.5 mg/L after 14 years of irrigation. Maximum nitrogen concentrations, when flow is

relatively low, also increase with time. For example, it is predicted that at 17 kgN/ha/yr will

result in nitrogen concentrations of about 5 mg/L greater than present at times of low flow

after 14 years of irrigation.


Nitrogen concentrations in groundwater are predicted to rise by less than 10 mg/L in 50 years

of 17 kgN/ha/yr irrigation assuming that nitrogen is fully mixed in the aquifer. Increases of

up to around 50 mg/L are predicted in 50 years with a 17 kg/ha/yr application and estimates of

partial mixing. The time-scale of changes in groundwater quality is also decadal, with some

groundwater quality showing predicted increases through the 50 years period of simulation.

14.0 REFERENCES

Chapelle, F. H., 1993. Ground-water Microbiology and Geochemistry. John Wiley & Sons,

N. Y. 424p.

Hawkes Bay Regional Council, 2001. State of the Environment Update 2001. Hawkes Bay

Regional Council. 106p.

Hawkes Bay Regional Council, 2001. Subsurface geology of the Ruataniwha Plains and

relation to hydrology. Environmental Management Group Internal Report EMI 0111.

17p + 10 figs.

Hawkes Bay Regional Council, 2000. Sate of the Environment Update 2000. Hawkes Bay

Regional Council. 102p.

Hawkes Bay Regional Council, 1999. Ruataniwha Plains conceptual hydrogeological model.

Environment Management Group Technical Report EMT 99/3. 56p.

Hawkes Bay Regional Council, 1998. Sustainable low flow project. Ruataniwha rivers

Waipawa-Tukipo-Tukituki. Environmental Management Group Technical Report

EMT 98/2. 106p.

Lincoln Environmental, 2002. Ruataniwha Plains Water Management Stage 1: potential

irrigation demand. Report No. 4477/2 (Parts 1 and 2).

Luba, L.D., 2001. Hawkes Bay. In Groundwaters of New Zealand. M.R. Rosen and P.A.

White (eds). New Zealand Hydrological Society Inc., Wellington. p367-380.


Murray, D.L., 2002. Modelling Ruataniwha Plains Groundwater. Report prepared for

Hawkes Bay Regional Council. 44p.

Sarrazin, U., 2002. Aquatic Habitat Survey of the Ruataniwha Plains. Draft report Hawkes

Bay Regional Council.


Extents of the Ruataniwha Plains

RiversRoads

N

0 4 8 Kilometers

Man

ga o nuku stream

Waipaw

a

Tukituki

Tukipo River

Makaretu Stream

Lake Hatuna

River

River

Figure 1. Ruataniwha Plains and location of rivers, streams and roads.

0 4 8 Kilometers


20m contours

N

Figure 2. Ruataniwha Plains - 20 m contours.


Extents of the Ruataniwha PlainsRivers40m contours

G Rivers and streams gain flow

L Rivers and streams lose flow

N No large loses or gains in flow

N

0 3 6 Kilometers

L

L

N

G

Figure 3. Regions of the Ruataniwha Plains where rivers and streams gain and lose flow.


Rivers, streams and overall ranking

1

2

3

4

0

40m contours

No assessment

Poor

Good

N

0 4 8 Kilometers

Figure 4. Rivers, streams and overall ranking (after Sarrazin, 2002).


Figure 5. Groundwater flow model grid of the Ruataniwha Plains with contours of

predicted groundwater level.


Figure 6. Scatter plot of ammonium vs. nitrate concentrations in Ruataniwha

groundwater samples. Very few samples contain measurable concentrations of

both ammonium and nitrate.

Figure 7. Scatter plot of nitrate vs. total nitrogen concentration for Ruataniwha

groundwater samples with more than 0.1 mg/l nitrate.


Figure 8. Scatter plot of ammonium vs. total nitrogen concentrations for Ruataniwha

groundwater samples with less than 0.1 mg/l nitrate.

Figure 9. Scatter plot of nitrate concentration vs. well depth for Ruataniwha groundwater

samples.


N

0 3 6 Kilometers


Zones

11

12

13

1415

16

17

18

21

19

22

25

2324

31

37

3635

34

33

32

39

38

Figure 10. Location of surface-water zones.

%

%

%

%

%%% %

%

%

%%

%

%%

%%

%

%

%

%

%

%

%

%

%

%

%

%

%

%

%

%

%%

%

%

%

%

%

%

%%

%

%%%% %

%%

%

17

18

19

20

2122 23

24

25

26

27

28

29

139

140

141

142

144

273

277

279

280

281

283

284

285

286

287

288

289

290

291

292

293, 294

356

397

398

402

405

410

413

414

659

2221

2292

2293

2294

2316

2403

22392315

% Surface water sites


40m contours

139

N

0 3 6 Kilometers

Figure 11. Location of surface-water monitoring sites.


Groundwater zones


Rivers

N

0 4 8 Kilometers

11

12

13

14

21,22,23

24

25

33

34

35

31, 32

Figure 12. Location of groundwater capture zones.

$$$$

$ $

$$$

$

$

$$

$$

$

$

$

$$

$

$

$

$

$

$

$

$

$

$$

$$

$

$

$$$

133

137

145

146

147

220221

222

223

224225

226

227

229

230

231

233

234

235

236

237

239

2431377

13851497

2224 2227

2229

2387

2597

2598, 2599

134135 136

138

1365

$ Groundwater sites


40m contours

236

N

0 3 6 Kilometers

Figure 13. Location of groundwater monitoring sites.


Figure 14. Estimated seepage velocity across the Ruataniwha Plains (m/day), non-

irrigated.

Figure 15. Estimated seepage velocity across the Ruataniwha Plains (m/day), irrigated

model.


0

2

4

6

8

10

12

0 200 400 600 800

Week number

N m

g/L

Figure 16. Predicted weekly nitrogen concentration in the Waipawa River @ RDS with a 92 kgN/ha/yr irrigation.


0

5

10

15

20

25

30

0 200 400 600 800

Week number

N m

g/L

Figure 17. Predicted weekly nitrogen concentration in the Tukituki River @ Tapairu Rd with a 92 kgN/ha/yr irrigation.


0

5

10

15

20

25

30

0 200 400 600 800

Week number

N m

g/L

Figure 18. Predicted weekly nitrogen concentration in the Kahahakuri @ Ongaonga with a 92 kgN/ha/yr irrigation.


0

0.5

1

1.5

2

0 200 400 600 800

Week number

N m

g/L

Figure 19. Predicted weekly nitrogen concentration in the Waipawa River @ RDS with a 17 kgN/ha/yr irrigation.


0

1

2

3

4

5

0 200 400 600 800

Week number

N m

g/L

Figure 20. Predicted weekly nitrogen concentration in the Tukituki River @ Tapairu Rd with a 17 kgN/ha/yr irrigation.


0

1

2

3

4

5

6

0 200 400 600 800

Week number

N m

g/L

Figure 21. Predicted weekly nitrogen concentration in the Kahahakuri @ Ongaonga with a 17 kgN/ha/yr irrigation.


Appendix 1. Copy of contract.

Ruataniwha Plains Nitrate Model

1. Study purpose

The principal aim of the study is to provide sufficient information to enable HBRC to

characterise the likely effects of the existing and two future land use scenarios on:

the nitrate concentrations in groundwater under both steady state and transient (non-steady

state) conditions;

the annual, seasonal and weekly river N concentrations at the points of discharge to

surface water, under both steady state and transient (non-steady state) conditions; and

the timeframes under which these effects to both groundwater and surface water are likely

to occur.

2. Project Scope

The scope of the study will be defined by the following;

The project area will be that area covered by the existing Ruataniwha Plains groundwater

model constructed by Dave Murray.

The land use scenarios to be modelled will be two potential future land use scenarios

identified for the future irrigation demand modelling conducted by Lincoln Ventures, and

the existing land use. Models of the scenarios to be completed by Lincoln Ventures by

the end of May 2002. To be paid for by HBRC separate to this contract.

Steady state conditions, in terms of groundwater quality and the effects on streams, will be

assumed to have occurred for the existing land use scenario (note: this assumption will

need to be confirmed during the study by a comparison of the groundwater travel time

estimations derived and a more detailed review of the existing SoE monitoring

information available).

Review of chemical conditions in the aquifer with a view to assessing whether nitrogen is

a conservative tracer

The nitrogen losses under various land uses in terms of kg/ha/yr will be derived from the

latest version of Overseer by Agresearch by the end of May 2002. To be paid by HBRC

separate to this contract.

Dave Murray will provide all groundwater flow model outputs required by the end of May

2002, to be paid for by HBRC, separate to this contract.

Realisation of the full extent of each of the future land use scenarios will be assumed to

occur over a 20 year period.

HBRC will provide information on, map locations of land uses, average and transient flow

in the receiving streams and rivers, groundwater and surface water quality data by the end

of May 2002.


This project will be expected to define groundwater capture zones if required.

3. Specific outputs required

The study outputs should include:

An estimate of the mass input of N, under both steady-state and transient conditions for

each scenario.

An estimate of groundwater flow direction and travel time across the Plains. This will be

linked to a separate HBRC project on water dating.

Estimates of groundwater outflow zones and contributing groundwater capture

zones/areas.

Estimates of in-groundwater mass N transport and concentrations by zone, and

aggregated, for each scenario.

Estimates of steady-state groundwater N concentrations.

Estimates of the steady-state nutrient mass discharge to each discharge area on an annual,

seasonal and weekly basis and aggregated, for each scenario

Estimates of transient mass discharge to each discharge area on an annual, seasonal and

weekly basis and aggregated, for each scenario

Estimates of the effects of the steady-state nutrient mass discharge from each discharge

area, on river N concentrations on an annual, seasonal and weekly basis, for each scenario.

Estimate of transient nutrient mass discharge from each discharge area, on river N

concentrations on an annual, seasonal and weekly basis, for each scenario.

4. Model calibration

The is groundwater quality information available for at least 12 long term groundwater

monitoring sites and up to 10 surface water sites. The model should aim to approximate the

actual results found at these sites under the existing land use.

5. Output presentation

The final report will include the information collated, model description, and present, and

discuss, the predictions. Graphical presentation of the information by way of contour maps

and the like will be used wherever possible.

Completion date: 31 August 2002, dependent on Lincoln Ventures, Agresearch, and

David Murray completing their work by the dates specified in this

contract.


Appendix 2. Hydrological data used in this study.

Physical hydrology - surface water (all sites include gauging measurements).

Table A2.1 TIDEDA file: Rpflow.mtd

Site Name On Off Comments

23205 Waipawa at Stewarts 990831 1011204

23207 Tukituki at Tapairu Rd 870529 1011031

23218 Makaroro at Burnt Bridge 750702 1011120

23220 Tukipo at SH50 (Punawai) 761229 1020123

23235 Waipawa at RDS 880422 1020122

23248 Kahahakuri at Ongaonga Rd Bridge 990826 1011227

23252 Tukituki at Folgers Lake 790511 1010903

1023211 Mangaonuku at Argyll Rd 991209 1011204

1223220 Waipawa at Pendle Hill 940624 1010815

Physical hydrology - groundwater.

Table A2.2 TIDEDA file: RPGWMan.mtd


402001 Ruataniwha Plains at Springfield 920123 1020417

403001 Ruataniwha Plains at Greene 920123 950219

403003 Ruataniwha Plains at H.B.F.M. 920123 1020417

404001 Ruataniwha Plains at Hatuma 920422 1020417

884001 Ruataniwha Plains at Te Papa 920121 1020417

884005 Ruataniwha Plains at Feedlot 920204 1020417

884007 Ruataniwha Plains at Springhill 920204 1020417

884009 Ruataniwha Plains at Riddel 920204 1020417

884011 Ruataniwha Plains at Atherstone 920123 1011227

884013 Ruataniwha Plains at Thompsen 920123 1020417

884015 Ruataniwha Plains at Ingrams 970123 1020320

884017 Ruataniwha Plains at Kowhai Main (Worsnops) 981105 1020320

885001 Ruataniwha Plains at Glen Athol 920226 1020417

886001 Poukawa (Brownrigg) at Barkers (Cato) 1000628 1001115

887001 Poukawa (Brownrigg) at Homestead (Primer) 1000628 1001115

893001 Ruataniwha Plains at Lutz 920123 1020320

893003 Ruataniwha Plains at Kindar 920226 1020417

893005 Ruataniwha Plains at Totaranui 920226 1020417

893007 Ruataniwha Plains at Forest Gate 920226 1020417

893011 Ruataniwha Plains at Eldarado House 970210 1020417

894001 Ruataniwha Plains at Golf Club 920123 1020417

894003 Ruataniwha Plains at Livingstone 920123 1020417

894005 Ruataniwha Plains at Grocorp 920226 1020417

894007 Ruataniwha Plains at Donald 920123 1020417

894009 Ruataniwha Plains at Hudson 970123 1020327

894011 Ruataniwha Plains at Pacific Orchard 970123 1020417

895001 Ruataniwha Plains at Punanui (deep) 920204 1020417

895003 Ruataniwha Plains at Punanui (shallow) 920204 1020417


Table A2.3 TIDEDA file: RPGWAuto.mtd


884001 Ruataniwha Plains at Te Papa 880816 1011127

884003 Ruataniwha Plains at Davenport Well 881123 920616

884005 Ruataniwha Plains at Feedlot 920302 1011003

884007 Ruataniwha Plains at Springhill 920310 1011127

884015 Ruataniwha Plains at Ingrams 961023 1000828

884017 Ruataniwha Plains at Kowhai Main (Worsnops) 971029 1011114

893011 Ruataniwha Plains at Eldorado House 961010 1000630

894007 Ruataniwha Plains at Eldorado Donalds 961023 1011128

894009 Ruataniwha Plains at Hudson 961002 1011128

894011 Ruataniwha Plains at Pacific Orchard 961211 1011003

895001 Ruataniwha Plains at Punanui No. 1 921001 1011128

895003 Ruataniwha Plains at Punanui No. 2 920703 940330

Table A2.4 Water chemistry - groundwater sites

SiteID SiteDesc

133 Takapau investigation bore 1



136 Smiths bore @ Oruawharo road

137 Fords bore @ Station road

138 Carmans house bore @ Station road

145 P Kings bore

146 Feedlot bore, Eastern Equities (Deer), Tikokino

147 EEC domestic bore

220 AR Eddy, Butler Rd, Tikokino

221 N Riddell, 300 Butler Rd, Tikokino

222 Glen Athol Farm, Tikokino Rd

223 Grocorp E, Thornton Orchard, Wakarara Rd, Ongaonga Pacific orchard, Plantation Rd

224 J Duncan McLean (Springhill), Wakarara Road, Ongaonga

225 OD Thompson, Wakarara Rd

226 Waipukurau Golf Club, Waipukarau

227 J Ormsby, Station Rd, Maharakeke

229 R Harrison, Paget Rd, Takapau

230 Lutz Kneesch, Fairfield Rd, Takapau

231 Livingstone, Ashcott Rd

233 Kinder, SH50 Ongaonga

234 HJ Talbot, Blackburn Rd, Ongaonga

235 Richmonds, HBFM Co, 116 Fraser Rd, Takapau

236 B Ingram, 1689 Tikokino Rd, Waipawa

237 Te Papa, Grocorp, SH50, Ongaonga

239 Grocorp, Goldwater Orchard, Swamp Rd, Ongaonga

243 Don & Allison Lynch, Oruawharo road, Takapau

1365 Grocorp Aitken (Pacific), Plantation Rd, Waipawa

1377 Carman's farm bore @ tap on pumpshed, Station road

1385 De Stackpoles house bore, Fraser road

1497 HBRC investigation bore, 1 st bore at oxidation pond, Fairfield road

2224 Jensen, Tukituki, Makaretu

2227 Meredith, Onga Onga

2229 Murdochs Well

2387 Warsnops, Corner SH50 and Makaroro Road

2597 Plantation Road Dairies, Bore 1 Plantation Road

2598 Plantation Road Dairies, Bore 2 Paddock 10 Sth side fence line

2599 Plantation Road Dairies, Bore 3 paddock 11 Nth side fence line


Table A2.5 Water chemistry - surface water sites.

SiteID SiteDesc

17 Tukituki river u/s Waipukurau oxidation pond discharge @ SH 2 bridge

18 Waipawa river @ Waipawa @ SH 2

19 Makaretu river @ SH 50

20 Tukituki river @ Onga Onga road bridge

21 Tukipo river @ Ashcott road

22 Tukituki river @ Pukeora

23 Tukituki river @ Coughlin road

24 Tukituki river u/s Waipukurau oxidation pond discharge

25 Tukituki river d/s Waipukurau pond discharge @ gauge station, Tapairu Rd

26 Waipawa river u/s solid waste site @ gauge station "RDS"

27 Waipawa river d/s Waipawa oxidation pond discharge @ Walker Road

28 Tukituki river d/s Waipawa river confluence

29 Tukituki river d/s Tamumu solid waste site

139 Maharakeke stream @ Station road

140 Maharakeke stream @ Oruawharo road

141 Makaretu stream @ Fairfield (Burnside) road

142 Makaretu stream @ Stubbs road

144 Tukipo river @ State Highway 50 @ gauge station (Punawai)

273 Mangamate stream @ State Highway 50 bridge

277 Mangatarata stream u/s Tukituki river confluence @ Mangatarata road bridge

279 Tukipo river u/s Maharakeke stream confluence @ Burnside road bridge

280 Waipawa river at SH 50

281 Tukituki river @ Tamumu bridge

283 Waipawa river at Wakarara road crossing (u/s Makaroro stream confluence)

284 Mangaonuku stream u/s Waipawa river at Tikokino road

285 Papanui stream u/s Tukituki river confluence @ Middle road bridge

286 Mangaonuku stream @ Argyll East road

287 Mangaonuku stream @ SH 50 bridge 3 u/s Mangamate stream confluence

288 Papanui stream @ Elsthorpe Road Bridge (u/s Otane waste water discharge)

289 Papanui stream d/s Otane waste water works discharge (alternative for Middle road bridge site?)

290 Makaretu river @ Paget/Ellison road bridge

291 Waipawa river @ North Block road

292 Makaroro stream u/s Dutch creek @ Wakarara road

293 Papanui stream 20m d/s Te Aute College waste water discharge

294 Papanui stream d/s Te Aute College waste water discharge @ SH2 culvert

356 Tukituki river @ State Highway 50 @ gauge station

397 Porangahau stream at Oruawharo road

398 Porangahau stream at Fraser road

402 Makaroro river at Makaroro road

405 Maharakeke stream at SH 2

410 Kahahakuri stream at Ongaonga (Fairfield) road

413 Porangahau stream tributary u/s Takapau dump

414 Porangahau stream tributary d/s Takapau dump

659 Kahahakuri stream @ Plantation road bridge

2221 Tukituki River 50 upstream Waipukurau oxidation pond outlet drain

2239 Tukituki River 50 metres downstream of confluence of the Waipukurau oxidation pond outlet

2292 Tukituki River 1000m downstream of the Waipukurau Oxidation Pond outlet drain confluence


2294 Tukituki River, 100m upstream Waipawa river confluence



2403 Tukituki River @ Shag Rock


Appendix 3. Mean flow and mean water chemistry values.

Table A3.1 Mean flow from TIDEDA records.

Site Location Period of record Monthly mean

flow m3/s

23205 Waipawa at Stewarts 31/08/99 - 04/12/01 9.4

23207 Tukituki at Tapairu Road 29/05/87 - 31/10/01 15.1

23218 Makaroro at Burnt Bridge 02/07/75 - May 1991 6.12

23220 Tukipo at SH50 29/12/76 - 23/01/02 1.5

23235 Waipawa at RDS 22/04/88 - 22/01/02 16.1

23248 Kahahakuri at Onga Onga Road Bridge 26/08/99 - 27/12/01 0.516

1023211 Mangaonuku @ Argyll Road 09/12/99 - 04/12/01 2.148

Table A3.2 Mean flows estimated by Geoff Wood from available gaugings or correlations.

Monitoring

points:

Surface

water

quality site

River/stream name Co-ordinates Estimated

mean

water flow

at site

m**3/s

273 Mangamate@SH50

bridge

2807300 6152300 0.47

284 Mangaonuku @

Tikokino Rd

2811300 6138400 3.6

287 Mangaonuku@SH50 2808573 6155198 0.28 reasonable estimate, R2 = 0.86

356 Tukituki @SH50 2796500 6135600 4.20

20 Tukituki at Ongaonga Br 2807326 6132667 4.22

659 Kahahakuri@Plant. Rd

Bridge

2805010 6137800 - insufficient data

279 Tukipo@Burnside 2794737 6132377 2.99

21 Tukipo@Ashcott 2808000 6131000 7.7

398 Porangahau@Fraser 2799853 6128722 - insufficient data. Only 1 concurrent

gauging with Tukipo site. Ratio of the

2 (58/361 * Tukipo mean of 1500)

gives mean at this site as 0.24 m3/s.

397 Porangahau@Oruawharo 2797710 6125906 0.67 rough r2 (0.79), probably reasonable

at mean flow - at least the mean is

interpolated and not extrapolated!.

405 Maharakeke@SH2 2806838 6129896 1.38 r2=0.87, n=5, valid 'x' flow range

=700l/s, "mean" x value =1500 l/s


Table A3.3 Mean nitrogen concentrations in groundwater.

Site Nitrate-nitrogen Period of record Ammonia-nitrogen* Period of record

133 21.6 14/04/83 - 18/09/92

134 5.4 25/05/84 - 18/04/95

135 13.8 03/05/82 - 22/09/92

136 14.03 14/04/88 - 18/04/95

137 1.66 30/07/81 - 18/09/92

138 31.5 30/07/81 - 18/04/95

145 0.75 28/04/93 - 12/06/95

146 1.00 28/04/93 - 27/11/01

147 0.38 28/04/93 - 20/09/94

220 0.35 06/09/94 - 28/11/01

221 0.15 06/09/94 - 28/11/01

222 0.48 05/10/94 - 17/05/95

223 6.26 18/11/88 - 26/11/97

224 0.09 22/11/94 - 27/11/01 0.27 21/04/95 - 27/11/01

225 0.90 06/09/94 - 12/06/95

226 0.04 06/09/94 - 03/10/01 0.38 05/10/94 - 03/10/01

227 0.07 06/09/94 - 11/02/97 0.37 05/10/94 - 11/02/97

229 0.08 06/09/94 - 28/11/01 0.55 05/10/94 - 28/11/01

230 0.04 06/09/94 - 02/10/00 0.46 05/10/94 - 02/10/00

231 4.41 06/09/94 - 28/11/01

233 0.42 06/09/94 - 13/06/00

234 0.15 06/09/94 - 23/03/95 2.03 05/10/94, 02/11/94

(two values)

235 0.01 06/09/94 - 20/03/02 0.54 05/10/94 - 28/11/01

236 0.58 18/11/83 - 28/11/01

237 0.14 09/03/94 - 27/11/01 0.02 14/02/97 - 27/11/01

239 0.07 09/03/94 - 28/11/01 0.82 11/02/97 - 28/11/01

243 0.09 06/09/94 - 21/03/01 1.77 05/10/94 - 21/03/01

1365 0.01 06/10/88 - 28/11/01 0.22 26/11/97 - 28/11/01

1377 22.6 30/07/81 - 15/08/89

1385 10.4 30/07/81 - 23/03/95

1497 1.22 08/12/82 - 06/08/87

2224 0.03 14/02/97 - 03/12/99 0.04 14/02/97 - 03/12/99

2227 3.52 14/02/97 - 03/10/01

2229 0.05 14/02/97 - 02/10/00

2387 1.5 02/10/00 (one value)

2597 6.29 22/01/01 - 07/03/02

2599 5.1 22/01/01 - 07/03/02

* Most wells have ammonia-nitrogen detections. The values here are means from wells

where NH4 is the indicator of nitrogen (Table 1).


Table A3.4 Mean nitrogen concentrations in surface water.

Mean Concentrations mg/L

Site Nitrate-

nitrogen Period of record

Dissolved

inorganic nitrogen Period of record

20 0.26 13/09/90 - 10/05/00 0.38 28/04/98 - 10/05/00

21 1.10 09/08/77 - 09/01/97 - -

23 0.99 13/09/90 - 20/07/94 - -

26 0.63 13/09/90 - 10/05/00 0.77 24/06/98 - 10/05/00

144 0.85 09/08/77 - 19/02/02 0.99 28/04/98 - 19/02/02

273 1.08 09/08/77 - 04/03/98 - -

279 2.08 20/07/94 - 09/01/97 - -

280 0.17 09/08/77 - 19/02/02 0.17 03/12/98 - 19/02/02

283 0.10 09/08/77 - 10/05/00 0.11 28/04/98 - 10/05/00

284 1.66 13/11/78 - 09/01/97 - -

286 1.64 11/07/94 - 19/02/02 1.29 16/08/00 - 19/02/02

287 0.53 09/08/77 - 10/05/00 1.23 28/04/98 - 10/05/00

290 0.07 11/07/94 - 10/05/00 0.20 03/01/98 - 10/05/00

356 0.8 09/08/77 - 19/02/02 0.23 28/04/98 - 19/02/02

397 1.91 09/08/77 - 10/05/00 1.51 28/07/98 - 10/05/00

398 3.53 30/06/81 - 23/03/95 - -

402 0.14 09/08/77 - 22/01/97 - -

405 1.96 09/08/77 - 07/01/77 - -

410 2.91 16/07/82 - 10/05/00 3.23 25/02/99 - 10/05/00

659 2.59 07/12/94 - 03/12/98 4.29 28/07/98, 03/12/98 (2 values)


Appendix 4. Contents of Excel spreadsheets and worksheets.

A4.1 Excel spreadsheet Ruasteadystate.xls

This contains worksheets for simulating water quality using non-irrigated seepage velocities

with broadly, the following purposes:

Model development: Ruatbase

MZ

SWZ

SWZ Mon

GW Inf Zones

GW Mon

Irricells

Irr SWZ

gwflow

streams

Running the model: N summary

surfqualpredicts

gwqualpredicts

N bkg

Nirri

Npoint

Intermediate workings: Z1A bkg, Z1B bkg, Z2 bkg, Z3A bkg, Z3B bkg, Bkg sum

Z1A irri, Z1B irri, Z2 irri, Z3A irri Z3B irri, Irr sum

N load all

Other: STDM+10, Wells Dm, Gwq.sites, Swg.sites, test

The worksheets used for running the model are updated should model properties change (e.g.

altering surface water zones is outlined in Appendix 5, Section 5.3).


Ruatbase - a base map of the region

A worksheet that represents the extent of the groundwater flow model of the Ruataniwha

Plains:

80 rows by 100 columns

500 m by 500 m cell size

Inactive cells = grey, and a value of 0

Active cells = white, and a value of 1

MZ - the major zones

Definitions of zones in the Ruataniwha Plains model:

0 = outside the model area, or stream beds

1 = zone where the water quality effects of landuse are likely to reside in the Waipawa

River, which includes the catchments of the:

Mangaonuku Stream, and other northern streams, Waipawa River

some land on the south side of the Waipawa River is included where there is

evidence that the land drains to the Waipawa River through surface water or

groundwater

2 = zone between the Waipawa River and Tukituki River where the water quality effects

of landuse are likely to reside in the Tukituki River. This excludes the Tukipo River.

3 = zone south of the Tukituki River where the water quality effects of landuse are likely

to reside in the Tukipo River catchment. The zone includes the catchments of the

Tukipo River, Makaretu River, Porangahau Stream, and Maharakeke Stream.

SWZ - surface-water zones

This worksheet defines sections of the Ruataniwha Plains where the effects of landuse may be

represented at surface water quality monitoring sites. Numbers are two-digit (Fig. 10, Table

A4.1), with the first digit being a major zone (1, 2 or 3). The pattern of stream flow gains and

losses (Fig. 3) are a consideration in defining these zones. Generally monitoring sites are

sited in gaining zones. This is because sites in losing zones are less likely to be affected by

land use.


A4.1. Location of surface-water zones.

Zone Number Location

11 Mangatahi Stream catchment

12 Te Heka Stream, and others east of Mangaonuku Stream

13 Catchments east of Mangaonuku Stream

14 Catchment of the Mangaonuku Stream

15 Catchment of the Mangaonuku Stream

16 Catchment of the Mangamate Stream

17 Catchment of the Mangamauku Stream + unnamed stream that crosses

SH50 at Richardson‟s Bridge

18 Area between Mangamauku Stream, Waipawa River

19 Area around Waipawa River, eastern Ruataniwha Plains

21 Area between Waipawa River and Lindsay water race (approx)

22 Area between Kahahakuri Stream and Lindsay water race

23 Area between Kahahakuri Stream and Tukituki River (west)

24 Tukituki River terraces u/s of SH50

25 Kahahakuri Stream to Tukituki River (east)

31 Tukituki River to Tukipo River

32 Tukipo River to Mangatewai River (north)

33 Tukipo River to south of Mangatewai River

34 Tukipo River to Makaretu River (west)

35 Makaretu River to Porangahau Stream (west)

36 Makaretu River to Maharakeke Stream

37 Tukipo River to Makaretu River (east)

38 Makaretu River at eastern Ruataniwha Plains

39 Tukituki River to Tukipo River (west)

SWZ Mon - surface-water quality monitoring sites

The locations of the following surface water quality monitoring sites (Fig. 11, Table A4.2) of

some sites listed in Appendix 2 and Appendix 3.

Table A4.2. Hawkes Bay Regional Council surface-water quality monitoring sites.

Site River/Stream

287 Mangaonuku

273 Mangamate

286 Mangaonuku

402 Makaroro

283 Waipawa

280 Waipawa

284 Mangaonuku

659 Kahahakuri

410 Kahahakuri

356 Tukituki

26 Waipawa

20 Tukituki

144 Tukipo

21 Tukipo


22 Tukituki

279 Tukipo

405 Maharakeke

23 Tukituki

398 Porangahau

397 Porangahau

290 Makaretu

GW Inf zones - groundwater capture zones

Cells that are within the capture zone of selected groundwater monitoring sites are tabulated.

Cells are identified through flowpaths identified by the Ruataniwha Plains MODFLOW

steady-state model. The capture zones (Fig. 12) are associated with groundwater monitoring

sites (Fig. 13) as in Table A4.3.

Table A4.3. Groundwater capture zones and monitoring wells.

Capture zone

number

Groundwater well

number (sheet

GWmon)

Depth of well

11 220 45.7

12 236 65.8

13 146 12.4

14 222 21.8

21 224 75

22 223 55.5

23 239 142

24 2227 ?

25 233 46.3

31 2229 ?

32 231 22.6

33 229 24.4

34 1497 ?

35 1377 7

The capture zone includes the cell of each monitoring well.

GW mon - location of groundwater quality monitoring sites

As with the sheet „GW inf zones‟ but with the locations and numbers, of all groundwater

quality monitoring sites indicated.


Table A4.4. Summary of all nitrogen loadings to the Ruataniwha Plains.

Point Sources

Surface

zones

Loading Irrigation Loading Number of cells

Total loading

1/cells Total output

kgN/ha/year kgN/ha/year per ha kgN/yr

11 0 11 0 0 0 25 0

12 0 12 0

13 0 13 0

14 0 14 0

15 0 15 0

16 0 16 0

17 0 17 0

18 0 18 0

19 0 19 0

21 0 21 0

22 0 22 0

23 0 23 0

24 0 24 0

25 0 25 0

31 0 31 0

32 0 32 0

33 0 33 0

34 0 34 0

35 0 35 0

36 0 36 0

37 0 37 0

38 0 38 0

39 0 39 0

kgN/yr kgN/yr kgN/yr

Sums 0 0 0

N balance

N inputs kgN/yr Check sum of N applied through land use

(from the Nloadall sheet) Background land use 0

Irrigation 0 kgN/ha/yr 1/cells/ha kgN/yr

Point sources 0

Sum land use 0 0 25 0

Through u/s stream/river boundaries: 94217


N outputs kgN/yr

Waipawa River

Through u/s boundary 28067

From land use 0

Sum 28067

Tukituki River

Through u/s boundary 45727

From Zone 2 0

From Zone 3 0

Sum 45727

Other u/s river boundaries 20423


Balance (Out –In) 0

Percent difference 0 %


N Summary - a summary of nitrogen inputs and outputs

A summary of all nitrogen applications (Table A4.4) to Ruataniwha Plains land as specified

by the three worksheets: „N bkg‟, „N irri‟, and „N point‟.

The nitrogen loadings are summarised by surface water zones from the three „nitrogen-

loading‟ worksheets as kg N/ha/yr. Sums are calculated as kg N/yr.

This sheet has a nitrogen balance calculation which has the following components:

Inputs: Land sources - background, irrigation and point sources.

Influent nitrogen through boundary river/streams: Mangaonuku, Mangamate,

Waipawa, Tukituki.

Outputs: Nitrogen output through rivers: Waipawa River and Tukituki River.

A balance equation measures the difference between outputs and inputs and expresses this as

kg N/yr and percentage difference. The difference between output and input should be less

than 10%. Differences should only arise because of rounding of numbers in the worksheets.

Surfqualpredicts

Predictions of steady-state nitrogen concentrations at 16 surface sites.

gwqualpredicts

Predictions of groundwater nitrogen concentrations at 14 groundwater sites.

N bkg - current nitrogen loadings to land

This worksheet (for example Table A4.5 for Zone 1) estimates the current nitrogen loadings

on land as evidenced by surface and groundwater quality.


Table A4.5. Nitrogen loadings to land in Zone 1.

Zone loads

Major zones Zone 1

Input data


Load kgN/ha/yr

Cells in sub zone

ha

Background Output

kgN/yr

Nloadall total output sum for zones

kgN/ha/yr

l/ cells per per ha

Total output kgN/yr

11 0 144 3600 0 0 25 0 12 0 119 2975 0 0 25 0

13 0 110 2750 0 0 25 0

14 0 47 1175 0 0 25 0 15 0 66 1650 0 0 25 0

16 0 128 3200 0 0 25 0

17 0 129 3225 0 0 25 0 18 0 616 15400 0 0 25 0

19 0 32 800 0 0 25 0

Monitoring points:

Surface water site

River/stream name N conc. in stream

at model boundary mg/L

water flow at model

boundary m**3/s

water flow at site

m**3/s


284 Mangaonuku @ Tikokino Rd 0.5 0.28 3.6 286 Mangaonuku @ Argyll Rd 0.5 0.28 2.1

287 Mangaonuku@SH50 0.5 0.28 0.28 26 Waipawa@RDS 0.1 8.9 16.1

Calc.stream N concentration

Monitoring points: Surface water

zone contributions

Cells in

sub zone

ha

bkgrd Mass

of N at RuaT

boundary

kgN/yr

Water flow

at RuaT boundary

m**3/yr

bkgrd

RuaT Output

kgN/yr

water flow

at site m**3/yr

bkgrd

Calc N at site

at site

mg/L

Obs

N

Nloadall

Total output

sum for

zones kgN/yr

Nloadall

Calc N at site

at site

mg/L

Surface water site

273 Mangamate@

SH50 bridge

16 128 3200 16007.6736 14821920 0 14821920 1.1 1.08 0 1.1

284 Mangaonuku @

Tikokino Rd

11+12+13+14+

15+16+17+18

1359 33975 4415.04 8830080 0 113529600 0 1.66 0 0

286 Mangaonuku @

Argyll Rd

13+14+15+16+

17

480 12000 4415.04 8830080 0 66225600 0.1 1.64 0 0.1

287 Mangaonuku@

SH50

14+15 113 2825 4415.04 8830080 0 8830080 0.5 0.53 0 0.5

26 Waipawa@RDS 11+12+13+14+

15+16+17+18+

19

1391 34775 28067.04 280670400 0 507729600 0.1 0.63 0 0.1

Groundwater site

Vert. capture ratio (1=fully penetrating) gw zone bkgrd Calc N

Obs N Nloadall Calc N

220 1 11 0 0.3 0

236 1 12 0 1 0 146 1 13 0 1 0

222 1 14 0 0.48 0

The format of this sheet is, for each zone:

1) a list of surface water subzones, loading rates (variable), zone size (ha) and calculated

nitrogen input (kg N/yr),

2) a list of surface water monitoring sites, influent nitrogen concentrations (if a stream

crosses the upstream model boundary), flow rates at the upstream model boundary,

and flow rates at the measuring sites,

3) calculated nitrogen concentration at observation sites, considering influent nitrogen

mass and mass-loading from landuse,


4) observed nitrate-nitrogen, or ammonia-nitrogen, concentrations at the observation site,

5) groundwater monitoring site and capture zone number,

6) calculated nitrogen concentration at observation sites, and

7) observed nitrate-nitrogen, on ammonia-nitrogen, concentrations at groundwater

monitoring sites.

A nitrogen balance model is at the foot of the worksheet. Zone-by-zone nitrogen inputs are

compared with nitrogen outputs. Estimated nitrogen concentrations in the Tukituki River,

downstream of the confluence with the Tukipo River, are compared with observations.

Nirri - nitrogen loadings to land from irrigation

This sheet is used to apply nitrogen for four types of land use to the surface water zones that

are within the Ruataniwha Plains groundwater model irrigated zone.

Table A4.6. Nitrogen application from irrigation.

Irrigation sources

N outputs Land use type A B C Fallow

kgN/ha/yr 100 20 10 0

Zone

number

Irrigate

d cells

in SWZ

Irrigated

ha in

SWZ

Land use type

ha

N application

kgN/yr

Total N kgN/yr Equiv zone irrig N

kgN/ha/yr

zone zones A B C Fallow A B C Fallow

11 0 0 0 0 0 0 0 0 0 0 0 0 12 0 0 0 0 0 0 0 0 0 0 0 0

13 0 0 0 0 0 0 0 0 0 0 0 0

14 0 0 0 0 0 0 0 0 0 0 0 0 15 0 0 0 0 0 0 0 0 0 0 0 0

16 18 450 0 0 0 450 0 0 0 0 0 0

17 25 625 0 0 0 625 0 0 0 0 0 0 18 251 6275 0 0 0 6275 0 0 0 0 0 0

19 23 575 0 0 0 575 0 0 0 0 0 0

21 149 3725 0 0 0 3725 0 0 0 0 0 0

22 47 1175 0 0 0 1175 0 0 0 0 0 0 23 110 2750 0 0 0 2750 0 0 0 0 0 0

24 25 625 0 0 0 625 0 0 0 0 0 0

25 69 1725 0 0 0 1725 0 0 0 0 0 0

31 173 4325 0 0 0 4325 0 0 0 0 0 0

32 19 475 0 0 0 475 0 0 0 0 0 0

33 86 2150 0 0 0 2150 0 0 0 0 0 0

34 47 1175 0 0 0 1175 0 0 0 0 0 0

35 45 1125 0 0 0 1125 0 0 0 0 0 0 36 98 2450 0 0 0 2450 0 0 0 0 0 0

37 44 1100 0 0 0 1100 0 0 0 0 0 0

38 3 75 0 0 0 75 0 0 0 0 0 0 39 7 175 0 0 0 175 0 0 0 0 0 0

sum 30975 0 0 0 30975 sum 0 0 0 0 0


The four landuses have associated nitrogen applications in the example in Table A4.6 have

associated nitrogen applications to groundwater as in Table A4.7.

Table A4.7. Nitrogen applications used to demonstrate Table A4.6.

The number of hectares used for each land use is listed by surface water zone (Table A4.6).

The N application (in kgN/yr) is calculated by multiplying the number of ha of a particular

land use by the N application rate (in kgN/ha/yr). The total N application is summed across

the different land uses for each surface water zone, and then back calculated to produce an

„average‟ N loading rate (in kgN/ha/yr) across the whole surface water zone.

Npoint - nitrogen application from point sources

This sheet contains a list of surface water and groundwater monitoring sites with observed

and calculated nitrogen concentrations (Table A4.8). The worksheet also contains a map of

the Ruataniwha Plains as cells. Each cell represents a point-source of nitrogen with an

application to land of kg N/ha/yr over the whole 500 m by 500 m (25 ha) cell. A table on this

worksheet (Table A4.8) compares predicted and observed nitrogen at surface and

groundwater monitoring sites.

Land use Nitrogen application

(kg N/ha/yr)

A 100

B 20

C 10

Fallow 0


Table A4.8. Site-by-site nitrogen summary on the N point worksheet.

Zone 3 Calc N at site Obs

Calc N at site Obs Surface water at site (tot) N

Surface water at site (tot) N sites mg/L

sites mg/L 144 0 0.85

273 1.1 1.08 279 0 2.08

284 0 1.66 21 0 1.1

286 0.1 1.64 398 0 3.53

287 0.5 0.53 397 0 1.91

26 0.1 0.63 405 0 1.96

23 temp

Groundwater

sites

220 0 0.3

236 0 1 Groundwater sites

146 0 1 2229 0 0.05

222 0 0.48 231 0 4.41

*229 0 0.55

Zone 2 Calc N at site Obs 1497 0 1.22

Surface water at site (tot) N 1377 0 22.6

sites mg/L

356 0.1 0.8 Calc N. conc Obs

20 0.1 0.26 at site (tot)

659 0 2.59

410 0 2.91 Tukituki@Site 23 0.1 0.99

Groundwater

sites

*224 0 0.27

223 0 6.26

*239 0 0.55

2227 0 3.52

233 0 0.42

Irricells

Cells that are irrigated in the Ruataniwha Plains groundwater model are set to a value of 2.

Irr SWZ

This lists the surface water zone cells that are irrigated in the Ruataniwha Plains groundwater

model.

Z1A bkg, Z1B bkg

Nitrogen application for existing landuse, by cell, in Zone 1, from sheet N bkg.

Z2 bkg



Z3A bkg, Z3B bkg


Bkg sum

Sum of nitrogen applications for existing landuse, by cell in all zones, from sheet N bkg.

Z1A irr, Z1B irr

Nitrogen application from irrigation, by cell, in Zone 1. Used by Nirr.

Z2 irr

Nitrogen application from irrigation, by cell, in Zone 2. Used by Nirr.

Z3A irr, Z3B irr

Nitrogen application from irrigation, by cell in Zone 3. Used by Nirr.

Irr sum

Sum of nitrogen applications from irrigation, by cell, from all zones, from sheet Nirr.

N load all

Sum of all cell-by-cell nitrogen loadings i.e. Bkg sum + Irr sum + N point.

gwflow

Predicted groundwater flow rates from Ruataniwha Plains MODFLOW model predictions of

e-w and n-s flow rates. Units are m3/s.

Streams

Stream locations.

STDM+10

Stream number - this is the Ruataniwha Plains groundwater model stream number plus 10

(Table A4.9).


Table A4.9. Stream number and stream name.

Stream

Number Stream Name

11 Mangamate

12 Mangaonuku1

13 Mangaonuku2

14 Mangamauku

15 Mangaonuku3

16 Mangaoho

17 Mangaonuku4

18 Mangatahi

19 Mangaonuku5

20 Makaroro

21 Waipawa

22 Waipawa2

23 Waipawa3

24 Tukituki1

25 Maharakeke1

26 Porangahau

27 Maharakeke2

28 Makaretu1

29 Makaretu2

30 Tukipo1

31 Tangarewai

32 Tukipo2

33 Mangatewai

34 Tukipo3

35 Tukipo4

36 Tukituki2

37 Kahahakuri

38 Tukituki3

WellsDm

Location of monitoring wells (=2) used in the Ruataniwha Plains MODFLOW groundwater

flow model.

Gwq.sites

Location of HBRC groundwater quality monitoring sites (=2).

Swq.sites

Location of HBRC surface water quality monitoring sites (=2).

test

Test worksheet.


A4.2. Excel spreadsheet Ruagwflux.xls

This spreadsheet is used to calculate groundwater seepage velocity of values from, and

derived values from the steady-state, non-irrigated model. The spreadsheet contains the

following worksheets with, broadly, the purpose of:

Input data: rhtflux

fntflux

heads

aquifer base

porosity

Worksheets: Totalflux (m3/day)

Totalflux (m3/s)

Gradient rht

Gradient fnt

Aq thick

Seepagevelrht

Seepagevelfnt

Combined seepage velocity

rhtflux right (west to east) groundwater flow from steady-state simulation

(m3/day), non-irrigated

fntflux front (north to south) groundwater flow from steady-state simulation

(m3/day), non-irrigated

Totalflux (m3/day) magnitude of vector of rhtflux and fntflux, non-irrigated

Totalflux (m3/s) magnitude of vector of rhtflux and fntflux, non-irrigated

Heads predicted groundwater level (m) from steady-state flow model

Gradient rht west to east head gradient across 1 km (three cells)

Gradient fnt north to south head gradient across 1 km (three cells)

Aquifer base base of aquifer used in steady-state flow model

Aq thick thickness of saturated zone in steady-state flow model

Porosity porosity, set to 0.2

Seepagevelrht calculated west to east seepage velocity = rhtflux/Aq thick/500/porosity


Seepage vel fnt calculated north to south seepage velocity = fntflux/Aq thick/500/

porosity

Combined seepage magnitude of the vector of seepagevelrht and seepagevelfnt (m/day),

velocity Fig. 14

A4.3 Excel spreadsheet Ruagwqualtrans.xls

This spreadsheet is used to predict groundwater quality changes with time without irrigation

and contains the following worksheets with, broadly, the purpose of:

Input data: Gwzones

Welldist

Seepage velocity

Irrig sum

Calculation worksheet: Travel time (d)

Travel time (yr)

tt le 1yr

tt le 2yr

tt le 5yr

tt le 10yr

tt le 20yr

tt le 30yr

tt le 50yr

Output: Zone loads

Trans conc

Summary

Gwzones groundwater capture zones, as in Rautsteadystate.xls

Welldist distances of cells from monitoring wells (m), all groundwater zones,

cell-centre to cell-centre


Seepage velocity seepage velocity (from Ruagwflux.xls) (m/day), non-irrigated

Travel time (d) predicted travel time (non-irrigated) to monitoring wells (days) =

Welldist/Seepage velocity. Note that the travel time for the whole travel

path is not considered, i.e. the seepage velocity in each cell is assumed

to represent the average seepage velocity over the travel path

Travel time (yr) predicted travel time to monitoring wells (years). Travel time set to 0.5

yr for the cells containing monitoring wells.

Irrig sum sum of irrigation from Ruatsteadystate.xls

tt le 1yr N applications of cells within 1 years travel time of monitoring wells

tt le 2 yr N applications of cells within 2 years travel time of monitoring wells






Zone loads Total N loadings on each zone, 1 to 50 years, non-irrigated

Trans conc Calculations of transient groundwater (1 to 50 years) quality with

irrigation applications

Summary Calculations of transient groundwater (1 to 50 years) quality with

irrigation applications

This spreadsheet is actively linked to Ruatsteadystate.xls.

A4.4 Excel spreadsheet Ruasurfflowdist.xls

This spreadsheet is used to calculate travel times of groundwater that discharges to surface

water, using non-irrigated groundwater seepage velocities. Input, calculation and output

worksheets are as follows:

Input data: surf zones

16 to 39

seep vel

Calculation worksheets: surf dist


Output: travel time (d)

travel time (y)

surf zones surface water zones as in Ruasteadystate.xls

16 to 39 travel distances to surface water, as follows:

Worksheet „destination‟ travel route to monitoring site:

Surface zone Destination cell Pathway

16 cell BU18 groundwater

17 cell BX22 groundwater

18 cell CA34 groundwater

19 cell CB52 groundwater

21 cell BW50 groundwater

22 cell BN46 groundwater

23 cell BU56 groundwater

24 cell AZ51 groundwater/surface water

25 cell BY58 groundwater

31 nearest river cell groundwater/surface water

32 cell AT57 groundwater

33 cell AY62 groundwater


35 cell BB71 groundwater

36 cell BT63 groundwater/surface water




surf dist all the distances from sheets 16 to 39

seep vel groundwater seepage velocity, non-irrigated

travel time (d) groundwater travel time, days, non-irrigated

travel time (y) groundwater travel time, years, non-irrigated


A4.5 Excel spreadsheet Ruasurfqualtrans.xls

This spreadsheet is used to predict groundwater quality changes with time for seepage

velocities with non-irrigated land and contains the following worksheets for the following

purposes:

Input data: surf zones

surf dist

seepage velocity

travel time (d)

travel time (yr)

Irrig sum

Calculation worksheets: tt le 1yr

tt le 2yr

tt le 5yr

tt le 10yr

tt le 20yr

tt le 50yr

Output: zone loads

trans conc

summary

surf zones surface water zones (from Ruasteadystate.xls)

surf dist distance of travel to surface water monitoring sites

seepage velocity groundwater seepage velocity, non-irrigated

travel time (d) groundwater travel times (days), non-irrigated

travel time (yr) groundwater travel times (years), non-irrigated

Irrigsum sum of cell-by-cell irrigation (kgN/ha/yr) (from Ruasteadystate.xls)

tt le 1 yr N applications of cells within a travel time of less than 1 year of surface

monitoring sites

tt le 2 yr N applications of cells within a travel time of less than 2 years of

surface monitoring sites










zone loads predicted loading from zones 1 to 50 years

trans conc predicted transient N concentrations for 14 surface-water monitoring

sites for 1 to 50 years due to irrigation. Concentrations due to irrigation

are added to the currently-observed concentration.

summary predicted transient N concentrations for 14 surface-water monitoring

sites for 1 to 50 years due to irrigation. Concentrations due to irrigation

are added to the currently-observed concentration.

A4.6 Excel spreadsheet Ruagwfluxirri.xls

The worksheets in this spreadsheet are the same as Ruagwflux.xls and use the MODFLOW

model steady-state solution with irrigation to calculate groundwater cell-by-cell seepage

velocities.

A4.7 Excel spreadsheet Ruagwqualtransirri.xls

The worksheets in this spreadsheet are the same as Ruagwqualtrans.xls and use the

MODFLOW model steady-state solution with irrigation to estimate transient groundwater

quality in response to irrigation.

A4.8 Excel spreadsheet Ruasurfflowdistirri.xls

The worksheets in this spreadsheet are the same as Ruasurfflowdist.xls and use predicted

groundwater travel times with irrigation.


A4.9 Excel spreadsheet Ruasurfqualtransirri.xls

The worksheets in this spreadsheet are the same as Ruasurfqualtrans.xls and use predicted

groundwater travel times with irrigation.


Appendix 5. Operation of the Excel spreadsheets.

1.0 NITROGEN LOADING

The are three ways to „load‟ nitrogen to the Ruataniwha Plains model: „background‟,

„irrigation‟, and „point sources‟. Data is entered in the Ruasteadystate.xls spreadsheet or the

Ruasteadystateirri.xls spreadsheet (Appendix 4). These two spreadsheets differ with

Ruasteadystate.xls representing groundwater flow in the Ruataniwha Plains without irrigation

and Ruasteadystateirri.xls representing groundwater flow with irrigation.

1.1 Background nitrogen loading

„Background‟ loading of nitrogen is used to match current land use with current mean

nitrogen concentrations in surface water and groundwater (section 7.8).

Nitrogen is applied to surface-water zones (Table A4.1) at the same rate through the zone and

is entered through worksheet „Nbkg‟.

Nitrogen loadings are entered for each zone in the blue-coloured cells in the worksheet. For

example 100 kgN/ha/yr is entered for zone 19 in Table A5.1 and predictions of surface water

quality (e.g. 0.59 mg/L at site 26, Table A5.2) and groundwater quality (e.g. 0.6 mg/L at site

222 Table A5.3). A vertical capture ratio (section 7.8.2, Table A5.3) may be defined so that

estimates of groundwater quality are based on partial mixing of nitrogen with groundwater.

Table A5.1 Entering nitrogen loadings in surface water zones, Nbkg worksheet.

Zone 1 Nloadall 1/ Nloadall

Input data Background Total output cells Total output mean loading

Surface water Load Cells in subzone ha Output sum for zones per ha

subzones kgN/ha/yr kgN/yr kgN/ha/yr kgN/yr kgN/ha/yr

11 0.5 144 3600 1800 72 25 1800 0.5

12 0.5 119 2975 1487.5 59.5 25 1487.5 0.5

13 0.5 110 2750 1375 55 25 1375 0.5

14 1 47 1175 1175 47 25 1175 1

15 1 66 1650 1650 66 25 1650 1

16 4 128 3200 12800 818 25 20450 6.4

17 28 129 3225 90300 4037 25 100925 31.3

18 5 616 15400 77000 7347 25 183675 11.9

19 100 32 800 80000 3591 25 89775 112.2

sum 267587.5

1.2 ‘Irrigation’ nitrogen

Nitrogen from irrigation is loaded through the worksheet Nirri. It is somewhat of a misnomer

to call this nitrogen from irrigation as the nitrogen can be from any land use, irrigated or non-

irrigated. Four types of land uses, with four nitrogen leaching rates (Table A5.4) are used to

represent the mix of land uses on the plains. The nitrogen outputs from each land use may be

adjusted by the user (Table A5.4).

Nitrogen leaching rates, and land areas, may be entered in the cells coloured blue on the

worksheet. In an example zone 19 (Table A5.4), with a total of 575 irrigable hectares in the

zone, has a land use scenario of: 100 ha of land use „A‟, 200 ha of land use „B‟, 100 ha of land

use „C‟ and 175 ha of „fallow‟ land.

N loading

on zone 19


The „fallow‟ land use is calculated from the total irrigable hectares in the zone minus the sum

of land uses „A‟, „B‟ and „C‟. An error will occur in this spreadsheet (Table A5.5) if the sum

of land uses A, B, and C is greater than the total irrigable area.

Table A5.2 Example of estimation of nitrogen concentration of surface water monitoring

site 26, „Nbkg‟ worksheet.

River/stream name N conc. in stream water flow water flow

at model boundary at model boundary at site

mg/L m**3/s m**3/s

Mangamate@SH50 bridge 0.5 0.2 0.47

Mangaonuku @ Tikokino Rd 0.5 0.1 3.6

Mangaonuku @ Argyll Rd 0.5 0.1 2.1

Mangaonuku@SH50 0.5 0.1 0.28

Waipawa@RDS 0.1 9.4 16.1

bkgrd bkgrd bkgrd

Mass of N at Water flow at RuaT water flow Calc N at site Obs

Surface water zone Cells in subzone ha RuaT boundary RuaT boundary Output at site at site N

contributions kgN/yr m**3/yr kgN/yr m**3/yr mg/L

16 128 3200 3153.6 6307200 12800 14821920 1.08 1.08

11+12+13+14+15+16+17+18 1359 33975 1576.8 3153600 187587.5 113529600 1.67 1.66

13+14+15+16+17 480 12000 1576.8 3153600 107300 66225600 1.64 1.64

14+15 113 2825 1576.8 3153600 2825 8830080 0.5 0.53

11+12+13+14+15+16+17+18+19 1391 34775 29643.84 296438400 267587.5 507729600 0.59 0.63

Table A5.3 Example of estimation of nitrogen concentration of groundwater at site 222,

„Nbkg‟ worksheet.

Groundwater site Vert. capture ratio (1=fully penetrating) gw zone Calc N Obs N

220 1 11 4 0.3

236 1 12 0.8 1

146 1 13 0.9 1

222 1 14 0.6 0.48

1.3 ‘Point source’ nitrogen

Nitrogen loadings from point sources may be entered through the „N point‟ worksheet. This

worksheet represents the Ruataniwha Plains as a set of 500 m by 500 m cells. The user can

enter nitrogen loading, as kgN/ha/yr, to cells representing the land on the plains coloured blue

on the worksheet. Point loadings may not be entered into cells representing rivers and

streams. These cells are coloured yellow on the worksheet.

2.0 NITROGEN BALANCE

Nitrogen loadings are summarised in the worksheet „Nsummary‟. loadings from background,

irrigation, and point sources are summed with nitrogen inputs from rivers across the western

boundary. These are compared with nitrogen outputs through the Waipawa and Tukituki

rivers (Table A5.6). In the example the total nitrogen loading to the plains is 817752 kgN/yr.

This includes 770763 coming from land use and 46989 kgN/yr coming into the Ruataniwha

Plains from boundary streams and rivers. A total of 297232 kgN/yr is predicted as leaving the

Ruataniwha Plains through Waipawa River and 515789 kgN/yr is predicted as leaving the

plains through the Tukituki River.

Note: The balance of nitrogen (e.g. Table A5.6) should always be close to zero. A balance

significantly different from zero probably means that a worksheet has been corrupted.

Estimated N

concentration at

site 26

Estimated N

concentration

at site 222


Table A5.4 Example of land use loadings of zone 19, „Nirri‟ worksheet.

Irrigation sources

N outputs

Land use type A B C Fallow

kgN/ha/yr 5 10 20 0

Adjust these values

Irrigable Irrigable Land use type N application Total N

Zone number cells in SWZ ha in SWZ ha kgN/yr kgN/yr


11 0 0 0 0 0 0 0 0 0 0 0

12 0 0 0 0 0 0 0 0 0 0 0

13 0 0 0 0 0 0 0 0 0 0 0

14 0 0 0 0 0 0 0 0 0 0 0

15 0 0 0 0 0 0 0 0 0 0 0

16 18 450 0 0 0 450 0 0 0 0 0

17 25 625 0 0 0 625 0 0 0 0 0

18 251 6275 0 0 0 6275 0 0 0 0 0

19 23 575 100 200 100 175 500 2000 2000 0 4500

Table A5.5 Example of an error in entering the land use areas for zone 19, „Nirri‟

worksheet.

Irrigation sources

N outputs

Land use type A B C Fallow

kgN/ha/yr 5 10 20 0

Adjust these values

Irrigable Irrigable Land use type N application Total N Equiv zone irrig N

Zone number cells in SWZ ha in SWZ ha kgN/yr kgN/yr kgN/ha/yr


11 0 0 0 0 0 0 0 0 0 0 0 0

12 0 0 0 0 0 0 0 0 0 0 0 0

13 0 0 0 0 0 0 0 0 0 0 0 0

14 0 0 0 0 0 0 0 0 0 0 0 0

15 0 0 0 0 0 0 0 0 0 0 0 0

16 18 450 0 0 0 450 0 0 0 0 0 0

17 25 625 0 0 0 625 0 0 0 0 0 0

18 251 6275 0 0 0 6275 0 0 0 0 0 0

19 23 575 200 200 200 ERROR 1000 2000 4000 #VALUE! #VALUE! #VALUE!

Nitrogen outputs

may be adjusted by

the user

Land use in zone 19 is:

100 ha of „A‟, 200 ha of

„B‟, 100 ha of „C‟ and

175 ha of „Fallow‟.

Irrigable hectares in zone 19


Table A5.6 Example of nitrogen balance calculations, „Nsummary‟ worksheet.

N balance

N inputs kgN/yr


Irrigation 0

Point sources 0

Sum land use 770763



N outputs kgN/yr

Waipawa River



Sum 297232

Tukituki River


From Zone 2 113700

From Zone 3 389475

Sum 515789




Percent difference 0 %

3.0 SURFACE WATER QUALITY PREDICTIONS

These are made on the „Nbkgrd‟ worksheet (Table A5.7) and echoed on the „Npoint‟

worksheet (Table A5.8).

4.0 GROUNDWATER QUALITY PREDICTIONS

These are made on the „Nbkgrd‟ worksheet (Table A5.9) and echoed on the „Npoint‟

worksheet.

5.0 MAINTENANCE OF SPREADSHEETS

5.1 Changing groundwater flow velocities

Groundwater flow velocities (Darcy) occur in the spreadsheets Ruagwflux.xls and

Ruagwfluxirri.xls. These spreadsheets are used to calculate the seepage velocities used in the

non-irrigated steady-state model, irrigated steady state model and transient models).

Sum of nitrogen inputs

Sum of nitrogen outputs

Balance


Table A5.7 Example of stream nitrogen concentrations, „Nbkgrd‟ worksheet.

Calc.stream N concentration bkgrd bkgrd bkgrd Nloadall Nloadall

Monitoring points: Mass of N at Water flow at RuaT water flow Calc N at site Obs Total output Calc N at site

Surface water Surface water zone Cells in subzone ha RuaT boundary RuaT boundary Output at site at site N sum for zones at site

site contributions kgN/yr m**3/yr kgN/yr m**3/yr mg/L kgN/yr mg/L

273 16 128 3200 3153.6 6307200 12800 14821920 1.08 1.08 12800 1.08

284 11+12+13+14+15+16+17+18 1359 33975 1576.8 3153600 187587.5 113529600 1.67 1.66 187587.5 1.67

286 13+14+15+16+17 480 12000 1576.8 3153600 107300 66225600 1.64 1.64 107300 1.64

287 14+15 113 2825 1576.8 3153600 2825 8830080 0.5 0.53 2825 0.5

26 11+12+13+14+15+16+17+18+19 1391 34775 29643.84 296438400 267587.5 507729600 0.59 0.63 267587.5 0.59

Surface water

monitoring sites

N conc. due to

„background‟

land use

Observed mean

concentration

N conc. due to:

„background‟ +

„irrigation‟ +

„point‟ land

uses


Table A5.8 Example of stream nitrogen concentrations, „Npoint‟ worksheet.

Zone 1

Surface water Calc N at site Obs

sites at site (tot) N

mg/L

273 1.08 1.08

284 1.67 1.66

286 1.64 1.64

287 0.5 0.53

26 0.59 0.63

Use the following procedure to calculate seepage velocity:

1) Paste-out Darcy west-to-east velocities from the steady-state Ruataniwha Plains model

output file into the „rhtflux‟ worksheet.

2) Paste-out Darcy north-to-south velocities from the steady-state Ruataniwha Plains

model output file into the „fntflux‟ worksheet.

3) Paste-out calculated groundwater heads from the steady-state Ruataniwha Plains

model output file into the „heads‟ worksheet.

4) Enter model of the aquifer base into to „Aquifer base‟ worksheet, if required.

5) Enter model porosities into „porosity‟ worksheet if required.

6) Copy the calculated non-irrigated seepage velocities in the „Total flux (m3/s)‟

worksheet into the Ruasteadystate.xls worksheet „gwflow‟.

7) Copy the calculated irrigated seepage velocities in the „Total flux (m3/s)‟ worksheet

into the Ruasteadystateirri.xls worksheet „gwflow‟.

5.2 Changing observed surface water and groundwater nitrogen concentrations

These are typed into the „Nbkd‟ worksheet (Table A5.7 and Table A5.9).

5.3 Adjusting the capture zones for surface water monitoring sites and groundwater

monitoring sites

Type the number of the zone into the „SWZ‟ worksheet to edit the surface water zone e.g.

change a boundary etc.

Type the number of the zone in the worksheet „GW inf zones‟ to edit the groundwater zone

e.g. change a boundary etc.

Surface water

monitoring site

N conc. due to

all land use

Observed mean

concentration


5.4 Adding new surface water or groundwater zones

First, delete a zone from the „SWZ‟ worksheet (surface water zones) or „GW inf zones‟

(groundwater zones). Type the code for the new zone into the same worksheet. Note that the

code number must be the same code number as the zone that has been deleted. This is

because a limited number of zone codes are available. The location of the zones is defined by

the codes on the worksheets.

Table A5.9 Example of groundwater nitrogen concentrations „Nbkgrd‟ worksheet.

bkgrd Nloadall

Groundwater site Vert. capture ratio (1=fully penetrating) gw zone Calc N Obs N Calc N

220 1 11 4 0.3 4

236 1 12 0.8 1 0.8

146 1 13 0.9 1 0.9

222 1 14 0.6 0.48 0.6

5.5 Update seepage velocities in ‘transient’ calculations

5.5.1 Groundwater travel times to monitoring wells The excel files Ruagwflowdist.xls and Ruagwflowdistirri.xls are used to estimate the travel

times in days and years through the groundwater system to monitoring wells.

Seepage velocities may require updating: paste seepage velocities (as m/day) out of

Ruagwflux.xls „combined seepage velocity‟ worksheet (for non-irrigated seepage velocities),

or Ruagwfluxirri.xls „combined seepage velocity‟ worksheet (for irrigated seepage velocities).

5.5.2 Travel times to surface water monitoring sites

Travel times through the groundwater system to surface water monitoring sites are calculated

in the Excel files Ruasurfflowdist.xls and Ruasurfflowdistirri.xls. Worksheets list the travel

distance to the surface water monitoring site using estimated groundwater flow directions and

seepage velocities derived from Ruataniwha Plains groundwater flow models. Travel times

use groundwater seepage velocity calculations from the non-irrigated, or irrigated,

groundwater flow models.

Seepage velocities may require updating: paste seepage velocities (as m/day) out of

Ruagwflux.xls „combined seepage velocity‟ worksheet (for non-irrigated seepage velocities)

or Ruagwfluxirri.xls „combined seepage velocity‟ worksheet (for irrigated seepage velocities)

into the spreadsheets Ruasurfflowdist.xls and Ruasurfflowdistirri.xls.

5.5.3 Transient calculations The sheets Ruagwqualtrans.xls, Ruagwqualtransirri.xls, Ruasurfqualtrans.xls and

Ruasurfqualtransirri.xls are used to estimate future nitrogen concentrations in groundwater

and surface water. Changes in water quality are estimated over a 50 year period following a

Groundwater

monitoring

site

N conc. due to

„background‟

land use

Observed

mean

concentration

N conc. due to:

„background‟ +

„irrigation‟ +

„point‟ land uses


change in land-use in the spreadsheets at the time of writing. Water quality is estimated at 1,

2, 5, 10, 20, 30 and 50 years in the current version of the spreadsheets.

The following input data sets will require updating if seepage velocities are updated.

Well dist (m) - from Ruagwflowdist.xls

Surf dist (m) - from Ruasurfflowdist.xls

Seepage velocity - either non-irrigated (from Ruagwflux.xls) or irrigated (from

Ruagwfluxirri.xls)

Travel time (d) - non-irrigated (from Ruasurfflowdist.xls) or irrigated (from

Ruasurfflowdistirri.xls)

Travel time (yr) - non-irrigated (from Ruasurfflowdist.xls) or irrigated (from

Ruasurfflowdistirri.xls).

The sheets Ruagwqualtrans.xls, Ruagwqualtransirri.xls, Ruasurfqualtrans.xls and

Ruasurfqualtransirri.xls can be used to estimate future nitrogen concentrations at any time as

follows:

Enter a new time, in years in cell b1 of the worksheets titled „tt le 1 yr‟ and

rename the worksheet with the correct year.

The „Trans conc.‟ worksheet contains the summary of surface and groundwater quality. This

calculates nitrogen concentrations above the background level. Adjust the figures in Column

G is the background levels require change.

6.0 ASSUMPTIONS

Assumptions used in the building of the spreadsheets are noted in this section.

6.1 Surface water zones

It is assumed that the nitrogen drainage in a surface water zone will be fully „captured‟ by the

monitoring site associated with the zone. Surface water monitoring sites are chosen in

reaches of rivers and streams where groundwater is thought to be flowing to the streams. It is

possible drainage-carrying groundwater discharges downstream of a monitoring point.

6.2 Groundwater capture

The groundwater capture zones are estimated by eye from contour lines predicted by the

Ruataniwha Plains groundwater model. It may be that local groundwater flow directions are

different from the modelled flow directions. It is also be possible that the capture zones are of

different extents than estimated in this work.

6.3 Mixing in surface water

It is assumed that the nitrogen in drainage is fully mixed with surface water. This may not be

the case. Stratification of nitrogen in surface water may give rise to higher local

concentrations of nitrogen.


6.4 Groundwater mixing ratio

Groundwater mixing ratios are chosen so that the mean of observed groundwater chemistry

measurements matches the predictions of nitrogen in drainage given by „calibration‟ of the

land use to observed surface water nitrogen concentrations. The vertical profile of nitrogen

concentrations is unknown locally, so the mixing ratio is therefore unknown.

Some clue to the vertical mixing of nitrogen in the Ruataniwha Plains is given by nitrogen

concentrations and well depth data (Fig. 9). It is unknown any regional pattern (e.g. Fig. 9) is

valid at the local scale.

6.5 Water balance

It is assumed that all water entering the Ruataniwha Plains leaves via the Waipawa and

Tukituki rivers. Water enters the Ruataniwha Plains in rivers across the northern and western

boundaries of the plains, and in rainfall recharge to groundwater on the plains.

6.6 Chemistry balance

It is assumed that all nitrogen entering the Ruataniwha Plains leaves via the Waipawa and

Tukituki rivers. Nitrogen enters the Ruataniwha Plains in rivers across the northern and

western boundaries of the plains, and in rainfall recharge to groundwater on the plains.

7.0 SPREADSHEET SECURITY

A number of worksheets have „locked‟ cells. This is to prevent users from inadvertently

corrupting worksheets. The code to unlock worksheet cells is „ruataniwha‟.

regional groundwater quality and surface water …€¦ · 5. nitrogen inputs to groundwater and...

Documents