Pergamon Phys. Chem. Earth (i?). Vol. 26, No. 7-8, pp. 523-527,200l

0 2001 Elsevier Science Ltd.

All rights reserved

1464-1909/01/$ -see front matter

PII: S1464-1909(01)00044-2

Computer Aided Evaluation of Planning Scenarios to Assess the Impact of Land-Use Changes on Water Balance

J. Terpstra and A. van Mazijk

Delhi University of Technology, Faculty of Civil Engineering and Geosciences, Section of Hydrology and Ecology, Stevinweg 1, P.O. Box 5048, 2600GA Delft, The Netherlands

Received 24 April 2000; revised 27 October; accepted 26 November 2000

Abstract. The impact of land use scenarios on water availability is part of the analysis and evaluation of water resources planning options for a river basin. An increasing demand for proper planning and development strategies indicates the need for robust methodologies to analyse the relation between spatial planning and water resources availability. Such a need can be met, for instance, by means of a simple water balance, which relies on a grid description of the river basin. The distributed character of the basin water balance modelling allows to evaluate the influence of land use changes on runoff availability at any point in the river network, this being limited only by the size of the grid mesh. A pilot study in this direction has been undertaken, aiming at establishing a modular framework, which allows developing a computer aided estimation of impacts of land use changes on total runoff. An application to an actual case area has highlighted major advantages and some limitations of using such an approach in the early phase of planning. An example of such planning illustrates the impact of reforestation on run off and consequently on the expansion of a wastewater treatment plant. Q 2001 Elsevier Science Ltd. AlI rights reserved

1 Introduction

One of the main objectives of water resources management is to match water availability and water use in a river basin. The water using activities are spatially distributed. If the supply and demand for water does not match at a certain location, water conducting infrastructure with or without storage capacity (reservoirs) is installed. The primary step in efficient spatial planning is to match

water demand with the spatial availability of water. An aspect often forgotten in this respect, is the impact of

Correspondence to: A. van Mazijk

Spatial plan

i’ I I 4 I L-m Evaluation of impacts

Fig. 1 Matching demand supply

the new land use on the hydrological availability of water. Locating for instance a new housing area close to a particular well, which is supposed to supply drinking water, could have an impact on the infiltration rate in the catchment of the well and so the project has to be redesigned.

Recognition that land use patterns and related water using activities do not only govern the demand for water, but also the water availability, has triggered the project to develop a tool helping the spatial or water resources planner to account for the whole water system (Fig. 1). At the Institute of Hydromechanics and Water Resources

Management of the ETH Zurich in co-operation with the Delft University of Technology the Land Use Change Impact Determination model (LUCID) has been developed in the framework of a master thesis (Terpstra, 2000). This model is a pilot tool for the evaluation of impacts of land use change scenarios on water availability in a river basin, i.e. the discharges in the river system. Since the aim of the

524 J. Terpstra and A. van Mazijk : Impact of Land-Use Changes on Water Balance

Fig. 2 The Glatt basin in Switzerland

tool is not to give insight in the flooding risks caused by changing the land use, but focussing on the impact of land use changes on the average river discharge, the model concept of this pilot tool is based on a monthly water balance.

In order to test this model concept and to act as a reference, a case area was selected: the Glatt river watershed. This catchment is located in the north-eastern part of Switzerland in the border region between the Kantons St.Gallen and Appenzell Ausserrhoden (Fig. 2 and Fig. 4). The basin covers an area of about 90~10~ m* and is characterised by an annual precipitation of about 1,500 mm, a total height difference of 600 m with the outlet of the basin at about 400 m and the peak at 1000 m above sea level. The land use is mainly agricultural with 62%, followed by forests with 26% and 11% urban area.

2 The LUCID-model

The LUCID-model is based on a grid representation of the river basin, For the modelled Glatt-river basin a grid- element size of 1OOxlOOm has been chosen. For the considered monthly water balance the most significant processes related to the land use are the evapotranspiration and the interception. Therefore the water balance per grid element concerns precipitation, evapotranspiration (including interception) and runoff. For the determination of the runoff, groundwater flow is not considered separately: the changes in ground water storage can be assumed to be negligible on the applied time scale.

The model input data per grid element consist of land use, precipitation and potential evapotranspiration. The appointed land use can differ from grid element to grid element. The land use data were derived from standard land use maps. In the developed model the different kinds of agricultural land use like wheat, corn and potatoes have been put together into one land use type: agriculture. Built- up area, on the other hand, is split up into high-, medium- and low-density housing, industry and traffic systems.

Using measured data and applying Thiessen polygons created a distributed precipitation map. For the estimation of.the actual evapotranspiration values the Bagrov method (DVWK, 1996; Dyck, 1983) was applied. The method uses three parameters: the precipitation P, the potential evapotranspiratibn ETp and the effectiveness parameter N, which depends on the land use and the soil type. For the determination of the actual evapotranspiration ETu the following relation is given

- (-1

dETazI_ ETa N

dP ETP

The model uses a graphical solution of Eq. (1) after DVWK (1996). It constitutes a simple relationship between the ratio of precipitation and potential evapotranspiration (PIETp) on one hand and the ratio of actual and potential evapotranspiration (ETdETp) on the other hand. This is considered per land use, i.e. N-value. For the Glatt basin one soil type was used, corresponding with an available soil moisture of 15% (Gurtz et al., 1997).

The potential evapotranspiration itself depends on the land use. This would imply that for every land use type separate values for the potential evapotranspiration have to be computed. However, a sensitivity analysis showed no significant influence on the resulting actual evapotranspiration (Terpstra, 2000). Therefore one reference value has been applied, being the mean value of the potential evapotranspiration values of the concerned land uses.

The river network was derived from a digital elevation map (DEM). The river system is divided into reaches of a 100 m coinciding with the grid elements of the land use map. A river reach is determined on the basis of a minimum upstream drainage area, which has been selected in the present application at 0.5~10~ m*, corresponding with 50 grid elements. Every element of the land use map drains into a specific reach of the river system. The direction of flow for every grid element was derived from the DEM, using steepest descent for the surrounding grid elements.

The computational framework was implemented in a computer program written in Visual Basic. The program consists of separate modules each representing a different step in the calculation process. A first module allows setting up and changing the land use map. In the next module the water balance per grid element is computed. This module gives the discharge as the result of precipitation and actual evapotranspiration, calculated according to the Bagrov method. A final module computes the flow routing for each grid element through the basin and the discharge per river grid-element. The model concept assumes linearity of the flow-routing system. Therefore the watershed response is equal to the sum of the responses of its sub-areas, e.g. grid elements (Olivera and Maidment, 1999).

J. Terpstra and A. van Mazijk : Impact of Land-Use Changes on Water Balance 525

Fig. 3 Interface for changing a land use scenario

Important element for the user is the interface by which land use scenarios can be built and evaluated (Fig. 3). For the set up of a land use scenario an interactive land use map of the considered area is available. Interactive options can be used to present discharges throughout the basin. Those facilitate an evaluation of the effects of land use scenarios.

il/ Oberbiiren (outlet)

‘kq Gossau

Wissenbach

I

\rrr.'O 3km &

Fig. 4 Glatt catchment: discharge gauging stations

3 Calibration and validation

The Glatt basin (Fig. 4) is used for calibration and validation of the model. The calibration has focussed on the applicability of the Bagrov method. As part of the calibration particular care was needed for checking of the input data. For calibration the measured and calculated values of the monthly discharges at the measuring stations Herisau, Wissenbach, Gossau and Oberbiiren (Fig. 4), were compared. The calibration was executed with data of the years 1993, 1994 and 1995. For the validation the period of 1984 up to 1992 was considered. In Fig. 5 the validation for the measuring station OberbUren is presented: the monthly

Discharge at Oberbiiren 5

1

Jan Feb Mar Apr May Jun Jul Aug Sap Ckt Nov Dee

-Measured - - - - Calculated

Fig. 5 Discharges at OberbUren per month averaged over the period of 1984 up to 1992

discharges, averaged over the years 1984 up to 1992 are compared with the calculated values. The computed monthly discharges are within a 10 to 20% interval around the measured values. On the year balance the difference is only 1 to 2%. This shows that for the considered area and the applied monthly time scale an explicit modelling of the groundwater component in the water balance is not necessary.

A comparison of the Bagrov method with the more sophisticated, physically based, Penman approach (Penman, 1948), proved that the Bagrov approach produces the same results under the circumstances concerned (Terpstra, 2000). Due to the fact that seasonal change of the interception depth is not included by the Bagrov method, the computed discharges should be adjusted. The model underestimates discharges in winter and spring, whereas summer and autumn discharges are overestimated. In summer, interception by leaves results in a higher evaporation and, hence, in a lower discharge than the model predicts. If this effect would be incorporated, the differences between measured and calculated values will become even smaller than the above indicated deviations (Fig. 5).

4 Application of the model

The operational LUCID-model is a tool to assess the impact on the river discharge, as a result of changes in land use of a grid element with an individual surface area of 0.01~10~ m2. Spatial planners can use this capability e.g. to analyse the effects of expanding urban areas or reforestation of agricultural areas. The model can also be used to control the available river discharge in certain river reaches for particular changes of land use conditions. This application deals for instance with the river discharge needed for drinking water supply or dilution of wastewater.

Referring to the last item the following scenario can be considered. At the village of Flawil (Fig. 3) a wastewater purification plant treats the water of about 20,000 people (AfU, 1998). Some kilometres upstream, at the village of

526 J. Terpstra and A. van Mazijk : Impact of Land-Use Changes on Water Balance

_ _ _ - Present discharge 4 -Present plant

-Expanded plant -Reforestation

Fig. 6 Monthly discharges at Flawil (averaged over the period 1984-1995) and the minimum prescribed river discharge related to the present capacity of the wastewater treatment plant (Flawil) and the expanded plant (Flawil + Gossau)

Gossau, another plant purifies the water of an equivalent of 18,000 people. The treatment facilities need to be improved. It is however too expensive to improve both facilities and therefore the village of Gossau will be connected to the improved plant at Flawil. The site at Flawil now discharges an average of 70 I/s into the Glatt. In Switzerland refined prescriptions of water conservation are in operation. For certain substances, e.g. ammonium, regulations require a dilution factor of at least 10. In the considered example of Flawil this means an average discharge of at least 700 l/s (Fig. 6). Connecting the Gossau area to this plant would mean an additional 60 l/s effluent from the plant at Flawil, requiring a minimum flow of 1.3 m3/s in the Glatt River. At present only the average discharge for March falls below this value. However, if part of the upstream catchment would be reforested the average discharges would decrease. In that case the situation is not only critical during the month March, but also in February and October. Moreover, the predicted discharges are monthly averages. This means that even for several days during the month the discharge is expected to be lower and the water quality worse.

The impact of land use change on the availability of water depends primarily on the average volume of rainfall excess. At present not more than 10% of the basin is covered with housing and industry. One of the tests executed by the developed model concerned the change of the complete catchment into urban area. The effect of this urbanisation scenario resulted into an increment of about 30% of the river discharge, which corresponds with about 300 mm precipitation a year. Due to the already high precipitation level of about 1,500 mm, the impact remains rather small. In a country like the Netherlands with 750 mm precipitation per year and an average evapotranspiration of 400 mm, an

increase of the discharge with 300 mm would mean a doubling of the runoff. When applying this on arid and semi-arid regions one can foresee even higher impacts.

5 Limitations

Change of land use has its impacts on the time lag between precipitation and river discharge. Expanding impervious areas speeds up the rainfall-runoff process, whereas regreening will slow down the process. The acceleration of the flow processes will likely also lead to higher peak flow rates. The applied monthly time step filters out the dynamics with shorter time scales. Excess water table heights and flow speeds can not be computed with the present model. It is clear that land use changes will affect these dynamic processes. There are scores of research projects in this particular field with comparable grid based computer tools developed, such as the flood ,risk analysis model ‘Flora’ (Burlando et al., 1994).

The water balance approach implemented in this model is based on the fact that for most medium and large river basins the time lag between precipitation and discharge at the outlet is smaller than one month. However, there are basins wherein this time lag is longer than a month, such as e.g. the Zambezi River basin. In that case the water balance approach, using a monthly time step as applied in this model, would not be appropriate.

The applied grid-element size depends on the resolution of the available data. In Switzerland as in most western countries, land use and other data are available on the scale of a 100x100 m element size or smaller with the latest satellite imagery technology. In large parts of the world a larger element size will be indicated because of data limitations.

A different level of detail can be observed for certain natural and hydrological data. Surface and atmospheric data are available at a much higher resolution than subsurface characteristics and parameters.

Care should be taken to apply the model in its present configuration to varying watershed conditions, especially because the groundwater influence is not explicitly considered. The computational framework is made up of separate modules allowing incorporating the effects of groundwater on the monthly water balance rather easily.

6 Conclusions and outlook

The LUCID-model shows that it is feasible to develop a tool on the basis of readily available information, which provides the required management information as defined at the start of the project: the connection of spatial planning and the availability of water resources. Because of the modular character of the model it is possible to improve it step by step. The calibration and validation shows that,

.I. Terpstra and .4. van Mazijk : Impact of Land-Use Changes on Water Balance 521

using the simple Bagrov method for the land use dependent actual evapotranspiration, the water balance approach even gives information about critical situations in relation to the availability of water and in succession its quality. The grid approach for the modelling of the river basin offers additional opportunities for easily exchanging information with other GIS oriented systems.

Certain changes in the land use have also some impacts on the water system. Changing natural and agricultural areas into urban ones changes the amount of evaporation, but human settlement also means extra point withdrawals affecting the water availability. New housing areas are accompanied by e.g. wastewater purification plants, drinking water facilities and hydroelectric power stations. These point withdrawals have their own impact on the river discharge. The impacts are concentrated in one point and differ in their characteristics from the changes in the land use. Their occurrence is however (spatially) coupled to a certain land use.

Incorporating point withdrawals and processes involved with groundwater into the computational framework would be a substantial improvement of the LUCID-model. A simple routine for groundwater storage could e.g. be added, accounting for the delays in part of the rainfall-runoff process. The effects of point withdrawals could e.g. be included by simply superimposing discharges or extractions on the flow routing. In addition, incorporation of a leave area index could improve the Bagrov approach.

In the future the LUCID-model could become an element of an overall framework for spatial/water resources planning e.g. linking it to the Ribasim model (Delft Hydraulics, 1997).

References

AfU, Zustand und Nutzung der Gewtiser im Kanton St.Gallen, Auswertung und Darstellung gewtisserrelevanten Daten 1982 bis 1996,

Baudepartement des Kantons St.Gallen, Amt fir Umweltschutz,

StGallen, 1998.

Burlando, P., Mancini, M., and Rosso, R., FLORA: a distributed flood risk

analyser in Computer Support for Environmental Impact Assessment,

(B-16), ed.: G. Guariso and B. Page, Elsevier Science, North-Holland,

1994.

Delft Hydraulics, Ribasim, Model description, Del& The Netherlands,

1997.

DVWK, MerkblXter zur Wasserwirtschati 238/1996, Ermittlung der

Verdunstung von Land- und Wasserflachen, Kommissionsvertrieb

Wirtschafis- und Verlagsgesellschafi Gas und Wasser mbH, Bonn, 1996.

Dyck, P., Grundlagen der Hydrologie, Ernst & Sohn, Verlag fir

Architektur und technische Wissenschafien, Berlin, 1983.

Gurtz, J., Baltensweiler, A., Lang, H., Menzel, L., and Schulla, J.,

Auswirkungen von klimatischen Variationen auf Wasserhaushalt und

Abfluss im Flussgebiet des Rheins, VDF Hochschulverlag AG an der

ETH Zurich, Zurich, 1997. Olivera, F. and Maidment, D., Geographical information systems (GIS)-

based spatially distributed model for runoff routing, Waler Resources

Research, ~01.35, no.4, 1155-l 164, 1999.

Penman, H.L., Natural evaporation from open water, bare soil and grass,

Proceedings Royal Society London, Series A 193, 120-145, 1948.

Terpstra, J, Impact of Land Use Change on River Discharge, Design of a

software tool to analyse the impacts of land use change on the

discharges in the Glatt basin, Switzerland, Master Thesis, Delfl University of Technology and Swiss Federal Institute of Technology

Zurich, 2000.