CHAPTER TWO: BASICS IN CHAPTER TWO: BASICS IN IRRIGATION ENGINEERINGIRRIGATION ENGINEERING

2.1. IRRIGATION ENGINEERING: This involves Conception, Planning, Design, Construction, Operation and Management of an irrigation system.

An irrigation engineer is one who has a long theoretical and practical training in planning, design, construction, operation and management of irrigation systems.

Considerations in Planning Considerations in Planning Irrigation SystemsIrrigation Systems

i) Location: The main point to consider in locating an irrigation project is the need to investigate available resources in the area e.g.

Climate, Adequate water in quality and quantity, Land with good agricultural potential and Good topography, Availability of labour (sophisticated or not), Land tenure, Marketing, Transport facilities etc.

Considerations in Planning Considerations in Planning Irrigation Systems Contd.Irrigation Systems Contd.

ii) Crops to be grown: Should be determined by available resources as well as marketability of the crops especially in terms of what people like to eat.

iii) Water Supply: Consider (a)Sources of water (b) Quantity and quality of water c) Engineering works necessary to obtain water e.g. if

underground, pumping is needed d) Conveyance System: can be by gravity e.g. open channels or

canals or by closed conduits e.g. pipes. (e) Water measuring devices e.g. weirs, orifice, flumes, current

meters

 

Other ConsiderationsOther Considerations iv) Systems of Applying Water: e.g. Surface (90% worldwide), Sprinkler(5%), Trickle and Sub-irrigation(5%).  v) Water Demand: The water requirement for the given crop has to

be determined. This is by calculating the evapotranspiration (to be treated later)

  vi) Project Management: Consider how to manage the irrigation

system

 

2.2 CROP WATER AND NET 2.2 CROP WATER AND NET IRRIGATION REQUIREMENTSIRRIGATION REQUIREMENTS

In irrigation, it is essential to know the amount of water needed by crops. This determines the quantity of water to be added by irrigation and

rainfall and helps in day to day management of irrigation systems.

Total water demand of crops is made up of:  i) Crop water use: includes evaporation and transpiration

(evapotranspiration described in section 2.3 below)  ii) Leaching requirement:   iii) Losses of water due to deep seepage in canals and losses due to

the inefficiency of application.

EVAPOTRANSPIRATIONEVAPOTRANSPIRATION 2.3.1 DEFINITIONS  a) Evaporation: The process by which water is changed from the

liquid or solid state into the gaseous state through the transfer of heat energy.

b) Transpiration: The evaporation of water absorbed by the crop which is used directly in the building of plant tissue in a specified time. It does not include soil evaporation.

  c) Evapotranspiration, ET: It is the sum of the amount of water

transpired by plants during the growth process and that amount that is evaporated from soil and vegetation in the domain occupied by the growing crop. ET is normally expressed in mm/day.

FACTORS THAT AFFECT FACTORS THAT AFFECT EVAPOTRANSPIRATIONEVAPOTRANSPIRATION

 Weather parameters, Crop Characteristics, Management and Environmental aspects are factors affecting ET

  (a)   Weather Parameters: The principal weather conditions affecting

evapotranspiration are: Radiation, Air temperature, Humidity and Wind speed.

CROP FACTORS THAT CROP FACTORS THAT AFFECT ETAFFECT ET

Crop Type

Variety of Crop

Development Stage

Crop Height

Crop Roughness

Ground Cover

Crop Rooting Depth

Management and Environmental Management and Environmental FactorsFactors

(a)   Factors such as soil salinity, Poor land fertility, Limited application of fertilizers, Absence of control of diseases and Pests and poor soil management May limit the crop development and reduce soil

evapotranspiration.

Other factors that affect ET are ground cover, plant density and soil water content. The effect of soil water content on ET is conditioned primarily by the magnitude of the water deficit and the type of soil. Too much water will result in waterlogging which might damage the root and limit root water uptake by inhibiting respiration.

EVAPOTRANSPIRATION EVAPOTRANSPIRATION CONCEPTSCONCEPTS

(a) Reference Crop Evapotranspiration (ETo): Used by FAO.

This is ET rate from a reference plant e.g. grass or alfalfa, not short of water and is denoted as ETo. The ET of other crops can be related to the Et of the reference plant.

ETo is a climatic parameter as it is only affected by climatic factors.

The FAO Penman-Monteith method is recommended as the sole method for determining ETo. The method has been selected because it closely approximates grass ETo at the location evaluated, is physically based, and explicitly

incorporates both physiological and aerodynamic parameters.

CROP ET UNDER STANDARD CROP ET UNDER STANDARD CONDITIONS CONDITIONS (ETc) (ETc)

This refers to crop ET under standard conditions, i.e. ET from disease-free, well-fertilized crops, grown in large fields, under optimum soil water conditions.

ETc can be derived from ETo using the equation: ETc = Kc . ETo where Kc is crop coefficient  Crop Evapotranspiration under non- standard

conditions as mentioned above is called ETc (adjusted). This refers to growth of crops under non-optimal conditions.

DETERMINATION OF DETERMINATION OF EVAPOTRANSPIRATIONEVAPOTRANSPIRATION

Evapotranspiration is not easy to measure. Specific devices and accurate measurements of various physical parameters or the soil water balance in lysimeters are required to determine ET. The methods are expensive, demanding and used for research purposes. They remain important for evaluating ET estimates obtained by more indirect methods.

ENERGY BUDGET METHODENERGY BUDGET METHODThis method like the water budget

approach involves solving an equation which lists all the sources and sinks of thermal energy and leaves evaporation as the only unknown. It involves a great deal of instrumentation and is still under active development. It is data intensive and is really a specialist approach.

Energy Budget Method Contd.Energy Budget Method Contd.

Water Balance MethodWater Balance Method The Water Balance or Budget Method is a

measurement of continuity of flow of water. This method consists of drawing up a balance sheet of

all the water entering and leaving a particular catchment or drainage basin.

The water balance equation can be written as:  ET = I + P – RO – DP + CR + SF + SW  Where: I is Irrigation, P is rainfall, RO is surface runoff,

DP is deep percolation, CR is capillary rise, SF and SW are change in sub-surface flow and change in soil water content respectively

Lysimeters For Water Balance Lysimeters For Water Balance MethodMethod

Lysimeters are normally adopted in water balance studies. By isolating the crop root zone from its environment and

controlling the processes that are difficult to measure, the different terms in the soil balance equation can be determined with greater accuracy.

Using Lysimeters, crop grows in isolated tanks filled with either disturbed or undisturbed soil.

In weighing lysimeters, water loss is directly measured by change in mass while

In non-weighing ones, the ET for a given time is determined by deducting the drainage water collected at the bottom of the lysimeters, from the total water input.

Non-Weighing LysimeterNon-Weighing Lysimeter

ET Computed from ET Computed from Meteorological DataMeteorological Data: :

ET is commonly computed from weather data. A large number of empirical equations have been developed for assessing crop or reference crop evapotranspiration from weather data. Some of these methods include the Blaney-Criddle, Penman, Thornthwaite, Radiation, Hargreaves, Turc and many others. Most of these methods have been found to only work in specific locations.

Following an Expert Consultation by Food and Agriculture Organization in May 1990, the FAO Penman-Monteith method is now recommended as the standard method for the definition and computation of the reference evapotranspiration. The FAO Penman-Monteith equation is described in the Notes.

ET Estimated from ET Estimated from Evaporation Pans:Evaporation Pans:

Evaporation from an open water surface provides an index of integrated effect of radiation, air temperature, air humidity and wind on evapotranspiration. However, differences in the water and cropped surface produce significant differences in the water loss from an open surface and the crop. The pan is used to estimate reference ETo by observing the evaporation loss from a water surface (Epan) and applying empirical coefficients (Kpan)to relate pan evaporation to Eto thus:

ETo = Kp x Epan

Standard Pan: United States Class A Standard Pan: United States Class A PanPan

The most common Evaporation Pan used is the United States Class A pan. This is made up of unpainted galvanized iron, 1.2 m in diameter and 25.4 cm deep. The bottom supported on a wooded frame, is raised 15.24 cm above the ground surface. The water surface is maintained between 5.0 and 7.6 cm below the rim of the pan and is measured daily with a gauge. The daily evaporation is computed as the difference between observed levels corrected for any precipitation measured in an adjacent or nearby standard rain gauge. A pan coefficient of 0.7 (0.6 - 0.8) is normally used to convert the observed value to an estimated value for lake or reservoirs. This is because the rate of evaporation in small areas is greater than that from large areas.

US Class A Evaporation PanUS Class A Evaporation Pan

Incoming Radiation q’ Absorbed By Water

EvaporationAir Flow

Conduction Through Walls of pan

q’ conv absorbed by the water

Incoming Radiation Heats Pan Wall q’’ rad

Convection q”conv heats up pan walls

          

 

Heat Transfer Mechanisms Involved In Heating Of Water In The Standard Pans (diameter D) And Their Walls (After Jagroop,2000).

 

Types of Evaporation PansTypes of Evaporation Pans

A Comparison of Standard Open A Comparison of Standard Open PansPans

Pan Dimensions Pan Coefficient

US Class A 1.2 m Diameter; 250 mm Deep

0.7 (0.6 to 0.8)

Australian Pan 900 mm Diameter; 900 mm Deep. Large Pan: 1200 mm Diameter and 850 mm Deep

0.9 ( 0.6 to 1.2)

British Tank 1.83 m Square 0.9 (Very Variable)

2.4. LEACHING REQUIREMENT2.4. LEACHING REQUIREMENT

Most irrigation water contain dissolved salts. Evaporation removes pure water leaving a

concentration of salt in soil. Salt concentration may reach a level that is detrimental

to the growth of the crop and should be controlled. The only practical way of achieving this is by leaching.

Leaching requirement is an extra water needed to pass through the root zone in addition to the normal requirement to ensure that salts are placed below the root zone.

LEACHING REQUIREMENT LEACHING REQUIREMENT CONTDCONTD..

acceptableEC

RainETirrigEc

ZoneRoottheinContentSaltAcceptableRainETWaterIrrigationinionConcentratSaltLR

)(

)(

Ec acceptable = 4 mmhos/cm. For water quality, Ec of 0.8 Mmhos/cm is medium, quality while Ec of 4 mmhos/cm is saline.

2.5. EFFECTIVE PRECIPITATION2.5. EFFECTIVE PRECIPITATION This is the component of rainfall that is

available to crops ie. does not runoff.

It can be estimated as 65% of total rainfall.

It can also be estimated as the rainfall value, which has 80% probability of being exceeded (D80).

2.6 NET IRRIGATION 2.6 NET IRRIGATION REQUIREMENT (Nir)REQUIREMENT (Nir)

This is the moisture that must be supplied by irrigation to satisfy evapotranspiration plus that needed for leaching and not supplied by off-season storage, and the effects of precipitation and groundwater storage.

Nir = ET + Wl - Ws - Re

Where: Nir is the net irrigation; ET is evapotranspiration, Wl is leaching requirement; Ws is off-season soil moisture carry-over. All parameters are in mm of water.

 

2.72.7        GROSS IRRIGATION GROSS IRRIGATION REQUIREMENT (Gir)REQUIREMENT (Gir)

Gross Irrigation Requirement is equal to:

Net Irrigation Requirement Divided by Irrigation Efficiency

Irrigation efficiency accounts for losses in storage and distribution systems, losses in application systems as well as operation and management losses.

Irrigation Efficiency depends on the Method of Applying Irrigation Water

2.8 IRRIGATION TERMS2.8 IRRIGATION TERMS2.8.1. Depth of Irrigation: This is the

depth of the readily available moisture. This is the net depth of water normally needed to be applied to the crops during each irrigation

Example 1Example 1 The Moisture Content at Field Capacity of a Clay Loam Soil is 28%

by Weight While that at Permanent Wilting Point is 14% by Weight. Root Zone Depth Is 1 m and the Bulk Density Is 1.2 g/cm3 . Calculate the Net and Gross Depth of Irrigation Required If the Irrigation Efficiency Is 0.7.

  Solution: Field Capacity = 28%; Permanent wilting point = 14% i.e. Available moisture = 28 - 14 = 14% by weight i.e. Pm Bulk density (Db) = 1.2 g/cm3 Root Zone depth (D) = 1 m = 1000 mm Equivalent depth of available water (d) = Pm . Db . D = 0.14 x 1.20 x 1000 mm = 168 mm This is the net depth of irrigation.

Solution to Example 1 contd.Solution to Example 1 contd.

Gross Water Application is equal to: Net Irrigation/Efficiency = 84/0.7 = 120 mm

Note: This is the actual water needed to be pumped for irrigation.

It is equivalent to: 120 /1000 mm x 10,000 m2 =

1200 m 3 per hectare.

2.8.2 2.8.2 Irrigation Interval (II): Irrigation Interval (II):

This is the time between successive irrigations.

Irrigation interval is equal to: Readily Available Moisture or Net Irrigation divided by

Evapotranspiration, ET The shortest irrigation interval is normally use in

design. The irrigation interval varies with ET. It is equivalent to Readily Available Water divided by the

Peak ET

Example 2Example 2 For the Last Example. the Peak ET is

7.5 mm/day, Determine the Shortest Irrigation Interval.

Solution: From Example 1, Readily Available Moisture (RAM) = 84 mm

i.e. Shortest irrigation interval = RAM/ Peak ET = 84/7.5 = 11 days.

Irrigation Period (IP)Irrigation Period (IP)This is the number of days allowed to

complete one irrigation cycle in a given area.

Irrigation Period Contd.Irrigation Period Contd.

Assuming water is applied in a border in a day, the total period of irrigation is then 11 days.

1 2 3 4 5 6 7 8 9 10

Irrigation Interval and PeriodIrrigation Interval and Period

In irrigation scheduling, the irrigation period should be less that the irrigation interval. This is because if the period is not smaller, before the latter parts of the area are to be irrigated, the earlier irrigated areas will need fresh irrigation.

At peak evapotranspiration (used in design), irrigation interval should be equal to irrigation period. i.e. Generally IP < II

2.8.4 Desired Irrigation Design 2.8.4 Desired Irrigation Design Capacity (Qc)Capacity (Qc)

This is the flow rate determined by the water requirement, irrigation time, irrigation period and the irrigation application efficiency.

It is the flow rate of flow of the water supply source e.g. pumps from a reservoir, or a borehole required to irrigate a given area.

Desired Irrigation Design Capacity Desired Irrigation Design Capacity (Qc)(Qc) Contd. Contd.

aEHFdA

cQ ...

Where:•Qc is the Desired Design Capacity; •d is the Net Irrigation Depth = Readily Available Moisture; •F is the number of Days to complete the Irrigation (Irrigation Period);•H is the number of Hours the System is perated (hrs/day) and• Ea is the Irrigation Efficiency

Example 3Example 3 A 12-hectare field is to be irrigated with a sprinkler

system. The root zone depth is 0.9 m and the field capacity of the soil is 28% while the permanent wilting point is 17% by weight. The soil bulk density is 1.36 g/cm and the water application efficiency is 70%. The soil is to be irrigated when 50% of the available water has depleted. The peak evapotranspiration is 5.0 mm/day and the system is to be run for 10 hours in a day.

Determine: (i) The net irrigation depth (ii) Gross irrigation ie. the depth of water to be pumped (iii) Irrigation period (iv) Area to be irrigated per day and (v) the system capacity.

Solution to Example 3Solution to Example 3 Solution: Field Capacity = 28%; Permanent

Wilting Point = 17% ie. Available Moisture = 28 - 17 = 11% , which is

Pm Root zone depth = 0.9 m; Bulk density = 1.36 g/cm3

Depth of Available Moisture, = Pm . Db. D = 0.11 x 1.36 x 900 = 135 mm Allowing for 50 % depletion of Available Moisture

before Irrigation, Depth of Readily Available Moisture = 0.5 x 135 mm = 67.5 mm

Solution of Example 3 Contd.Solution of Example 3 Contd. i) Net irrigation depth = Depth of the Readily Available Moisture =

67.5 mm  ii) Gross Irrigation = Net irrigation Application efficiency = 67.5/0.7 = 96.4 mm

iii) Irrigation interval = Net irrigation or RAM Peak ET = 67.5/5 = 13.5 days = 13.5 days = 13 days (more critical)

In design, irrigation interval = irrigation period ie. irrigation period is 13 days

Solution of Example 3 ContdSolution of Example 3 Contd.. iv) Total area to be irrigated = 12 hectares Area to be irrigated per day = Total area / irrigation

period = 12 ha/ 13 days = 1 ha/day   v) System Capacity, Qc = A. d m3 /s F. H. Ea Area, A = 12 ha = 12 x 10000 m2 = 120,000 m2

Net irrigation depth, d = 67.5 mm = 0.0675 m Irrigation period , F = 13 days Number of hours of operation, H = 10 hrs/day Irrigation efficiency, Ea = 0.78

Solution of Example 3 ConcludedSolution of Example 3 Concluded System capacity, Qc = 120,000 m2 x 0.0675 m

13 days x 10 hrs/day x 0.7   = 89.01 m 3/hr

Recall: 1 m 3 = 1000 L and 1 hr = 3600 s ie. 89.01 m3 /hr = {89.01 x 10 3 L}/3600 secs = 24.73 = 25 L/s The pump to be purchased for sprinkler irrigation must

have capacity equal to or greater than 25 L/s. Alternatively, more than one pump can be purchased.  

2.9. IRRIGATION EFFICIENCIES2.9. IRRIGATION EFFICIENCIES

These irrigation efficiencies are brought about by the desire not to waste irrigation water, no matter how cheap or abundant it is.

The objective of irrigation efficiency concept is to determine whether improvements can be made in both the irrigation system and the management of the operation programmes, which will lead to an efficient irrigation water use.

2.9.1 Application Efficiency2.9.1 Application Efficiency

EWater in root zone after irrigation

Total volume of water applieda

Total vol of water applied Vol of Tailwater Vol of deep percolation

Total water applied

. ( . . )

Ea is inadequate in describing the overall quantity of water

since it does not indicate the actual uniformity of irrigation, the amount of deep percolation or the magnitude of under-irrigation. See diagrams in text. 

Example 4Example 4 Delivery of 10 m3/s to a 32 ha farm is continued for 4

hours. The tail water is 0.27 m3/s. Soil probing after irrigation indicates that 30 cm of water has been stored in the root zone. Compute the Application Efficiency.

  Solution: Total volume of water applied = 10 m3/s x 4 hrs x 3600s/hr = 144,000 m3

Total tail water = 0.27 x 4 x 3600 = 3888 m3

Total water in root zone = 30 cm = 0.3 m x 32 ha x 10,000 m2/ha = 96,000 m3

Solution to Example 4 Contd.Solution to Example 4 Contd.

= 96,000/144,000 = 66.7%.

EWater in root zone after irrigation

Total volume of water applieda

2.9.2 Water Conveyance Efficiency2.9.2 Water Conveyance Efficiency

EWater delivered to the Farm W

Water of water diverted from a stream reservoir or well Wcd

s

( )

, ( )

Farm

Water lost by evapAnd seepage Ws

Wd

Stream

Example 5Example 5 45 m3 of water was pumped into a farm distribution system.

38 m3 of water is delivered to a turn out (at head ditch) which is 2 km from the well. Compute the Conveyance Efficiency.

Solution:

EWater delivered to the Farm W

Water of water diverted from a stream reservoir or well Wcd

s

( )

, ( )

= 38/45 = 84%

2.9.3. Christiansen Uniformity 2.9.3. Christiansen Uniformity Coefficient (CCoefficient (C

uu))

CX

m nu 100 10( ./ /

)

This measures the uniformity of irrigation

W here: is the summation of deviations from the mean depth infiltered

m is the mean depth unfiltered and n is the number of observations.

// X

Example 6Example 6 A Uniformity Check is taken by probing many

stations down the border. The depths of penetration (cm) recorded were: 6.4, 6.5, 6.5, 6.3, 6.2, 6.0, 6.4, 6.0, 5.8, 5.7, 5.5, 4.5, 4.9. Compute the Uniformity Coefficient.

  Solution: Total depth of water infiltered =

76.7 cm Mean depth = 76.7/13 = 5.9 cm

Locations Depths (cm) Deviations from Mean

1 6.4 0.5

2 6.5 0.6

3 6.5 0.6

4 6.3 0.4

5 6.2 0.3

6 6.0 0.1

7 6.4 0.5

8 6.0 0.1

9 5.8 0.1

10 5.7 0.2

11 5.5 0.4

12 4.5 1.4

13 4.9 1.0

Example 6 ConcludedExample 6 Concluded

This is a good Efficiency. 80% Efficiency is acceptable.

 

/ /X

CX

m nu 100 10( ./ /

)

Cu

100 106 2

5 9 13( .

.

.)

= 6.2

m = 5.9 cm; n = 13

= 92%

2.9.4 Water Storage Efficiency (E2.9.4 Water Storage Efficiency (Ess))

2.9.5 Irrigation Efficiency

EVolume of water in the root zone after irrigation

Volume of water needed in root zone to avoid total water moisture depletions

E Steady stateET W R W

W

Net Irrigation

Water divertedil e s

i

( )

ET is Evapotranspiration; Wl is Leaching Requirement;

Re is Effective Precipitation;

is change in storage; Wi is water diverted, stored or pumped for irrigation.

sW

2.10 IRRIGATION SCHEDULING 2.10 IRRIGATION SCHEDULING

This means Predicting when to Irrigate and how much to Irrigate

For efficient water use on the farm, the farmer needs to be able to predict when his crops need irrigation. This can be done by:

Observing the plants; Keeping a Water Balance Sheet By Measuring the Soil Moisture Content or Computer Software 

2.10.1 2.10.1 Observing the Plants: Observing the Plants:

This is a direct way of knowing when the crops need water.

The farmer observes the plants for any signs of wilting or change in leaf colour or growth rate.

The method is simple but its major disadvantage is that the signs of shortage appear after the optimum allowable depletion has already been exceeded.

2.10.2. Keeping a Water Balance 2.10.2. Keeping a Water Balance SheetSheet

This approach works on the principle that the change in water content of the soil is represented by the difference between water added by irrigation(or rainfall) and the amount lost by evapotranspiration.

The records are kept for each farm and crops as shown in Table 2.4 below.

The method requires no equipment and is easy to operate.

It can be operated on a daily or weekly or 10 day basis.

Table 2.3: Table 2.3: Example of a Water Balance Example of a Water Balance SheetSheet

Date Estimated ET (mm)

Rainfall (mm)

Accumulated Deficit (mm)

Irrigation Period

5.1.05 4.2 - 4.2

6.1.05 3.5 - 7.7

7.1.05 3.8 - 11.5

8.1.05 4.5 - 16.0

9.1.05 5.2 - 21.2

10.1.05 5.1 2.0 24.3

11.1.05 5.5 - 29.8

12.1.05 5.1 - 4.9 (34.9) 30.0

13.1.05 4.9 - 9.8

etc.

Irrigation Plan: Apply 30 mm of water at 30 mm deficit.

2.10.3 Measuring Soil Moisture2.10.3 Measuring Soil Moisture This is the best scheduling and the most widely used.

Soil moisture can be indirectly measured using devices and instruments eg. tensiometers, resistance blocks or neutron probes.

Direct measurement of soil moisture can be by weighing or the gravimetric method.

These methods are either too expensive or complicated. The simplest and most practical method is to estimate

the moisture content by the 'feel and appearance' of the soil.

Soil is collected at the root zone and checked to guess the right time to irrigate.

2.11 IRRIGATION WATER: SOURCES, QUALITY & 2.11 IRRIGATION WATER: SOURCES, QUALITY & MEASUREMENTMEASUREMENT

2.11.1 Sources of Irrigation Water Supply i) Rainfall or Precipitation: This is a practical

and dominant factor. The supply varies with time and place e.g. while

Grenada receives 2,100 mm annual rainfall, Antigua receives only 1,100 mm. Trinidad receives 1, 950 mm (Data supplied by Gumbs, 1987).

To be of greatest benefit for crop production, the rainfall amount should be enough to replace water in the root zone on a regular basis.

Mean Annual Rainfall of Caribbean Countries

1127

1500 1524

1983

4500

2263 2253

20571980

1372

1971 1990

2500

2054

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Antigu

a & B

arbud

a

The Bah

amas

Barbad

os

Belize

Domini

ca

Gre

nada

Guy

ana

Haiti

Jam

aica

St. Kitts

& N

evis

St. Lu

cia

St. Vinc

ent &

the G

rena

dines

Surin

ame

Trinida

d & T

obago

Mea

n A

nn

ual

Rai

nfa

ll (

mm

)

Sources of Irrigation Water Sources of Irrigation Water Contd.Contd.

ii) Underground water sources: This can be shallow or bore holes.

  iii) Surface Sources: Streams, rivers, lakes, farm ponds

etc. Streams should be gauged to ensure that there is enough

water for irrigation. Rivers or streams can also be dammed to raise the height

of flow and make more water available for irrigation. Farm ponds can also be dug to store water from rivers or

channels (e.g. field station) or to collect water from rainfall 

Sources of Irrigation Water Sources of Irrigation Water Contd.Contd.

iv) Springs and waste water e.g. industrial water and sewage: Determine quality before use.

(For details of harnessing water for irrigation in the Caribbean, see Gumb's Soil & Water Conservation Methods, Chapter 7).

2.11.2 Irrigation Water 2.11.2 Irrigation Water Quality:Quality:

Irrigation water quality depends on i) Amount of suspended sediment eg.

silt contentii) The chemical constituents of water

i) Amount of Suspended i) Amount of Suspended Sediment:Sediment:

The effect of sediment may depend upon the nature of the sediment and the characteristics and soil conditions of the irrigated area.

Silt content in irrigation may be beneficial if it improves the texture and fertility of say sandy soil.

It can also be detrimental if it is derived from a sterile sub-soil, and applied to a fertile soil.

Silt accumulation can cause aggradation in canals or distribution systems. In sprinkler systems, silt can cause abrasion.

ii) The Chemical Constituents ii) The Chemical Constituents of Water:of Water:

There are three main elements or compounds that can cause hazards in irrigation water. They include:

Sodium, Boron and Salts.

a) Salinity Hazards:a) Salinity Hazards:

The units of salt concentration in irrigation water can be parts per million (p.p.m), milli equivalents/litre(ME/litre) or electrical conductivity.

On the basis of salinity, irrigation water can be classified as C1 to C4(see chart).

They refer to low, medium, high and very high salinity levels respectively.

While C1 water can easily be used for irrigation without need for leaching requirement,

C4 water is not useable, except in permeable soils where adequate leaching and drainage is possible and for highly tolerant crops.

b) Sodium Hazard:b) Sodium Hazard:

It is Measured in Sodium Absorption Ratio (S.A.R).

SAR is defined as the proportion of sodium relative to other cations.

SARNa

Ca Mg

2

Parameters are measured in ME/litre.

Sodium Hazard Contd.Sodium Hazard Contd.

Irrigation Water is also divided into S1 to S4 in terms of Sodium (SAR) Content.

S1 Water can be used readily S2 and S3 can be used with adequate Leaching

and Drainage and addition of Organic and Chemical amendment.

S4 Water has very high Sodium Content and is unsuitable for irrigation except where calcium, gypsum or other chemical amendments are possible.

(See water quality chart).

BoronBoron

See Table 2.4 in Note Book for Permissible limit of Boron for several classes of irrigation water

2.11.3 Measurement of Irrigation 2.11.3 Measurement of Irrigation WaterWater

Water is the most valuable asset of irrigated agriculture and can be detrimental if used improperly.

An accurate measurement permits an intelligent use.

The methods to use for measurement should depend on the flow, environmental conditions and the degree of accuracy required.

Methods of Measuring Irrigation Methods of Measuring Irrigation WaterWater

a) Direct method: Collect water in a contained of known volume e.g. bucket. Measure the time required for water from an irrigation source e.g. siphon to fill the bucket.

Flow rate = Volume/time m3/hr or L/s etc.   b) Weirs: Weirs are regular notches over which water flows. They are used to regulate floods through rivers, overflow dams

and open channels. Weirs can be sharp or broad crested; made from concrete

timber, or metal and can be of cross-section rectangular, trapezoidal or triangular.

Sharp crested rectangular or triangular sections are commonly used on the farm.

Weirs Contd.Weirs Contd. The discharge through a weir is usually expressed as: Q = C L Hm

where Q is the discharge; C is the coefficient dependent on the nature of weir crest and

approach conditions; L is the length of crest; H is the head on the crest and m is an exponent depending on weir opening.

Weirs should be calibrated to determine these parameters before use eg. for trapezoidal weirs(Cipoletti weir),

Q = 0.0186 L H1.5 Q is discharge in L/s; L, H are in cm.

Methods of Measuring Irrigation Methods of Measuring Irrigation Water Contd.Water Contd.

c) Orifices: An orifice is an opening in the wall of a tank containing water.

The orifice can be circular, rectangular, triangular or any other shape.

The discharge through an orifice is given by: Q = C A 2 g h

Where Q is the discharge rate; C is the coefficient of discharge (0.6 - 0.8); A is the area of the orifice; g is the acceleration due to gravity and h is the head of water over an orifice.

Methods of Measuring Irrigation Methods of Measuring Irrigation Water Contd.Water Contd.

d) Flumes: Hydraulic flumes are artificial open channels or sections of natural channels.

Two major types of hydraulic flumes are Parshall or Trapezoidal ones.

Flumes need to be calibrated after construction before use. See Chapter 6 for further information.

  e) For streams, use gauging. A current meter is used to

measure velocity at 0.2 and 0.8 Depth or at only 0.6 depth. Measure areas of all sections using trapezoidal areas. Q = ai vi

Methods of Measuring Irrigation Methods of Measuring Irrigation Water Contd.Water Contd.

Using Floats: A floating object is put in water and observe the time it takes the float e.g. a cork to go from one marked area to another.

  Assuming the float travels D meters in t secs Velocity of water at surface = ( D/t ) m/s Average velocity of flow = 0.8 (D/t) Flow rate, Q = Cross sectional area of flow x velocity.

 

D

Object