CHAPTER TWO: BASICS IN

IRRIGATION 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

Irrigation 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

Irrigation Systems Contd. ii) Crops to be grown: Should be determined by available

resources as well as marketability of the crops especially interms 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. ifunderground, pumping is needed

d) Conveyance System: can be by gravity e.g. open channelsor canals or by closed conduits e.g. pipes.

(e) Water measuring devices e.g. weirs, orifice, flumes, currentmeters

Other 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 thegiven crop has to be determined. This is bycalculating the evapotranspiration (to be treated later)

vi) Project Management: Consider how to managethe irrigation system

2.2 CROP WATER AND NET

IRRIGATION REQUIREMENTS

In irrigation, it is essential to know the amount of water neededby crops.

This determines the quantity of water to be added by irrigationand rainfall and helps in day to day management of irrigationsystems.

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 lossesdue to the inefficiency of application.

EVAPOTRANSPIRATION 2.3.1 DEFINITIONS

a) Evaporation: The process by which water is changed fromthe liquid or solid state into the gaseous state through thetransfer of heat energy.

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

c) Evapotranspiration, ET: It is the sum of the amount ofwater transpired by plants during the growth process and thatamount that is evaporated from soil and vegetation in thedomain occupied by the growing crop. ET is normallyexpressed in mm/day.

FACTORS THAT AFFECT

EVAPOTRANSPIRATION

Weather parameters, Crop Characteristics,Management and Environmental aspects arefactors affecting ET

(a) Weather Parameters:

The principal weather conditions affecting evapotranspiration are:

Radiation,

Air temperature,

Humidity and

Wind speed.

CROP FACTORS THAT

AFFECT ET Crop Type

Variety of Crop

Development Stage

Crop Height

Crop Roughness

Ground Cover

Crop Rooting Depth

Management and Environmental

Factors (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 soilevapotranspiration.

Other factors that affect ET are ground cover, plant density andsoil water content. The effect of soil water content on ET isconditioned primarily by the magnitude of the water deficit andthe type of soil. Too much water will result in waterloggingwhich might damage the root and limit root water uptake byinhibiting respiration.

EVAPOTRANSPIRATION

CONCEPTS (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

CONDITIONS (ETc) This refers to crop ET under standard conditions, i.e.

ET from disease-free, well-fertilized crops, grown inlarge 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- standardconditions as mentioned above is called ETc(adjusted). This refers to growth of crops under non-optimal conditions.

DETERMINATION OF

EVAPOTRANSPIRATION 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 METHOD

This 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.

Water 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 sheetof all the water entering and leaving a particularcatchment 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 surfacerunoff, DP is deep percolation, CR is capillary rise,SF and SW are change in sub-surface flow andchange in soil water content respectively

Lysimeters For Water Balance

Method 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 Lysimeter

ET Computed from

Meteorological 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

Evaporation Pans: Evaporation from an open water surface provides an

index of integrated effect of radiation, airtemperature, air humidity and wind onevapotranspiration. However, differences in thewater and cropped surface produce significantdifferences in the water loss from an open surfaceand the crop. The pan is used to estimate referenceETo by observing the evaporation loss from a watersurface (Epan) and applying empirical coefficients(Kpan)to relate pan evaporation to Eto thus:

ETo = Kp x Epan

Standard Pan: United States Class A

Pan

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 Pan

Incoming Radiation

q’ Absorbed By

Water

Evaporation

Air 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 Pans

A Comparison of Standard Open

Pans

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 REQUIREMENT

Most irrigation water contain dissolved salts.

Evaporation removes pure water leaving aconcentration of salt in soil.

Salt concentration may reach a level that isdetrimental to the growth of the crop and should becontrolled. The only practical way of achieving this isby leaching.

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

LEACHING REQUIREMENT

CONTD.

acceptableEC

RainETirrig

Ec

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 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

REQUIREMENT (Nir)

This is the moisture that must be supplied by irrigation to satisfyevapotranspiration plus that needed for leaching and notsupplied by off-season storage, and the effects of precipitationand 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.

2.7 GROSS IRRIGATION

REQUIREMENT (Gir)

Gross Irrigation Requirement is equal to:

Net Irrigation Requirement Divided by Irrigation Efficiency

Irrigation efficiency accounts for losses in storageand distribution systems, losses in applicationsystems as well as operation and managementlosses.

Irrigation Efficiency depends on the Method ofApplying Irrigation Water

2.8 IRRIGATION TERMS

2.8.1. 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 1 The Moisture Content at Field Capacity of a Clay Loam Soil is

28% by Weight While that at Permanent Wilting Point is 14% byWeight. Root Zone Depth Is 1 m and the Bulk Density Is 1.2g/cm3 . Calculate the Net and Gross Depth of IrrigationRequired 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.

Gross Water Application is equal to:

Net Irrigation/Efficiency = 84/0.7 = 120 mm

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

It is equivalent to:

120 /1000 mm x 10,000 m2 =1200 m 3 per hectare.

2.8.2 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 2

For the Last Example. the Peak ET is7.5 mm/day, Determine the ShortestIrrigation Interval.

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

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

Irrigation Period (IP)

This is the number of days allowed to

complete one irrigation cycle in a given

area.

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 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

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

(Qc) 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 3 A 12-hectare field is to be irrigated with a sprinkler

system. The root zone depth is 0.9 m and the fieldcapacity of the soil is 28% while the permanentwilting point is 17% by weight. The soil bulk densityis 1.36 g/cm and the water application efficiency is70%. The soil is to be irrigated when 50% of theavailable water has depleted. The peakevapotranspiration is 5.0 mm/day and the system isto 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 3

Solution: Field Capacity = 28%; PermanentWilting Point = 17%

ie. Available Moisture = 28 - 17 = 11% , which isPm

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 Moisturebefore Irrigation, Depth of Readily Available Moisture

= 0.5 x 135 mm = 67.5 mm

Solution of Example 3 Contd.

i) Net irrigation depth = Depth of the Readily AvailableMoisture = 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 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 Concluded

System capacity, Qc = 120,000 m2 x 0.0675 m13 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 irrigationmust have capacity equal to or greater than 25 L/s.

Alternatively, more than one pump can bepurchased.

2.9. IRRIGATION EFFICIENCIES

These irrigation efficiencies are brought aboutby the desire not to waste irrigation water, nomatter how cheap or abundant it is.

The objective of irrigation efficiency conceptis to determine whether improvements can bemade in both the irrigation system and themanagement of the operation programmes,which will lead to an efficient irrigation wateruse.

2.9.1 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 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 afterirrigation indicates that 30 cm of water has beenstored in the root zone. Compute the ApplicationEfficiency.

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 hax 10,000 m2/ha = 96,000 m3

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 Efficiency

EWater delivered to the Farm W

Water of water diverted from a stream reservoir or well Wc

d

s

( )

, ( )

Farm

Water lost by evap

And seepage Ws

Wd

Stream

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

system. 38 m3 of water is delivered to a turn out (athead ditch) which is 2 km from the well. Compute theConveyance Efficiency.

Solution:

EWater delivered to the Farm W

Water of water diverted from a stream reservoir or well Wc

d

s

( )

, ( )

= 38/45 = 84%

2.9.3. Christiansen Uniformity

Coefficient (Cu)

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 6

A Uniformity Check is taken by probing manystations down the border. The depths ofpenetration (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 Concluded

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

/ /X

CX

m nu

100 10( .

/ /)

Cu

100 106 2

59 13( .

.

.)

= 6.2

m = 5.9 cm; n = 13

= 92%

2.9.4 Water Storage Efficiency (Es)

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 divertedi

l 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

This means Predicting when to Irrigate andhow much to Irrigate

For efficient water use on the farm, the farmerneeds to be able to predict when his cropsneed 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 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

Sheet

This approach works on the principle that thechange in water content of the soil isrepresented by the difference between wateradded by irrigation(or rainfall) and the amountlost by evapotranspiration.

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

The method requires no equipment and iseasy to operate.

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

Table 2.3: Example of a Water Balance

Sheet

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 Moisture

This is the best scheduling and the most widely used.Soil moisture can be indirectly measured usingdevices and instruments eg. tensiometers, resistanceblocks or neutron probes.

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

These methods are either too expensive orcomplicated.

The simplest and most practical method is toestimate the moisture content by the 'feel andappearance' of the soil.

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

2.11 IRRIGATION WATER: SOURCES, QUALITY &

MEASUREMENT

2.11.1 Sources of Irrigation Water Supply

i) Rainfall or Precipitation: This is apractical and dominant factor.

The supply varies with time and place e.g.while Grenada receives 2,100 mm annualrainfall, Antigua receives only 1,100 mm.Trinidad receives 1, 950 mm (Data suppliedby Gumbs, 1987).

To be of greatest benefit for crop production,the rainfall amount should be enough toreplace water in the root zone on a regularbasis.

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

Ant

igua

& B

arbud

a

The Bah

amas

Bar

bados

Bel

ize

Dom

inica

Gre

nada

Guy

ana

Haiti

Jam

aica

St.

Kitts

& N

evis

St.

Lucia

St.

Vince

nt &

the G

rena

dine

s

Sur

inam

e

Trinidad

& T

obago

Mean

An

nu

al

Rain

fall

(m

m)

Sources of Irrigation Water

Contd. ii) Underground water sources: This can be

shallow or bore holes.

iii) Surface Sources: Streams, rivers, lakes, farmponds etc.

Streams should be gauged to ensure that there isenough water for irrigation.

Rivers or streams can also be dammed to raise theheight of flow and make more water available forirrigation.

Farm ponds can also be dug to store water fromrivers or channels (e.g. field station) or to collectwater from rainfall

Sources of Irrigation Water

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

Quality:

Irrigation water quality depends on

i) Amount of suspended sediment eg.

silt content

ii) The chemical constituents of water

i) Amount of Suspended

Sediment: The effect of sediment may depend upon the

nature of the sediment and the characteristicsand soil conditions of the irrigated area.

Silt content in irrigation may be beneficial if itimproves the texture and fertility of say sandysoil.

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

Silt accumulation can cause aggradation incanals or distribution systems. In sprinklersystems, silt can cause abrasion.

ii) The Chemical Constituents

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:

The units of salt concentration in irrigation water canbe parts per million (p.p.m), milliequivalents/litre(ME/litre) or electrical conductivity.

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

They refer to low, medium, high and very high salinitylevels respectively.

While C1 water can easily be used for irrigationwithout need for leaching requirement,

C4 water is not useable, except in permeable soilswhere adequate leaching and drainage is possibleand for highly tolerant crops.