Notes on Net irrigation in water resources engineering

Irrigation

1. Crop Water Requirement (CWR)-(ET
c)
2/12/2023 MISOF 2
CWRisdefinedasthedepthofwaterneededtomeetthewaterlossthroughevapotranspiration(ET
c)ofadisease-freecrop,
growinginlargefieldsundernon-restrictingsoilconditionsincludingsoilwaterandfertility,andachievingfullproduction
potentialunderthegivengrowingenvironment.
ET
c=ET
oxK
c
Where:
ET
c=Cropevapotranspiration(mm/day)
ET
o=Referencecropevapotranspiration(mm/day)
K
c=Cropcoefficient
ThevaluesofETcandCWRareidentical
Cropevapotranspirationisinfluencedbyseveralmajorfactors
namely:
•Planttemperature,
•Ambientairtemperature,
•Solarradiation(sunshineduration/intensity),
•Windspeed/movement,
•Relativehumidity/vaporpressure,
•Soil water availability
CropFactorsThatAffectET
•CropType
•VarietyofCrop
•DevelopmentStage
•CropHeight
•CropRoughness
•GroundCover
•CropRootingDepth

2. 2/12/2023 MISOF 3
EstimationofreferencecropET
O ET
O=K
panxE
pan
ET
C = K
cx ET
O = K
cx E
panx K
pan
but ET
o= E
panx K
pan
Therefore, ET
C = K
cx E
panx K
pan
Formostcrops,K
cincreasesfromalowvalue(0.5–0.9)duringtheinitialstagesofgrowth,toa
maximumvalue(0.9–1.2)duringtheperiodwhenthecropreachesfulldevelopment,anddeclines
again(0.3–0.9)asthecropmatures.
Thecropgrowingseasoncanbedividedintofourstages,asfollows:
a)Initialstage:germinationto10%groundcover
b)Cropdevelopmentstage:from10%to80%groundcover
c)Mid-seasonstage:80%groundcovertostartofripening
d)Latestage:fromstartofripeningtoharvest.
Calculationofwaterrequirementperday
Type of pan: Class A evaporation pan
Water depth in pan on day 1 = 150 mm
Water depth in pan on day 2 = 144 mm
Rainfall (during 24 hours) = 0 mm
K
pan= 0.75
CALCULATION
Formula: ET
o= K
pan ×E
pan
E
pan= 150 -144 = 6 mm/day
ET
o= 0.75 ×6 = 4.5 mm/day

3. 2/12/2023 MISOF 4

4. Crop Water and Irrigation Requirements
2/12/2023 MISOF 5
IrrigationRequirements(IR)referstothewaterthatmustbesuppliedthroughtheirrigationsystemtoensurethatthecrop
receivesitsfullcropwaterrequirements.Ifirrigationisthesolesourceofwatersupplyfortheplant,theirrigationrequirement
willalwaysbegreaterthanthecropwaterrequirementtoallowforinefficienciesintheirrigationsystem.Ifthecropreceives
someofitswaterfromothersources(rainfall,waterstoredintheground,undergroundseepage,etc.),thentheirrigation
requirementcanbeconsiderablylessthanthecropwater.
NetIrrigationWaterRequirement
Thenetirrigationwaterrequirementisdefinedasthewaterrequiredbyirrigationtosatisfycropevapotranspirationand
auxiliarywaterneedsthatarenotprovidedbywaterstoredinthesoilprofileorprecipitation.
The irrigation system must be able to supply net water requirements plus expected losses of deep percolation, runoff, wind drift,
and evaporation.
It must account for the efficiency of the irrigation decision maker to schedule the right amount of water at the right
time and the ability of an irrigation system to uniformly apply that water across a field.

5. 2/13/2023 MISOF 6
Pe=Effectivedependablerainfall(mm)
Gw=Groundwatercontributionfromwatertable(mm)
SM=Waterstoredinthesoilatthebeginningofeachperiod(mm)
LR=Leachingrequirement(mm)
Ageneralformulatocalculatenetirrigationrequirementsmaybegivenas:
NIR=ETc-(Pe+Gw+SM)+LR
Where:
NIR:Netirrigationrequirement(mm)
ETc:Cropevapotranspiration(mm)

6. 2/12/2023 MISOF 7
Partoftherainwaterpercolatesbelowtherootzoneoftheplantsandpartitflowsawayoverthesoilsurface
asrun-off.Thisdeeppercolationwaterandrun-offwatercannotbeusedbytheplants.Inotherwords,partof
therainfallisnoteffective.Theremainingpartisstoredintherootzoneandcanbeusedbytheplants.This
remainingpartistheso-calledeffectiverainfall.Iftherainfallishigh,arelativelylargepartofthewateris
lostthroughdeeppercolationandrun-off.
Inmanycountries,formulaehavebeendevelopedlocallytodeterminetheeffectiveprecipitation.Such
formulaetakeintoaccountfactorslikerainfallreliability,topography,prevailingsoiltypeetc
Effectiveprecipitation(ER)isimportantinirrigationschedulingdecisions,isusedtodesignnewirrigation
systems,andisaguidingfactorforplanningcropproductionpractices.
Numerousmethodsforestimatingeffectiverainfallhavebeenproposedinthepast,including:direct
measurementtechniques;empiricalmethods;andsoilwaterbalancemethods

7. EffectiveRainfall
•Inirrigation,effectiverainfallistheportionof
precipitationwhichremainsinthesoilandis
availableforconsumptiveuse.Iftherainfallis
sufficienttocoverthewaterneedsofthecrops,
irrigationisnotrequired.Ifthereisnorainfall,
allthewaterthatthecropsneedhastobe
suppliedbyirrigation.
•Ifthereissomerainfall,butnotenoughto
coverthewaterneedsofthecrops,irrigation
waterhastosupplementtherainwaterinsucha
waythattherainwaterandtheirrigationwater
togethercoverthewaterneedsofthecrop.This
isoftencalledsupplementalirrigation.
2/13/2023 MISOF 8

8. 2/13/2023 MISOF 9
▪Thetermeffectiverainfallhasadifferentmeaningtodifferentusers.
▪Forexampleforhydrologisteffectiverainfallmeansrunoff,whereasforirrigationengineersofagriculturist
effectiverainfallmeansusefulorutilizablerainfallforthepurposeofcropgrowth.
▪Dastane(1974)hasdefinedeffectiverainfallas“thatportionofthetotalannualorseasonalrainfallwhichis
usefuldirectlyand/orindirectlyformeetingthecropwaterneedsincropproductionatthesitewhereitfalls
butwithoutpumping”.
▪Thus,itistheportionofrainfallthatdoesnotincludelossesduetosurfacerunoff,unnecessarydeep
percolationandresidualmoistureafterharvest.
▪Thisconceptofeffectiverainfallissuggestedforuseinplanningandoperationofirrigationprojects.
▪Anumberoffactorsaffectseffectiverainfallinclude,rainfallcharacteristics,landtopography,soilandcrop
characteristics,managementpractices,carryovermoisturecontentandgroundwatercontribution.
▪Anumberofmethodsareinpracticefordeterminingeffectiverainfall.Theseinclude,fieldwaterbalance
approach,drumcultureapproachforrice,andempiricalrelationship(SCSmethod).
▪ThewaterrequirementofdifferentcropsaregiveninTable2.

9. 2/13/2023 MISOF 10
Table 2: Water Requirement of Different Crops

10. Typical water extraction pattern in uniform soil profile
2/13/2023 MISOF 11
Croprootingdepth:Croprootingdepthdetermineshowmuchsoilwatercanbeaccessedbythecrops.Ashallow-
rootedcrophaslessaccesstosoilwaterascomparedtoadeep-rootedcrop.
Figure 1. Soil water reservoir components

11. 2/13/2023 MISOF 12
Thecrop’srootingdepthdeterminesthedepthofthesoilprofilefromwhichthecropcanextractsoilwater.Water
thatmovesbeyondthisdepthisunavailabletothecrop.
Forareaswherethewatertableisshallow,watermovesupintotherootzonethroughcapillaryaction.Fordesign
andmanagementdecisions,onlythewaterwithintherootzoneisconsideredinthecalculations.About70%ofthe
wateruptakeiswithinthetop50%oftherootzone.

12. Irrigation Interval and Period
2/13/2023 MISOF 13
Inirrigationscheduling,theirrigationperiodshouldbelessthattheirrigationinterval.Thisisbecauseifthe
periodisnotsmaller,beforethelatterpartsoftheareaaretobeirrigated,theearlierirrigatedareaswillneedfresh
irrigation.Atpeakevapotranspiration(usedindesign),irrigationintervalshouldbeequaltoirrigationperiod
DesiredIrrigationDesignCapacity(Q
c)
Thisistheflowratedeterminedbythewaterrequirement,irrigationtime,irrigationperiodandtheirrigation
applicationefficiency.
•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.
Where:
Q
c
=theDesiredDesignCapacity;
d=theNetIrrigationDepth=ReadilyAvailableMoisture;
F=thenumberofDaystocompletetheIrrigation(IrrigationPeriod);
H=thenumberofHourstheSystemisoperated(hrs/day)and
E
a=theIrrigationEfficiency

13. 2/13/2023 MISOF 14
OR
Delta:Itisthedepthofwaterrequiredtoraiseacropoveraunitareaacrossthebase
period.Itisgenerallyexpressedincmorm.
Duty:Itisthenumberofhectaresoflandirrigatedforfullgrowthofagivencropbya
supplyof1cumec(1.0m
3
)ofwatercontinuouslyduringtheentirebaseperiodofthatcrop.

14. 2/13/2023 MISOF 15
Importance of Duty
Ithelpsusindesigninganefficientcanalirrigationsystem.Knowingthetotalavailablewaterattheheadofamaincanal,and
theoveralldutyforallthecropsrequiredtobeirrigatedindifferentseasonsoftheyear,theareawhichcanbeirrigatedcanbe
workedout.
Inversely,ifweknowthecropsarearequiredtobeirrigatedandtheirduties,wecanworkoutthedischargerequiredfor
designingthechannel.

15. 2/13/2023 MISOF 16
Example1
Determinationofmonthlywaterrequirementsofgroundnuthavingmonthlycropcoefficient
1.12andET
o=5.06mm/day.Considermonthwith30days.
Solution:
DailywaterrequirementET
c=ET
oxK
c=5.06x1.12=5.66mm
Monthlywaterrequirement=5.66mmx30=170.016mm
Example 2
If rice requires about 10 cm depth of water at an average interval of about 10 days, and the crop period for rice is 120 days, find
out the delta for rice.
Solution:
No. of watering required = 120days/10 = 12
Total depth of water required in 120 days = 10cm ×12 = 120 cm
Δ for rice = 120 cm

16. 2/13/2023 MISOF 17
ExampleOne
TheMoistureContentatFieldCapacityofaClayLoamSoilis28%byWeightwhilethatatPermanentWiltingPointis14%by
Weight.RootZoneDepthis1.0mandtheBulkDensityIs1.2g/cm
3
.CalculatetheNetandGrossDepthofIrrigation
RequiredIftheIrrigationEfficiencyis70%.Stateanyassumptionsused.
Solution:FieldCapacity(FC)=28%;Permanentwiltingpoint(PWP)=14%
Availablemoisture=FC–PWP=28%-14%=14%byweighti.e.P
m
•Bulkdensity(D
b)=1.2g/cm
3
•RootZonedepth(D)=1m=1000mm
•Equivalentdepthofavailablewater(d)=Pm.D
b.D
=0.14x1.20x1000mm
=168mm(Thisisthenetdepthofirrigation).[Assume50%MAD]
Readilyavailablewater=0.5x168mm=84mm
•GrossWaterApplication=NetIrrigation/Efficiency=84/0.7=120mm
FromExampleone,ifthePeakETis7.5mm/day,determinetheShortestIrrigationInterval.Solution:From
Example1,ReadilyAvailableMoisture(RAM)=84mm
i.e.Shortestirrigationinterval=RAM/PeakET=84/7.5=11.2days

17. Tutorial
1)If wheat requires about 7.5 cm of water after every 28 days, and the base period for wheat is 140
days, find out the value of delta for wheat.
2)Find the delta for a crop when its duty is 864 hectares/cumecon the field, the base period of this
crop is 120 days.
3) After how many days will you supply water to soil in order to ensure sufficient irrigation of the
given crop, if,
• Field capacity of the soil = 28%
• Permanent wilting point = 13%
• Dry density of soil = 1.3 gm/cc
• Effective depth of root zone = 70 cm
• Daily consumptive use of water for the given crop = 12 mm.
2/13/2023 MISOF 18