CHAPTER TWO Water and Soil Requirements Water and Soil Requirements.Water Requirement of crops For the design of water harvesting systems, it is necessary to assess the: water requirement of crops and plants to be grown. Water requirement of crops may be defined as the quantity of water required by a crop or diversified pattern of crop in a given period of time for its normal growth or development under field conditions. It refers to the amount of water required to raise a successful crop in a given period. It includes consumptive use (C!, water loss due to evapo"transpiration andwater loss due to infiltration and deep percolation.1. Evapotranspiration!ET"#isdefinedasthelossofwateras evaporation from the crop field and crop surfaces. $. Consumptive use: refers to the loss of water due to #$ and the water that is used by the plant for metabolic activities. .%incetheamountofwaterthatisusedformetabolicactivityis insignificant (less than &'! the term C is equivalent to #$. C% & ET ' (eta)olic *ater + ET$hus, the determination of water requirement of crops refers to the determination of #$.Consumptiveuseofcropsmaybeestimatedforanyspecific period and it can be classified into three based on specific period. Water and Soil Requirements.Consumptive usea" ,ail-C%(istheamountofwaterusedduring)*hoursperiodby the crop. It is important to decide the type of crop or tree intended to be growing and to estimate pea+ period C rate of crops. )" Pea. period C% ( is the average daily water use rate of the crop or tree during a few days (usually , to &- days! of highest consumptive useinthegrowingseasonofthecroportree.Itisusuallyoccurs when the vegetation is abundant, temperature is high and crops are in thefloweringstage.Itisalsoimportanttodesignthewater harvesting technique and decide.select the type of crops or tree to be grown. c" SeasonalC%(istheamountofwaterusedbythecropsortrees duringtheentiregrowingseasonorcropperiod.Itisessentialto evaluateanddecideseasonalwaterdemandofthecropandwater supply of the area for W/ or irrigation pro0ect. Water and Soil Requirements.Consumptive use$hedeterminationofconsumptiveuseofcrops(water requirementsofcrops!isreferstodeterminationof evapotranspirationrateofcropsandtrees.$he determinationof#$rateofthecropisbasicallyusedto calculatethetotalwatercropwaterrequirementusing1c values of a given crop type.2ifferentplantspecieshavedifferentcropwater requirement.Cropwaterrequirementcanbeinfluencedby the following factors:Climate type and element Crop type (1c! and Crop growing stage (period! Water and Soil Requirements.PotentialEvaporation!PET":isthema3imumrateof evapotranspiration(#$!byactivelygrowingcropor vegetationwhichcompletelyshadeorcovertheground surfaceofthefieldwiththepresenceofabundantwater supply. 4#$ is the ma3imum rate of water loss as evaporation from the field with particular vegetation in abundant water supply or without shortage of available soil moisture. Water and Soil Requirements.ActualEvapotranspiration!AET":itindicatesthewater requirementofcropsunderthenormalfieldconditionofa place. It is the rate of evapo"transpiration by a particular crop in agivenperiodundertheprevailingsoilwaterandatmospheric condition. 5#$ rate of a crop or tree is #$ rate of a crop or tree underthee3istingsoilmoisturestatusandatmospheric conditionofthearea.Itisusuallylessthanthe4#$rateofa crop or tree. 5ctual #$crop varies under different conditions: a. %oil moisture condition ( high and low abundant soil moistureb. 5tmospheric condition ( related to wind, humidity and temperature elements c. Crop growth stage ( initial, development, mid"season(flowering! and late season staged. 6eographical location Water and Soil Requirements.Reference Crop EvapoTranspiration !ETo": is the rate of #$ fromalargeareacoveredwithactivelygrowinggreengrass which completely shade the ground under adequate soil moisture level also 4#$. $heconceptofreferencecropevapo"transpiration(#$o!is introducedtostudytheevaporateddemand(difference!ofthe particular crop with related to climate regardless of the soil type. $herateofwaterwhichevapotranspiredependsontheclimate condition. $hus,t/e/i0/estvalueofEToisfoundareaswhicharehot, dry, windy and sunny where as t/e lo*est ETo values are found in areas where it is cool, humid and cloudy with no wind. ETcrop & ETo 1 2cWater and Soil Requirements.T/ere are different met/ods to determine ETcrop3A. ,irect met/od#7ysimeter e3pert Field e3perimental plots%oil moisture depletion studies Water balance method 4.Pan evaporation met/od C.5ndirect met/ods 8lancy Criddle method$hornthwaite formula 9odified 4enman formula :adiation formula Water and Soil Requirements.A.,irect measurement met/ods1.6-simeter: is a device by which an e3perimental volume of soil planted with vegetation is located in a container to isolate it hydrological from the surrounding soil. .$hemethodinvolvesgrowingofcropsinalargecontainer(7ysimeter! whichinstalledincropfieldsandmeasurethewaterbalance(gainand loss! during the crop growing period. .$here are different components of water balance such as: .precipitation or effective rainfall, .irrigation water, .changes in soil water storage (available water range!, .runoff and deep percolation. .$omeasuretheevaporationfromasoilwhosesurfaceiswithinthe capillary border, tan+s equipped to maintain the water table at any desired elevation may be used. .$hesoilevaporationisdeterminedbyweighingthetan+satstated intervals and +nowing the amount of water that was added in the interim.Water and Soil Requirements.Where, #$c is evapotranspiration rate of the crop or consumptive use of water, I:w is irrigation water added to 7ysimeter, :f is rainfall or precipitation, 4e is deep percolation of water from 7ysimeter and %w is soil water storage (soil moisture status!.

Lysimeter for soil evaporationWater and Soil Requirements.$.7ield e1perimented plots: Field e3periments are carried with treatments having various level of irri0ation *ater to determine the seasonal consumptive use of water. Water is added to field plots and -ield o)tained from t/ese different plots are correlated with the total seasonal amount of water used. It is based on the principle that -ield increases as water used increases for some limit and then decreases with further increase in water. $he quantit- of *ater used to produce a most profita)le -ield is ta+en as the consumptive use of *ater for the given crops.Water and Soil Requirements.8.Soil moisture studies: is determination of C% of crops by themeasurementofsoilmoisturestatus)eforeandafter each irrigation application event. %oilwateriscontinuouslydepletedore3tractedfromthe soilinacropfield,andtheamountofe3tractedfromthe soilinagivenspecificperiodindicatethewatere3traction rate of the crop.Water and Soil Requirements.9.Water )alance met/od: is used to determine consumptive use of large areas is determination of C of water from inflo* and outflo*. Inflow water ; Irrigation water (Iw! < 6round water (6w! < :ainfall (4!=utflow water ; #vapotranspiration (#$! < %urface runoff (:o! < 2eep percolation %w!From this, Inflow water in to the field ; =utflow water from the filedIW < 6w < 4 ; #$ < := < 4# < >%w

;? #$c ; ( IW < 6w < 4! ( (:= < 4# < >%w!In most cases ground water contribution is usually minimum (less significant! then:;? #$c ; (Iw < 4! ( (:= < 4# < >%wWater and Soil Requirements.4.5ndirect measurement met/ods:Empirical formulas 1.4lane-Criddlemet/od:itstatesthatthemonthly consumptive use is given by@Where,Cuismonthlyconsumptiveuseincm,2iscropfactor, determinedbye3perimentsforeachcrop,undertheenvironmental conditions of the particular area, t is mean monthly temperature in -C and Pis monthly percent of annual day light hours that occur during the period.If represented by f, we get

Water and Soil Requirements.A B) C . & D*-+ tP= f k Cu$. Har0reavesClassApanevaporationmet/od:inthis method, evapotranspiration (consumptive use! is related to pan evaporationbyaconstant1,calledCcoefficient.$he formula can be written as:.$hepanevaporation(#p!canalsobedeterminedbyusingthe Christiansen formula, which states.Where,Rise3tra"terrestrialradiationinthesameunitsasEpincmor mm, Ct is coefficient for temperature, and given by:.Where,Tcisthemeantemperaturein -C,C*iscoefficientforwind velocity, and is given asWater and Soil Requirements.ppE K Cu ORE n evaporatio PanCu or ETo piration EvapotransK .! (! (= =Where,Wisthemeanwindvelocityat-.Emabovethegroundin 1m.day, C/ is coefficient for relative humidity, and is given by:Where, H is the mean percentage relative humidity at noon or average relative humidity for && and &C hours, and Cs is coefficient for percent of possible sunshine and is given by:Where,SisthemeansunshinepercentageandCeiscoefficientof elevation given as:Where, E is the elevation in &-- metersWater and Soil Requirements.8.Penman;sequation:incorporatingsomeofthemodifications suggested by other investigators, is given as:Where, #t is daily potential evapo"transpiration, A isslopeofthesaturationvapourpressurevs.temperaturecurveat the mean air temperature, and values are given in $able B.&, Hnisnetincomingsolarradiationorenergy,e3pressedinmmof evaporable water per day, Eaisaparameterincludingwindvelocityandsaturationdeficitin mm.day and is psychromatic constant (i.e. -.*F mm of /g.-C!. Water and Soil Requirements.$he net radiation (Hn! is also e3pressed as:Where,Hcismeanincidentsolarradiationatthetopoftheatmosphereona horiGontalsurface,e3pressedinmmofevaporablewaterperdayanditsvalues are given in $able B.B. $hisvalue is a function of latitude ( !of theplace and the period of the year asper the mean monthly values, risreflectioncoefficient(albedo!ofthegivenarea.sualvaluesofthis coefficient for different types of areas are given in $able B.), aisaconstantdependinguponthelatitude( !andisgivenas: a; -.)Fcos , ) is a constant having average value ; -.E), n is actual duration of bright sunshine in hours and