Irrigation Water Requirements Part 1

Irrigation Water RequirementsPenman Monteith Implementation1Irrigation Water RequirementsWhat it doesMonthly and seasonal irrigation water requirementsPeak ET valuesWhat it doesnt doIts not a schedulerIt doesnt calculate daily ET values2USDA-NRCS NEH Part 623 Chapter 2 Irrigation Water RequirementsPenman-MonteithRadiationTemperatureEvaporation Pan3Overview of IWRStrictly in conformance with USDA-NRCS NEH Part 623 Chapter 2 Irrigation Water RequirementsIWR can calculate evapo-transpiration using two methods:FAO 56 Penman MonteithSCS Blaney Criddle (TR-21)4Evapo-transpirationETEvaporationDoes not pass through the plantOccurs from wet soils and/or wet plantsAfter 70-80% canopy occurs, evaporation represents 10-25% of ETDoes not include spray losses.5Evapo-transpirationETTranspirationWater that enters the plant via the plant roots and leaves the plant through stomata.6A Quick Summary of ET MeasurementsPan EvaporationLysimetersVariety of ET Calculations Based on Weather DataPenman Monteith

7What is the equation?

=?Crop CoefficientPan Coefficient=?=8Evapo-transpirationET

9Evapo-transpirationET

10Various ET Equations

11Penman Monteith

12Estimating Crop ETReference crop methodETc=Kc ET0ETc = actual crop evapo-transpiration rateKc = crop coefficientET0 =evapo-transpiration rate for a grass reference crop13Estimating Crop ETThe reference crop is either grass or alfalfaGrass: 3 to 6 inches tall (NRCS standard)Alfalfa: 12 inches tallWell watered and healthy14Evapo-transpiration (ET)Knowing the ET rate of a crop allows us to:Design irrigation systems that will provide adequate water to the cropIrrigate at the appropriate times (scheduling)15Basal Crop Coefficient (Kcb)A coefficient used to relate the ET from a crop, that is not stressed for water and where the soil surfaces are dry, to that of a grass reference crop.16

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19Average Crop CoefficientKa (average crop coefficient)The average crop coefficient used to compute ET for a period of time where average conditions are used to account for the effect of water stress and evaporation from wet soil surfaces.20Evaporation from Wet Soil SurfacesIn order to determine this, you need the wetting interval (example: 7 days)Also needed: soil typeAlso needed: method of irrigation21Evaporation from Wet Soil SurfacesHow much water will evaporate from the soil surface?

The fraction of the soil surface that is wetted22Evaporation from Wet Soil SurfacesMethodFwRain1.0Above canopy sprinklers1.0Furrow IrrigationLarge application depth1.0Small application depth0.5Every other row irrigated0.5Trickle Irrigation0.2523Evaporation from Wet Soil Surfacestd = days required for soil surface to dryClay: 10 daysSilt Loam: 5 daysSand: 2 days

24Evaporation from Wet Soil Surfaces

Wet soil excel chart here

25Effect of water stressThe water use by stressed crops is very complex and requires extensive information to predict.26Average Crop CoefficientKa= Ks Kcb + Kw

27Wind and HumidityThe crop coefficients are then adjusted for wind and humidity:

Moderate Wind: (wind run < 250 miles/day)(less than 10.4 mph)Strong Wind: (wind run > 250 miles/day)Arid: < 20% Relative HumidityHumid: > 70 % Relative Humidity28Dja VuETc = Kc ET0ETc = actual crop evapo-transpiration rateKc = crop coefficientET0 =evapo-transpiration rate for a grass reference crop29

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Chart11111110.680.620.550.50.420.290.550.470.370.290.1800.450.350.230.13030.370.240.110440.290.1505550.230.0766660.16077770.11888880.059999901010101010

ClayClay LoamSilt LoamSandy LoamLoamy SandSandTime since wetting (days)Wet Soil Surface Decay Function

Sheet1Kcb0.25tKwtKwtKwFw100.7500.7500.75td1010.5110.4110.3220.4120.2820.1430.3430.1730.0040.2840.0850.2250.0060.1770.1280.0890.04100.00

Sheet1000000000000000000000

Clay

Use ThisTable 2-29Time since WettingClayClay LoamSilt LoamSandy LoamLoamy SandSand01.001.001.001.001.001.0010.680.620.550.500.420.2920.550.470.370.290.180.0030.450.350.230.130.0040.370.240.110.0050.290.150.0060.230.0770.160.0080.1190.05100.00

Use This000000000000000000000000000000000000000000000000000000000000000000

ClayClay LoamSilt LoamSandy LoamLoamy SandSandTime since wetting (days)Wet Soil Surface Decay Function

Sheet3

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