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Irrigation Water Requirements. Penman Monteith Implementation. Irrigation Water Requirements. What it does Monthly and seasonal irrigation water requirements Peak ET values What it doesn’t do It’s not a scheduler It doesn’t calculate daily ET values. USDA-NRCS NEH Part 623 Chapter 2. - PowerPoint PPT Presentation

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  • Irrigation Water RequirementsPenman Monteith Implementation

  • Irrigation Water RequirementsWhat it doesMonthly and seasonal irrigation water requirementsPeak ET valuesWhat it doesnt doIts not a schedulerIt doesnt calculate daily ET values

  • USDA-NRCS NEH Part 623 Chapter 2 Irrigation Water RequirementsPenman-MonteithRadiationTemperatureEvaporation Pan

  • Overview of IWRStrictly in conformance with USDA-NRCS NEH Part 623 Chapter 2 Irrigation Water RequirementsIWR can calculate evapo-transpiration using three methods:TemperatureRadiationSCS Blaney Criddle (TR-21)Strictly 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)

  • Evapo-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.

  • Evapo-transpirationETTranspirationWater that enters the plant via the plant roots and leaves the plant through stomata.

  • A Quick Summary of ET MeasurementsPan EvaporationLysimetersVariety of ET Calculations Based on Weather DataPenman Monteith

  • What is the equation?

  • Evapo-transpirationET

  • Evapo-transpirationET

  • Various ET Equations

  • Penman Monteith

  • Estimating Crop ETReference crop methodETc=Kc ET0ETc = actual crop evapo-transpiration rateKc = crop coefficientET0 =evapo-transpiration rate for a grass reference crop

  • Estimating Crop ETThe reference crop is either grass or alfalfaGrass: 3 to 6 inches tall (NRCS standard)Alfalfa: 12 inches tallWell watered and healthy

  • Evapo-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)

  • Basal 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.

  • Average 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.

  • Evaporation from Wet Soil SurfacesIn order to determine this, you need the wetting interval (example: 7 days)Also needed: soil typeAlso needed: method of irrigation

  • Evaporation from Wet Soil SurfacesHow much water will evaporate from the soil surface?

    _1069599803.unknown

  • Evaporation from Wet Soil SurfacesMethodFwRain1.0Above canopy sprinklers1.0Furrow IrrigationLarge application depth1.0Small application depth0.5Every other row irrigated0.5Trickle Irrigation0.25

  • Evaporation from Wet Soil Surfacestd = days required for soil surface to dryClay: 10 daysSilt Loam: 5 daysSand: 2 days

  • Evaporation from Wet Soil SurfacesWet soil excel chart here

    Chart1

    111111

    0.680.620.550.50.420.29

    0.550.470.370.290.180

    0.450.350.230.1303

    0.370.240.11044

    0.290.150555

    0.230.076666

    0.1607777

    0.1188888

    0.0599999

    01010101010

    Clay

    Clay Loam

    Silt Loam

    Sandy Loam

    Loamy Sand

    Sand

    Time since wetting (days)

    Wet Soil Surface Decay Function

    Sheet1

    Kcb0.25tKwtKwtKw

    Fw100.7500.7500.75

    td1010.5110.4110.32

    20.4120.2820.14

    30.3430.1730.00

    40.2840.08

    50.2250.00

    60.17

    70.12

    80.08

    90.04

    100.00

    Sheet1

    000

    000

    000

    000

    00

    00

    0

    0

    0

    0

    0

    Clay

    Use This

    Table 2-29

    Time since WettingClayClay LoamSilt LoamSandy LoamLoamy SandSand

    01.001.001.001.001.001.00

    10.680.620.550.500.420.29

    20.550.470.370.290.180.00

    30.450.350.230.130.00

    40.370.240.110.00

    50.290.150.00

    60.230.07

    70.160.00

    80.11

    90.05

    100.00

    Use This

    000000

    000000

    000000

    000000

    000000

    000000

    000000

    000000

    000000

    000000

    000000

    Clay

    Clay Loam

    Silt Loam

    Sandy Loam

    Loamy Sand

    Sand

    Time since wetting (days)

    Wet Soil Surface Decay Function

    Sheet3

    MBD004A6BBF.doc

    _1069599803.unknown

  • Effect of water stressThe water use by stressed crops is very complex and requires extensive information to predict.

  • Average Crop CoefficientKa= Ks Kcb + Kw

  • Wind 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 Humidity

  • Dja VuETc = Kc ET0ETc = actual crop evapo-transpiration rateKc = crop coefficientET0 =evapo-transpiration rate for a grass reference crop

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