Irrigation Water Requirements

Crops to be IrrigatedFactors for considerationRooting depth of crop ( Irr. Guide pp.. 3-8,3-9) Are there any soil barriers rock, hard pan, etc.establishment and shallow root depths require more frequent irrigationsCrop Heightwheel or riser heightabove canopy or within canopy applicationWater tolerancewater sensitive crops develop diseases (beans)What is Management Allowable Depletion (MAD)?Mad is defined as the percentage of the available soil water that can be depleted between irrigations without serious plant moisture stress. MAD is expressed as:a percentage of the total Available Water Content (AWC) the soil will hold in the root zonea soil-water deficit (SWD) in inches, oran allowable soil-water tension levelMore information Irrigation Guide pg.. 3-7ExampleGiven: Silt Loam soil (AWC= 2.1/ft), growing potatoes with rooting depth of 2.5and a MAD of 35%. Find : The maximum amount of water depletion before irrigation is necessary?2.1*2.5*.35 = 1.84Evapotranspiration OverviewRecently with more weather station and a greater demand for the available water there has been a great push to update ET values. The new ET values are needed to provide a more accurate picture of what is actually happening in the field.What is EVAPOTRANSPIRATION?DefinitionEvaporation of water from the soil and plant surfaces and transpiration from the stomatal cavities of plantsWhat are some methods for determining ET?Estimated crop evapotranspiration ETcBlaney-Criddle, etc.Direct measurementaerodynamic methoddetailed soil moisture monitoringlysimetryplant porometersregional inflow-outflow measurementsWhat influences the method you would select?Type, accuracy, and duration of available climatic dataNatural pattern of evapotranspirationIntended use of the evapotranspiration estimatesClimatic DataTypeTemperature, radiation, wind, humidityQualityLength of RecordNatural Pattern of Crop Water UseCrop ET varies from day to day Fluctuating climaticPlant growthDaily average Vs. average for a period1 day Vs 5 dayFrequency Distributions

ET for an Averaging Period

Intended UseIrrigation SchedulingSystem DesignReservoir operationVarious MethodsNRCS endorses four methodsPenman-MonteithRadiation methodTemperature MethodClass A evaporation panPenman-Montieth Method

More reliable for any length period daily, monthly, or seasonalIf adequate data available

Radiation Method

Not for daily ET, but for average daily ET over a period of days ~ 5day periodGood for monthly and SeasonalTemperature Method

Not for daily ET, but for average daily ET over a period of days ~ 5day periodGood for monthly and SeasonalET0 = evapotranspiration for grass reference cropCe = elevation adjustment factorat = climate adjustment factorbt = climate adjustment factorp = mean daily percent of annual daytime hoursT = mean air temperatureEvaporation Pan MethodET0=kpEpanET0 = evapotranspiration for grass reference cropkp = pan coefficientEpan = evaporation from panGood for monthly and SeasonalThings to Consider before changing ET ValuesLegal ramificationsQuantity of water supplySystem efficiencySCS TR21Where does TR21 fit in?Water Rights of many States Based on TR21Less AccurateWhat is the difference between ET0 and Consumptive Use?CU = Crop coefficient*ET0Use reference ET for specific CropCrop CurvesNRCS has switched from an Alfalfa based crop reference to a Grass crop referenceTo convert use a multiplier factor , usually 1.15New ones and procedures found in NEH part 623 chapter 2 Consumptive Use CalculationsField by fieldCU = crop coefficient * ETFarm CU - multiple fields, multiple cropsweighted CU based on percentage of cropsProject CUgpm/acre weighted by percentWeighted Consumptive Use

CUw= 80/230*.31+100/230*.21+50/230*.25Net Irrigation RequirementsFn = ETc + Aw - Pe - GW - DSW

Fn = net irrigation requirement for season ETc = crop evapotranspiration Aw = auxiliary water - leaching, temperature modification, crop quality Pe = effective precipitation GW = ground water contribution DSW= soil water depleted during seasonEffective precipitationThe part of rainfall that can be used to meet the evapotranspiration of growing crops.Does not include surface runoff or percolation below the crop root zone

System SizingSimply put Q = Fg*A/t

Q = to system flowrateFg = Gross irrigation requirementA = irrigated areat = time to irrigate the fieldWhat is the difference between Net and Gross?Cg = Cn Ea(1-Dt ) 100Cg = gross system capacityCn = net system capacityEa = application efficiencyDt = system downtimeMany efficiencies come in to play Field efficiencyFarm efficiencyConveyance efficiencyProject efficiency etc.Things influencing Field efficiencyDeep percolationSurface runoffSpray, drift lossesWhere to get more informationNRCS NEH 623 Chapter 2 Irrigation Water RequirementsET0 = The evapotranspiration for grass reference crop

( = heat of vaporization

Rn = net radiation

G = soil heat flux

( = slope of the vapor pressure curve

( = psychrometric constant

( = density of air

BP = mean barometric pressure

e0z = average saturated vapor pressure

ez = actual vapor pressure

(* = ((1+rc/ra)

rc = surface resistance to vapor transport

ra = aerodynamic resistance to sensible heat and vapor transport

K1 = the dimension coefficient

ET0 = The evapotranspiration for grass reference crop

( = heat of vaporization

( = slope of the vapor pressure curve

( = psychrometric constant

( = density of air

br = adjustment factor for wind and humidity

Rs = incoming solar radiation

CROPACRESCU%alfalfa80.3135wheat100.2143beets50.2522Total230.25100