Crops to be Irrigated Factors for consideration Rooting 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 irrigations Crop Height wheel or riser height above canopy or within canopy application Water tolerance water 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 zone a soil-water deficit (SWD) in inches, or an allowable soil-water tension level More information Irrigation Guide pg.. 3-7

Example Given: Silt Loam soil (AWC= 2.1”/ft), growing potatoes with rooting depth of 2.5’and a MAD of 35%. Find : The maximum amount of water depletion before irrigation is necessary? 2.1*2.5*.35 = 1.84”

Evapotranspiration Overview Recently 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? Definition Evaporation of water from the soil and plant surfaces and transpiration from the stomatal cavities of plants

What are some methods for determining ET? Estimated crop evapotranspiration ETc Blaney-Criddle, etc. Direct measurement aerodynamic method detailed soil moisture monitoring lysimetry plant porometers regional inflow-outflow measurements

What influences the method you would select? Type, accuracy, and duration of available climatic data Natural pattern of evapotranspiration Intended use of the evapotranspiration estimates

Climatic Data Type Quality Length of Record Temperature, radiation, wind, humidity Quality Length of Record

Natural Pattern of Crop Water Use Crop ET varies from day to day Fluctuating climatic Plant growth Daily average Vs. average for a period 1 day Vs 5 day

Frequency Distributions

ET for an Averaging Period

Intended Use Irrigation Scheduling System Design Reservoir operation

Various Methods NRCS endorses four methods Penman-Monteith Radiation method Temperature Method Class A evaporation pan

Penman-Montieth Method More reliable for any length period daily, monthly, or seasonal If adequate data available

Radiation Method Not for daily ET, but for average daily ET over a period of days ~ 5day period Good for monthly and Seasonal

Temperature Method ET0 = evapotranspiration for grass reference crop Ce = elevation adjustment factor at = climate adjustment factor bt = climate adjustment factor p = mean daily percent of annual daytime hours T = mean air temperature Not for daily ET, but for average daily ET over a period of days ~ 5day period Good for monthly and Seasonal

Evaporation Pan Method ET0=kpEpan ET0 = evapotranspiration for grass reference crop kp = pan coefficient Epan = evaporation from pan Good for monthly and Seasonal

Things to Consider before changing ET Values Legal ramifications Quantity of water supply System efficiency

SCS TR21 Where does TR21 fit in? Water Rights of many States Based on TR21 Less Accurate

What is the difference between ET0 and Consumptive Use? CU = Crop coefficient*ET0 Use reference ET for specific Crop

Crop Curves NRCS has switched from an Alfalfa based crop reference to a Grass crop reference To convert use a multiplier factor , usually 1.15 New ones and procedures found in NEH part 623 chapter 2

Consumptive Use Calculations Field by field CU = crop coefficient * ET Farm CU - multiple fields, multiple crops weighted CU based on percentage of crops Project CU gpm/acre weighted by percent

Weighted Consumptive Use CUw= 80/230*.31+100/230*.21+50/230*.25

Net Irrigation Requirements Fn = 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 season

Effective precipitation The 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 Sizing Simply put Q = Fg*A/t Q = to system flowrate Fg = Gross irrigation requirement A = irrigated area t = time to irrigate the field

What is the difference between Net and Gross? Cg = Cn Ea(1-Dt ) 100 Cg = gross system capacity Cn = net system capacity Ea = application efficiency Dt = system downtime Many efficiencies come in to play Field efficiency Farm efficiency Conveyance efficiency Project efficiency etc. Things influencing Field efficiency Deep percolation Surface runoff Spray, drift losses

Where to get more information NRCS NEH 623 Chapter 2 “Irrigation Water Requirements”