1. 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)

2. 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

3. 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”

4. 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.

5. What is EVAPOTRANSPIRATION?
Definition
Evaporation of water from the soil and plant surfaces and transpiration from the stomatal cavities of plants

6. 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

7. 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

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

9. 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

10. Frequency Distributions

11. ET for an Averaging Period

12. Intended Use
Irrigation Scheduling
System Design
Reservoir operation

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

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

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

17. 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

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

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

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

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

22. 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

23. 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

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

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

26. 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

28. 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

29. 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

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