1. Irrigation Water Requirements
Penman Monteith Implementation

2. 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

3. USDA-NRCS NEH Part 623 Chapter 2
Irrigation Water Requirements
Penman-Monteith
Radiation
Temperature
Evaporation Pan

4. Overview of IWR
Strictly in conformance with USDA-NRCS NEH Part 623 Chapter 2 Irrigation Water Requirements
IWR can calculate evapo-transpiration using two methods:
FAO 56 Penman Monteith
SCS Blaney Criddle (TR-21)
Strictly in conformance with USDA-NRCS NEH Part 623 Chapter 2 Irrigation Water Requirements
IWR can calculate evapo-transpiration using three methods:
Temperature
Radiation
SCS Blaney Criddle (TR-21)

5. Evapo-transpiration ET
Evaporation
Does not pass through the plant
Occurs from wet soils and/or wet plants
After 70-80% canopy occurs, evaporation represents 10-25% of ET
Does not include spray losses.

6. Evapo-transpiration ET
Water that enters the plant via the plant roots and leaves the plant through stomata.

7. A Quick Summary of ET Measurements
Pan Evaporation
Lysimeters
Variety of ET Calculations Based on Weather Data
Penman Monteith

8. What is the equation? = ?
Pan Coefficient = = ?
Crop Coefficient

9. Evapo-transpiration ET

10. Evapo-transpiration ET

11. Various ET Equations

12. Penman Monteith

13. Estimating Crop ET Reference crop method ETc=Kc ´ ET0
ETc = actual crop evapo-transpiration rate
Kc = crop coefficient
ET0 =evapo-transpiration rate for a grass reference crop

14. Estimating Crop ET The reference crop is either grass or alfalfa
Grass: 3 to 6 inches tall (NRCS standard)
Alfalfa: 12 inches tall
Well watered and healthy

15. Evapo-transpiration (ET)
Knowing the ET rate of a crop allows us to:
Design irrigation systems that will provide adequate water to the crop
Irrigate at the appropriate times (scheduling)

16. 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.

20. Average Crop Coefficient
Ka (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.

21. Evaporation from Wet Soil Surfaces
In order to determine this, you need the wetting interval (example: 7 days)
Also needed: soil type
Also needed: method of irrigation

22. Evaporation from Wet Soil Surfaces
How much water will evaporate from the soil surface?
The fraction of the soil surface that is wetted

23. Evaporation from Wet Soil Surfaces
Method Fw
Rain
Above canopy sprinklers 1.0
Furrow Irrigation
Large application depth 1.0 Small application depth 0.5 Every other row irrigated 0.5
Trickle Irrigation

24. Evaporation from Wet Soil Surfaces
td = days required for soil surface to dry
Clay: 10 days
Silt Loam: 5 days
Sand: 2 days

25. Evaporation from Wet Soil Surfaces
Wet soil excel chart here

26. Effect of water stress
The water use by stressed crops is very complex and requires extensive information to predict.

27. Average Crop Coefficient
Ka= Ks Kcb + Kw

28. Wind and Humidity
The 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 Humidity
Humid: > 70 % Relative Humidity

29. Déja Vu ETc = Kc ´ ET0 ETc = actual crop evapo-transpiration rate
Kc = crop coefficient
ET0 =evapo-transpiration rate for a grass reference crop