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 Irrigation Water Requirements Penman-Monteith Radiation Temperature Evaporation Pan

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)

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.

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

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

What is the equation? = ? Pan Coefficient ´ = = ? Crop Coefficient ´

Evapo-transpiration ET

Evapo-transpiration ET

Various ET Equations

Penman Monteith

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

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

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)

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 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.

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

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

Evaporation from Wet Soil Surfaces Method Fw Rain 1.0 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 0.25

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

Evaporation from Wet Soil Surfaces Wet soil excel chart here

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

Average Crop Coefficient Ka= Ks Kcb + Kw

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

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