irrigation water requirements
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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 three methods:– Temperature– Radiation– 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 two methods:– FAO 56 Penman Monteith– SCS Blaney Criddle (TR-21)
Evapo-transpirationET
• 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-transpirationET
• Transpiration– 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
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?
dcbww t
tKFK 11
The fraction of the soil surface that is
wetted
Evaporation from Wet Soil Surfaces
Method FwRain 1.0Above canopy sprinklers
1.0Furrow Irrigation
Large application depth 1.0Small application depth 0.5Every 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 hereWet Soil Surface Decay Function
0.00
0.20
0.40
0.60
0.80
1.00
0 2 4 6 8 10
Time since wetting (days)
Clay
Clay Loam
Silt Loam
Sandy Loam
Loamy Sand
Sand
Effect of water stress
• The water use by stressed crops is very complex and requires extensive information to predict.
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