Irrigation Water Requirements

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Irrigation Water Requirements

• Penman Monteith Implementation

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Irrigation Water Requirements

• What it does
• Monthly and seasonal irrigation water
  requirements
• Peak ET values
• What it doesnt do
• Its not a scheduler
• It doesnt calculate daily ET values

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USDA-NRCS NEH Part 623 Chapter 2

• Irrigation Water Requirements
• Penman-Monteith
• Radiation
• Temperature
• Evaporation Pan

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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)

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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.

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Evapo-transpirationET

• Transpiration
• Water that enters the plant via the plant roots
  and leaves the plant through stomata.

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A Quick Summary of ET Measurements

• Pan Evaporation
• Lysimeters
• Variety of ET Calculations Based on Weather Data
• Penman Monteith

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What is the equation?
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Evapo-transpirationET
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Evapo-transpirationET
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Various ET Equations
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Penman Monteith
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Estimating Crop ET

• Reference crop method
• ETcKc ET0
• ETc actual crop evapo-transpiration rate
• Kc crop coefficient
• ET0 evapo-transpiration rate for a grass
  reference crop

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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

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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)

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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.

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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.

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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

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Evaporation from Wet Soil Surfaces

• How much water will evaporate from the soil
  surface?

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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
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Evaporation from Wet Soil Surfaces
td days required for soil surface to dry Clay
10 days Silt Loam 5 days Sand 2 days
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Evaporation from Wet Soil Surfaces
Wet soil excel chart here
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Effect of water stress

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

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Average Crop Coefficient
Ka Ks Kcb Kw
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Wind and Humidity

• The crop coefficients are then adjusted for wind
  and humidity

Moderate Wind (wind run lt 250 miles/day) (less
than 10.4 mph) Strong Wind (wind run gt 250
miles/day) Arid lt 20 Relative Humidity Humid gt
70 Relative Humidity
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Déja Vu

• ETc Kc ET0
• ETc actual crop evapo-transpiration rate
• Kc crop coefficient
• ET0 evapo-transpiration rate for a grass
  reference crop