EVAPOTRANSPIRATION
humidity. The measurements from the weather instruments are used in various equations that have been developed over the years to estimate the rate of water use by a reference crop.
There are two reference crops used — alfalfa that is about 18 inches tall or a cool season grass that is 4-6 inches tall. Grass reference is typically 20%-30% lower than alfalfa reference, depending on location.
Most weather station networks that calculate ET will provide a daily reference value. The values for these reference crops are then modified by the grower or irrigation manager with crop coefficients that have been developed over the years to more closely estimate the amount of water being used
by a crop. By summing the values for each day since the last irrigation, modified for the specific crop, and then subtracting effective rainfall, you get a representation of the amount of water that should be applied to refill the soil reservoir by irrigation.
In addition to these methods for estimating ET, many different ET equations have been developed over the years. The Standardized Penman-Monteith Evapotranspiration Equation is frequently used.
Brent Mecham, CID, CLWM, CIC, CLIA, CAIS, is the industry development director for the Irrigation Association.
The most
common method for estimating ET is with weather stations that are measuring sunlight (solar radiation), wind, temperature and humidity.
Measurements provided by weather stations are used to calculate ET.
Photo credit: Stephen W. Smith, PhD, FASIC, CAIS, CLIA
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irrigationtoday.org Winter 2020 | Irrigation TODAY 21
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