Evapotranspiration

         Evapotranspiration

Evapotranspiration (ET) refers to  the combined loss of water due to evaporation from soil surfaces (E) and transpiration from plants (T). It is not possible to directly measure ET, but we can  calculate it  from weather data.  This calculated value is used to determine  the  irrigation required to replace the water used by a crop.

More than 20 equations have been developed to calculate the potential  ET from weather data. The most detailed approach  is the Penman-Monteith equation.  This is based on biophysical principles and requires hourly averages of solar radiation, temperature, humidity, and wind. Not all weather stations, however, record the measurements to calculate ET from the PM equation.

The potential, or reference ET is not the same as the actual ET. Rather, it is the largest value of ET that would occur if soil water is not limiting.

Many studies have been conducted to find relationships between actual and potential  ET for specific crops. The actual ET is determined by  multiplying the reference ET  by a value called the crop coefficient, which varies from zero to one. There is  considerably more error associated with the estimate of the crop coefficient than with the estimation of ET.

The potential  ET from this station is calculated using the Penman-Monteith equation. The "short" reference crop is  2.5 cm tall grass, and the "tall" reference crop is 30 cm tall alfalfa.

Additional, detailed information on evapotranspiration and methods used to calculate it are available at:

http://www.fao.org/docrep/X0490E/x0490e04.htm