Evapotranspiration
Evapotranspiration (ET) is the sum of evaporation and plant transpiration. Potential evapotranspiration (PET) is the amount of water that could be evaporated and transpired from a reference crop, usually a cool season grass, if there was plenty of water available. Actual evapotranspiration is usually no greater than precipitation, except in areas with high water tables, and will usually be less because some water will run off. If potential evapotranspiration is greater than actual precipitation, then soils are extremely dry during at least a major part of the year, unless irrigation is used. Potential ET is multiplied by a crop coefficient for the particular plant in question to estimate actual ET. Estimates of evapotranspiration are used in irrigation scheduling.The potential evapotranspiration can be graphed during the year. It follows an approximate bell curve around the seasons. Factors that affect evapotranspiration include the plant's growth stage or level of maturity, percentage of soil cover, solar radiation, humidity, temperature, and wind.
The most widely used equation for calculating ET is the Penman equation, but it is also the most complicated. The simpler Blaney-Criddle equation was popular in the Western United States for many years but it is not as accurate in regions with higher humidities.