Evapotranspiration (ET) is the sum of evaporation and plant transpiration. Potential evapotranspiration (PET) is the amount of water that could be evaporated and transpired if there was plenty of water available. Actual evapotranspiration is usually no greater than precipitation, with some buffer in time depending on the soils ability to hold water. It will usually be less because some water will be lost due to percolation or surface run off. An exception is areas with high water tables, where capillary rise can cause water from the groundwater to rise through the soil matrix to the surface. If potential evapotranspiration is greater than actual precipitation, then soil will dry out, unless irrigation is used.

Potential evapotranspiration is usually measured indirectly, from other climatic factors, but also depends on the surface type, such free water (for lakes and oceans), the soil type for bare soil, and the vegetation. Often a value for the potential evapotranspiration is calculated at a nearby climate station on a reference surface, conventionally short grass. This value is called the reference evapotranspiration, and can be converted to a potential evapotranspiration by multiplying with a surface coefficient. In agriculture, this is called a crop coefficient. The difference between potential evapotranspiration and precipitation is 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 general and widely used equation for calculating ET is the Penman equation. The Penman-Monteith variation is the FAO recommendation. 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. Other solutions used includes Makkink, which is simple but must be calibrated to a specific locale, and Hargreaves.

External links

• California Irrigation Management Information System (CIMIS)
• Texas Evapotranspiration Network
• Use and Construction of a Lysimeter to Measure Evapotranspiration