ETP

ETp, short for potential evapotranspiration, is a hydrological term used to denote the atmospheric demand for water from soil and vegetation under conditions of unlimited soil moisture. It represents the maximum amount of water that could be removed from a given area by evaporation from soil and by transpiration from plants, assuming there is no moisture limitation.

In many languages ETp is used interchangeably with other terms such as PET (potential evapotranspiration) or ETo (reference evapotranspiration) in English-language literature. ETp serves as a baseline or upper limit in water balance analyses, crop planning, and drought assessment, helping to distinguish atmospheric demand from actual water loss that occurs under moisture stress.

Calculation of ETp is most commonly performed with the FAO Penman-Monteith equation, which integrates net radiation, air temperature, humidity, and wind speed to estimate evapotranspiration. Data requirements typically include daily or hourly mean air temperature, relative humidity, wind speed at a standard height, and solar radiation or a proxy such as sunshine duration. When data are sparse, alternative methods like Hargreaves-Samani or Thornthwaite may be used, though they generally require fewer inputs and may be less accurate in some climates.

Applications of ETp are widespread in agriculture and water resources. It is used to determine crop water requirements by multiplying ETp by crop coefficients, informing irrigation scheduling and water budgeting. In hydrology, ETp contributes to rainfall-runoff models, reservoir planning, and climate-change impact studies. It is distinct from actual evapotranspiration (ETa), which reflects water actually lost given soil moisture and plant water status.