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# What Is Potential Evapotranspiration?

Plant leaves lose water though small openings called stomata that are found on the leaf surface.
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• Written By: Liz Thomas
• Edited By: Shereen Skola
2003-2015
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Potential evapotranspiration (PET) is the amount of evapotranspiration, or evaporation, that may occur if plenty of water is present in soil, a water source, or plants. The actual amount of evapotraspiration is limited by the water source and is used for water management in agriculture, building, and drainage studies. It is possible to estimate this amount using equations.

Evapotranspiration refers to the evaporation and transpiration that occurs. The potential refers to the total amount of evaporation that could possibly occur if there is plenty of water available. In many cases, water is not available and irrigation becomes necessary. Calculating the potential evapotranspiration ensures that the necessary amount of water is used in irrigation methods.

Estimates of PET can be calculated in three ways. The first equation, the Thornthwaite Equation, was developed in 1948. The Penman Equation was also developed in 1948. The most commonly used equation is the Penman-Monteith Equation developed in 1965, years later than the first two.

Modeling of PET is typically done using the Penman-Monteith equation. This is the standard method used by the Food & Agriculture Organization of the United Nations (FAO). The Soil and Water Assessment Tool (SWAT) and the Association of Civil Engineers (ASCE) use this equation as a standard method, though with one or two alterations.

In order to properly estimate potential evapotranspiration, certain factors are required. Solar radiation, wind speed, daily mean temperature and relative humidity are needed to complete the calculation. These factors can change daily, which means the potential evapotranspiration changes every time a factor shifts.

This calculation is not exact, as the type of crop will alter the necessary water. Each plant contains stomata that acts differently. The stomata are the pores on the leaf surface that release water vapor and oxygen into the atmosphere. Stomata resistance, or the ability of water vapor to pass through the leaf, is an unknown factor of PET estimation.