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What Is Potential Evapotranspiration?

By Liz Thomas
Updated Mar 05, 2024
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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.

Much of the current research investigates the affects of plants on potential evapotranspiration. It is possible that different plant factors will need to be added to the Penman-Monteith equation to achieve a more realistic estimate. This variation is why PET is rarely used in agricultural reports. In addition to irrigation estimations, potential evapotranspiration is also used to determine the type of drainage that is needed in fields. When constructing roads, buildings, and structures, PET is commonly used to determine how those structures will affect the environment. Any industry or action that requires water management may use PET.

Frequently Asked Questions

What is potential evapotranspiration and why is it important?

Potential evapotranspiration (PET) is the amount of water that could evaporate and transpire from a vegetated surface if there were sufficient water available. It's crucial for agricultural planning, water resource management, and understanding climate dynamics because it represents the upper limit of water demand by plants and evaporation from the soil.

How is potential evapotranspiration measured?

Potential evapotranspiration is estimated using mathematical models that consider various climatic factors such as temperature, humidity, solar radiation, and wind speed. The Penman-Monteith equation is widely regarded as the standard method for calculating PET, as recommended by the Food and Agriculture Organization of the United Nations.

What factors influence potential evapotranspiration rates?

Potential evapotranspiration rates are influenced by climatic conditions including solar radiation, air temperature, humidity, and wind speed. Geographic factors like altitude and latitude also play a role. For instance, higher temperatures can increase PET, while high humidity levels can reduce it.

Can potential evapotranspiration predict droughts?

Potential evapotranspiration can be an indicator of drought conditions when it significantly exceeds actual evapotranspiration due to water limitations. By comparing PET with precipitation data, researchers can assess drought severity and help in drought forecasting and water resource management.

How does potential evapotranspiration affect irrigation requirements?

Irrigation requirements are directly related to PET because it represents the maximum water demand of crops. By estimating PET, farmers can determine the optimal amount of water needed for irrigation to prevent both water stress in plants and water wastage, leading to more efficient water use in agriculture.

Does climate change impact potential evapotranspiration?

Climate change significantly impacts PET, as it alters the climatic variables that determine evapotranspiration rates. According to research, rising temperatures and changing precipitation patterns can increase PET in many regions, which may lead to greater water stress and affect water availability for ecosystems and human use.

AllThingsNature is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.

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