Evapotranspiration - A Combination of Evaporation and Transpiration

Condensation on a leaf

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Evaporation is the process of water changing from a liquid into a gas or vapor. Transpiration is the evaporation of water from a plant's leaves, stem, flowers, or roots back into the atmosphere. When combined as a sum, the two create evapotranspiration — an important component in the movement of water and water vapor through the hydrologic cycle.

Evapotranspiration and the Hydrologic Cycle Factors Affecting It

Relative humidity (the amount of water vapor in the air) is also an important consideration in evapotranspiration rates because as the air becomes more and more saturated, less water is able to evaporate into that air. Therefore, as the relative humidity increases transpiration decreases.

The movement of wind and air across an area is the third factor affecting evapotranspiration rates. As the movement of air increases, evaporation and transpiration does as well because moving air is less saturated than stagnant air. This is because of the movement of air itself. Once saturated air moves, it is replaced by drier, less saturated air which can then absorb water vapor.

The moisture available in a plant's soil is the fourth factor affecting evapotranspiration because when soil is lacking moisture, plants begin to transpire less water in an effort to survive. This, in turn, decreases evapotranspiration.

The final factor affecting evapotranspiration is the type of plant involved in the transpiration process. Different plants transpire water at different rates. For example, a cactus is designed to conserve water. As such, it does not transpire as much as a pine tree would because the pine does not need to conserve water. Their needles also allow water droplets to gather on them which is later lost to evaporation in addition to the normal transpiration.

Geographic Patterns of Evapotranspiration

Evapotranspiration rates are also highest in areas with a hot and dry climate. In the Southwest United States for instance, evapotranspiration is about 100% of the total precipitation for the area. This is because the area has a large amount of warm, sunny days throughout the year paired with little precipitation. When these combine, evaporation is at its highest.

By contrast, the Pacific Northwest's evapotranspiration is only about 40% of yearly precipitation. This is a much colder and wetter climate so evaporation is not as prevalent. In addition, it has a higher latitude and less direct solar radiation.

Potential Evapotranspiration

Potential evapotranspiration is monitored by hydrologists because it is useful in predicting the evapotranspiration of an area and as it usually peaks in the summer, it is helpful in monitoring potential drought situations.

Potential evapotranspiration combined with examining the factors contributing to actual evapotranspiration gives hydrologists an understanding of what an area's water budget will be after water is lost to this process. Because so much water is lost and drought is always a concern for many areas around the globe, evapotranspiration is an important topic in the study of both physical and human geography.