Evapotranspiration (ET) is a term describing the transport of water into the atmosphere from surfaces, including soil (soil evaporation), and from vegetation (transpiration). The latter two are often the most important contributors to evapotranspiration. Other contributors to evapotranspiration may include evaporation from wet canopy surface (wet-canopy evaporation), and evaporation from vegetation-covered water surface in wetlands.
The process of evapotranspiration is one of the main consumers of solar energy at the Earth's surface. Energy used for evapotranspiration is generally referred to as latent heat flux; however, the term latent heat flux is broad, and includes other related processes unrelated to transpiration including condensation (e.g., fog, dew), and snow and ice sublimation. Apart from precipitation, evapotranspiration is one of the most significant components of the water cycle.
The evaporation component of ET is comprised of the return of water back to the atmosphere through direct evaporative loss from the soil surface, standing water (depression storage), and water on surfaces (intercepted water) such as leaves and/or roofs. Transpired water is that which is used by vegetation and subsequently lost to the atmosphere as vapor. The water generally enters the plant through the root zone, is used for various biophysiological functions including photosynthesis, and then passes back to the atmosphere through the leaf stomates. Transpiration will stop if the vegetation becomes stressed to the wilting point, which is the point in which there is insufficient water left in the soil for a plant to transpire, or if the plant to atmosphere vapor concentration gradient becomes prohibitive to plant physiological processes (e.g. photosynthesis).
Factors affecting evapotranspiration
The rate of evapotranspiration at any location on the Earth's surface is controlled by several factors:
- Energy availability. The more energy available, the greater the rate of Evapotranspiration. It takes about 600 calories of heat energy to change 1 gram of liquid water into a gas.
- The humidity gradient away from the surface. The rate and quantity of water vapor entering into the atmosphere both become higher in drier air.
- The wind speed immediately above the surface. The process of evapotranspiration moves water vapor from ground or water surfaces to an adjacent shallow layer that is only a few centimeters thick. When this layer becomes saturated evapotranspiration stops. However, wind can remove this layer replacing it with drier air which increases the potential for Evapotranspiration. Winds also affect evapotranspiration by bringing heat energy into an area. A 5-mile-per-hour wind will increase still-air evapotranspiration by 20 percent; a 15-mile-per-hour wind will increase still-air evapotranspiration by 50 percent
- Water availability. Evapotranspiration cannot occur if water is not available.
- Physical attributes of the vegetation. Such factors as vegetative cover,plant height, leaf area index and leaf shape and the reflectivity of plant surfaces can affect rates of evapotranspiration. For example coniferous forests and alfalfa fields reflect only about 25 percent of solar energy, thus retaining substantial thermal energy to promote transpiration; in contrast, deserts reflect as much as 50 percent of the solar energy, depending on the density of vegetation.
- [Stomatal resistance]]. Plants regulate transpiration through adjustment of small openings in the leaves called stomata. As stomata close, the resistance of the leaf to loss of water vapor increases, decreasing to the diffusion of water vapor from plant to the atmosphere.
- Soil characteristics. Soil characteristics that can affect evapotranspiration include its heat capacity, and soil chemistry and albedo.
Seasonal trends of evapotranspiration within a given climatic region follow the seasonal declination of solar radiation and the resulting air temperatures. Minimum evapotranspiration rates generally occur during the coldest months of the year. Maximum rates generally coincide with the summer season. However since evapotranspiration depends on both solar energy and the availability of soil moisture and plant maturity the seasonal maximum evapotranspiration actually may precede or follow the seasonal maximum solar radiation and air temperature by several weeks.
Geographical patterns of evapotranspiration
Assuming that moisture is available, evapotranspiration is dependent primarily on the availability of solar energy to vaporize water. Evapotranspiration therefore varies with latitude, season of year, time of day, and cloud cover. Most of the evapotranspiration of water on the Earth's surface occurs in the subtropical oceans (Figure 1). In these areas, high quantities of solar radiation provide the energy required to convert liquid water into a gas. Evapotranspiration generally exceeds precipitation on middle and high latitude landmass areas during the summer season. Once again, the greater availability of solar radiation during this time enhances the evapotranspiration process.
Estimates of average nationwide evapotranspiration for the conterminous United States range from about 40 percent of the average annual precipitation in the Northwest and Northeast to close to 100 percent in the Southwest. During a drought, the significance of evapotranspiration is magnified, because evapotranspiration continues to deplete the limited remaining water supplies in lakes and streams and the soil.
The lower 5 miles of the atmosphere transports an average of about 40,000 billion gallons of water vapor over the conterminous United States each day. Slightly more than 10 percent of this moisture, however, is precipitated as rain, sleet, hail, or snow. The greatest proportion, about 67 percent, is returned to the atmosphere through evapotranspiration. About 29 percent is discharged from the conterminous United States as surface-water flowing into the Pacific and Atlantic Oceans and across the borders into Canada and Mexico, about 2 percent is discharged as groundwater outflow, and about 2 percent is consumed by people, animals, plants, and used for industrial and commercial processes. For most of the United States, evaporation returns less moisture to the atmosphere than does transpiration. Globally, evaporation processes are resposible for an overwhelming majority of the water returned to the atmosphere.
Actual evapotranspiration (AE or AET) is the quantity of water that is actually removed from a surface due to the processes of evaporation and transpiration.
Potential evapotranspiration or PE is a measure of the ability of the atmosphere to remove water from the surface through the processes of evaporation and transpiration assuming no control on water supply. Since PET assumes that water availability is unlimited, vegetation would never reach the wilting point (the point in which there is not enough water left in the soil for a plant to transpire). Therefore, the only limit to the transpiration rate of the plant is due to the physiology of the plant and not due to any atmospheric or soil moisture restrictions. Therefore, PET is considered the maximum ET rate possible with a given set of meteorological and physical parameters. On this basis, any irrigation that supplies more water than PET can accommodate could be viewed as wasted water.
Evapotranspiration (ET) is also sometimes discussed in terms of consumptive water use.
The overall resistance to water vapor transport due to soil, plant and surface factors is often termed bulk surface resistance.
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