Evapotranspiration and Evaporation

Evapotranspiration (ET) is the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies. Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapor through stomata in its leaves. Evapotranspiration is an important part of the water cycle. An element (such as a tree) that contributes to evapotranspiration can be called an evapotranspirator. Potential evapotranspiration (PET) is a representation of the environmental demand for evapotranspiration and represents the evapotranspiration rate of a short green crop, completely shading the ground, of uniform height and with adequate water status in the soil profile. It is a reflection of the energy available to evaporate water, and of the wind available to transport the water vapour from the ground up into the lower atmosphere. Actual evapotranspiration is said to equal potential evapotranspiration when there is ample water. Evapotranspiration and Evaporation relation to agriculture How does this relate to agriculture? Figure B Image from Bridget Lassiter If you can predict evapotranspiration rates, you will be able to estimate the water demands of the crop. This may help you to determine whether or not to irrigate, for example. If crops do not receive enough water, their leaves may curl and their production decline as the plants fight to conserve what water they can. Knowledge of predicted temperature and wind conditions from weather forecasts can give you a clue to how strong the evapotranspiration rates will be. Evaporation may also directly affect soil moisture conditions. If there is too much moisture in the soil, the farm machinery can get bogged down because it has to work too hard. The weight of the machinery can also compact the wet soil, leading to lack of air for healthy root systems to develop. If the soil is too dry, however, the plants may be easily stressed due to the lack of available water and a crust may sometimes form on top of the soil. This crust may be so impermeable that when it rains on top of the crusty soil, the rain runs right off rather than soaking in. SIGNIFICANCE OF EVAPOTRANSPIRATION Apart from precipitation, the most significant component of the hydrologic budget is evapotranspiration. Evapotranspiration varies regionally and seasonally; during a drought it varies according to weather and wind conditions. Because of these variabilities, water managers who are responsible for planning and adjudicating the distribution of water resources need to have a thorough understanding of the evapotranspiration process and knowledge about the spatial and temporal rates of evapotranspiration. Estimates of average statewide evapotranspiration for the conterminous United States range from about 40 percent of the average annual precipitation in the Northwest and Northeast to about 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 (U.S. Geological Survey, 1984). Slightly more than 10 percent of this moisture, however, is precipitated as rain, sleet, hail, or snow. The disposition of this precipitation in the conterminous United States is illustrated in figure 1. Figure 1. Average disposition of 4200 billion gallons per day of precipitation in the conterminous United States. (source: Data from U.S. Geological Survey, 1990). As shown, the greatest proportion, about 67 percent, returns to the atmosphere through evapotranspiration, about 29 percent is discharged from the conterminous United States as net surface-water outflow into the Pacific and Atlantic Oceans and across the borders into Canada and Mexico, about 2 percent is discharged as ground-water outflow, and about 2 percent is consumed by people, animals, plants, and industrial and commercial processes (U.S. Geological Survey, 1990). For most of the United States, evaporation returns less moisture to the atmosphere than does transpiration. EVAPOTRANSPIRATION PROCESS Evapotranspiration is the water lost to the atmosphere by two processes-evaporation and transpiration. Evaporation is the loss from open bodies of water, such as lakes and reservoirs, wetlands, bare soil, and snow cover; transpiration is the loss from living-plant surfaces. Several factors other than the physical characteristics of the water, soil, snow, and plant surface also affect the evapotranspiration process. The more important factors include net solar radiation, surface area of open bodies of water, wind speed, density and type of vegetative cover, availability of soil moisture, root depth, reflective land-surface characteristics, and season of year. Assuming that moisture is available, evapotranspiration is dependent primarily on the solar energy available to vaporize the water. Because of the importance of solar energy, evapotranspiration also varies with latitude, season of year, time of day, and cloud cover. The distribution of mean daily solar radiation for the United States (fig. 2) shows a regional variation similar to that of mean annual lake evaporation (fig. 3) and mean annual air temperature. The areas that receive the maximum solar radiation and have the greatest lake evaporation in the conterminous United States are in the Southwest; the areas that receive the minimum solar radiation and have the least lake evaporation are in the Northeast and Northwest. According to the 1980 Bureau of Census data (U.S. Bureau of the Census, 1987, p. 181), the area of open-water bodies in the 48 conterminous States totals 38.4 million acres. Mean annual lake evaporation ranges from about 20 inches in parts of Maine, Oregon, and Washington to about 80 inches in parts of Arizona, California, and Nevada. SUMMARY Apart from precipitation, evapotranspiration is the major component in the hydrologic budget. Evapotranspiration involves the process of evaporation from open bodies of water, wetlands, snow cover, and bare soil and the process of transpiration from vegetation. The principal climatic factors influencing evapotranspiration are solar radiation and wind speed. In the conterminous United States, evapotranspiration averages about 67 percent of the average annual precipitation and ranges from 40 percent of the precipitation in the Northwest and Northeast to about 100 percent of the precipitation in the Southwest. Estimates of the mean annual evapotranspiration have been derived from hydrologic budgets for each State. These estimates indicate that statewide evapotranspiration within the conterminous United States ranges from about 10 inches per year in the semiarid Southwest to about 35 inches per year in the humid Southeast. However, in selected areas of the Southwest where moisture is available and solar radiation is high, evapotranspiration rates in saltcedar have been estimated to be about 56 inches per year. Seasonal trends in evapotranspiration follow the seasonal trends in air temperature-maximum rates occur during the summer months, and minimum rates during the winter months. Advanced Very High Resolution Radiometer instruments installed on polar-orbiting satellites provide relative measurements of plant vigor, density of vegetation cover, and the seasonal duration of vegetation growth. These measurements also have been used to monitor the spatial and temporal persistence of drought for large areas. Changes in evapotranspiration during a drought depend largely on the availability of moisture at the onset of a drought and the severity and duration of a drought. Evaporation from open bodies of water during a drought increases, but transpiration by plants, particularly shallow-rooted plants, generally decreases. To effectively manage the Nation's water resources, water managers need to understand the significance of evapotranspiration in the hydrologic budget. Knowledge of the regional and seasonal variability of evapotranspiration and its change during a drought also is important.