By on August 11, 2010

Evapotranspiration (ET) is the loss of water to the atmosphere through evaporation and transpiration by plants. Because it is a significant component of the water budget in many ecosystems, estimating or measuring evapotranspiration is often critical in ensuring effective water and groundwater management. For instance, ET data helps minimize water loss by developing effective irrigation schedules. The data is also critical in determining water balances necessary to maintaining a variety of ecosystems, such as wetlands and reservoirs.

Evapotranspiration is dependent on factors pertaining to the weather, vegetation, soil, and water characteristics, and management practices. Meteorological parameters affecting ET rates include temperature, humidity, wind speed and direction, rainfall, and net solar radiation.

The phenomenon of evapotranspiration is evident in data collected at an automated water quality station at the Silver Lake Wetlands. The graph demonstrates daily fluctuations in which water level decreases as the solar charge on a battery increases. Minimum water levels correspond to the latest peaks of solar energy charge, and maximum water levels correspond to minimum solar energy. Although solar charge on the battery is an indicator of sunlight, the installation of a solar radiation sensor would quantify net radiation, which is among the more significant factors determining ET rates.

Monitoring Evapotranspiration

ET is a major component of the hydrologic cycle, but it is one of the most difficult to quantify accurately. Direct measurements can be made with lysimeters, which are instruments that weigh or measure the amount of water loss by an enclosed system. Direct methods, however, are expensive and demanding, and the results only apply to the exact situation in which they were measured.

Because direct methods are impractical for constant use on a large scale, ET is commonly estimated. Techniques used to estimate ET include calculating it from meteorological data or using pan evaporation methods.

Evaporation pan measurement is the traditional method of estimating ET. US Weather Bureau Class A specifications call for a 10-foot-deep by 48-foot-diameter galvanized iron pan. The rate of water loss is recorded either manually or electronically. The results are generally larger than actual ET. A general rule of thumb is to multiply pan evaporation values by 0.7 in the winter and 0.6 in the summer.

Estimating evapotranspiration by collecting climate data from automated weather stations is recognized as the most efficient and effective method for determining ET rates. This climate data is then used in mathematical equations to derive an ET value. Over the years, numerous algorithms have been used to calculate ET from meteorological data. Choosing the most appropriate method depends on the type of climate data available and site-specific conditions. The Penman-Monteith equation is commonly used in agricultural areas. The Bowen ratio energy balance method and the Eddy correlation method are other methods that are frequently used at both open water and terrestrial sites.

Data collected by weather monitoring systems typically includes temperature, solar radiation, rainfall, relative humidity, and wind speed and direction.

Image Credit: Wikimedia, SCM Waterproof Porous


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