Evapotranspiration Evapotranspiration Echion Category="Agronomy"Category="Climatology"Category="Ecology"Category="Hydrology"Evapotranspiration (ET) is the sum of Evaporation and Plant Transpiration. In evaporation, water is evaporated off of lake or pond surfaces, and from rain droplets caught in the leaves of trees. Transpiration takes water out of the watershed by evaporation of water through the pores in leaves. The trees aquire water through their roots exclusively, with the exception of trees in cloud forests. Evapotranspiration is a significant water loss from a watershed. Types of vegetation and land use significantly effect evapotranspiration, and therefore the amount of water leaving a watershed. Because water transpired through leaves comes from the roots, plants with deep reaching roots can more constantly transpire water. Thus Herbaceous plants transpire less than woody plants because herbaceous plants lack a deep taproot. Also, woody plants keep their structure over long winters while herbaceous plants must grow up from seed in the spring in seasonal climates, and will contribute almost nothing to evapotranspiration in the spring. Conifer forests tend to have much higher rates of evapotranspiration than Deciduous forests. This is because their needles give them superior surface area, resulting in more pores for transpiration, and allowing for more droplets of rain to be suspended in and around the needles and branches, where the some of the droplets can then be evaporated.

Evapotranspiration cannot be measured directly. Pan evaporation data can be used to estimate lake evaporation, but transpiration and evaporation of intercepted rain on vegetation are unknown. Evapotranspiration is usually found by creating an equation of known or estimated inputs into a watershed, such as Precipitation and outputs such as streamflow, groundwater flow, and evapotranspiration. If the other inputs and outputs are known, hydrologists can solve for evapotranspiration. Potential evapotranspiration (PET) is the amount of water that could be evaporated and transpired if there was plenty of Water available. Actual evapotranspiration is usually no greater than precipitation, with some buffer in time depending on the soil's ability to hold water. It will usually be less because some water will be lost due to Percolation or surface run off. An exception is areas with high water tables, where capillary rise can cause water from the groundwater to rise through the soil matrix to the surface. If potential evapotranspiration is greater than actual precipitation, then soil will dry out, unless Irrigation is used.

Potential evapotranspiration is usually measured indirectly, from other climatic factors, but also depends on the surface type, such free water (for lakes and oceans), the Soil type for bare soil, and the Vegetation. Often a value for the potential evapotranspiration is calculated at a nearby climate station on a reference surface, conventionally short grass. This value is called the reference evapotranspiration, and can be converted to a potential evapotranspiration by multiplying with a surface coefficient. In agriculture, this is called a crop coefficient. The difference between potential evapotranspiration and precipitation is used in Irrigation scheduling.

The potential evapotranspiration can be graphed during the year. It follows an approximate Bell curve around the seasons. Factors that affect evapotranspiration include the plant's growth stage or level of maturity, percentage of soil cover, Solar radiation, Humidity, Temperature, and Wind.

The most general and widely used equation for calculating ET is the Penman equation. The Penman-Monteith variation is the Food and Agriculture Organization's recommendation. The simpler Blaney-Criddle equation was popular in the Western United States for many years but it is not as accurate in regions with higher humidities. Other solutions used includes Makkink, which is simple but must be calibrated to a specific locale, and Hargreaves.

External links