Evapotranspiration
Evapotranspiration is the combined movement of water from land to the atmosphere through evaporation and plant transpiration. It connects soil moisture, vegetation, irrigation, drought, weather, and climate because it moves both water and energy through landscapes.
What evapotranspiration is
Evapotranspiration, often shortened to ET, is the total transfer of water from land surfaces to the atmosphere through evaporation and transpiration. Evaporation moves water from soil, water bodies, wet leaves, and shallow groundwater into the air. Transpiration moves water taken up by plant roots and released as vapor through leaves.
Evaporation plus transpiration
The two parts are grouped because they are hard to separate across real landscapes. A field, forest, wetland, or city park may lose water from bare soil, puddles, plant surfaces, and living leaves at the same time. Together, those flows show how much water is leaving the land system and returning to the atmosphere.
What controls ET
Evapotranspiration depends on available energy, air temperature, humidity, wind, soil moisture, plant type, leaf area, growth stage, and rooting depth. Hot, dry, windy conditions can raise atmospheric demand, but ET cannot stay high if soil water runs out or plants close their stomata to reduce water loss.
Role in the water cycle
ET is a major water-cycle flux. It links land, vegetation, and atmosphere by returning water vapor to the air after precipitation has reached the ground. NASA describes most atmospheric moisture as coming from evaporation over water bodies, with a smaller but important share released by plants through transpiration. Over land, ET strongly affects soil moisture and runoff.
Crops and irrigation
Farmers and water managers use ET to estimate how much water crops are using and how much irrigation may be needed. Reference evapotranspiration and crop coefficients help translate weather conditions and crop growth into practical water budgets. This is especially important where irrigation demand competes with rivers, aquifers, cities, and ecosystems.
Drought and heat
During drought, ET can fall because plants and soils lack water. During heat waves, atmospheric demand can rise, drying landscapes faster when water is available. Because evaporation consumes energy, ET also affects surface temperature. A well-watered field or forest can cool itself more than a dry bare surface, but that cooling comes with water loss.
Measuring from ground and space
ET can be estimated with weather stations, lysimeters, eddy-covariance towers, water-balance models, crop models, and satellites. NASA-supported tools such as OpenET use satellite data and models to estimate water use across fields and landscapes. No method is perfect, so estimates depend on scale, assumptions, and local calibration.
Why it matters
Evapotranspiration sits at the intersection of water supply, food production, ecosystem health, and climate. It helps explain why vegetation changes can alter streamflow, why irrigation planning needs weather data, why drought stress can intensify quickly, and why water and energy budgets have to be studied together.