Evaporation (transformation of liquid water to water vapor) and transpiration (water vapor emission from plant surfaces) are outflow processes of water budgets. Evapotranspiration (ET) is the combined process of water surface evaporation, soil moisture evaporation, and plant transpiration. Stormwater management applications may include water surfaces (e.g., pond, wetland, etc.), vegetation, or both, and therefore may require an estimation of evaporation, transpiration, or both to estimate water level changes between storms. For example, a wetland system includes vegetation, open water surfaces, and exposed moist soils. The combined effects of water surface evaporation, soil moisture evaporation, and plant transpiration for this system are often significant components of annual water budgets. Evaporation tends to lower water level in a pond or wetland over time, and evapotranspiration acts to dry out the soil before the next storm. During storms, however, evaporation and evapotransporation are typically not significant compared to precipitation, discharge and infiltration, and are often not considered.

Evapotranspiration is a function of meteorological conditions, such as air temperature, wind speed, relative humidity, and solar radiation; and of evaporating/transpiring surface conditions, such as albedo (i.e., fraction of reflected incident sunlight), water temperature, roughness, and water availability. The effective surface conditions of plants are especially complex. Stomata openings in plant leaves are essential for the movement of water vapor and other gases. The number of these openings varies with plant type. The size of these openings varies with changes to the pressure in