Evapotranspiration is the combined process by which water is transferred from the land surface to the atmosphere through two primary mechanisms, evaporation and transpiration. In stormwater management and hydrology, evapotranspiration represents a critical pathway by which precipitation is returned to the atmosphere, thereby reducing the volume of runoff and influencing the overall water balance of a site or watershed.
Evaporation is the physical process by which water is converted from a liquid to a vapor and released into the air from surfaces such as soil, pavement, water bodies, and vegetation. Transpiration, on the other hand, is a biological process in which water is absorbed by plant roots, transported through the plant, and released as water vapor through small openings in the leaves called stomata. Together, these processes function as a continuous mechanism for removing water from the ground and returning it to the atmosphere.
In natural systems, evapotranspiration plays a major role in regulating soil moisture, groundwater recharge, and surface runoff. A significant portion of precipitation is typically lost to evapotranspiration before it can contribute to streamflow or infiltration. However, in developed areas with high percentages of impervious surfaces, evapotranspiration is greatly reduced because there is less exposed soil and vegetation. This reduction contributes to increased runoff volumes and altered hydrologic behavior.
In the context of stormwater management, evapotranspiration is a key design consideration in green infrastructure and Low Impact Development (LID) practices. Systems such as rain gardens, bioswales, green roofs, and vegetated filter strips are specifically designed to maximize evapotranspiration by incorporating vegetation and engineered soils that retain and slowly release water. By promoting evapotranspiration, these practices help reduce runoff volume, lower peak flows, and improve water quality.
The rate of evapotranspiration depends on several factors, including temperature, solar radiation, humidity, wind speed, soil moisture availability, and the type and density of vegetation. It is often estimated using hydrologic models or empirical equations and is typically expressed as a depth of water over time, such as inches per day or millimeters per year.
Evapotranspiration is a fundamental component of the hydrologic cycle and a critical process in stormwater management, as it directly influences how much precipitation becomes runoff versus how much is naturally returned to the atmosphere.