What Is Evapotranspiration, And Why It Matters

Among the many problems associated with extended periods of drought is the inability of plants to extract water at a rate fast enough to keep up with the rates of evapotranspiration (the combined loss of water from plant transpiration and soil evapotranspiration) that atmospheric conditions will allow, according to Jeff Andresen and Aaron Pollyea in the Michigan State University Extension Department of Geography. The rate of potential evapotranspiration (PET), the amount of water that could potentially be lost to evaporation over a vegetated surface given meteorological conditions at the time, is dependent on the intensity of solar radiation, air temperature, humidity, and wind speed.

Of these meteorological factors, solar radiation and air temperatures are most important in a state like Michigan, with somewhat less influence from humidity and wind speed. Three of the factors (solar radiation intensity, air temperature, and wind speed) are positively correlated with PET (i.e., the greater the intensity of solar radiation, the greater the PET), while humidity is negatively correlated (i.e., the greater the amount of water vapor already in the atmosphere, the less that can evaporate). PET also depends on the amount of crop type and its leaf area, the phenological stage, and the amount of plant-available soil moisture in the rooting zone.