US Geological Survey


Date of this Version



Published in Remote Sensing of Drought: Innovative Monitoring Approaches, edited by Brian D. Wardlow, Martha C. Anderson, & James P. Verdin (CRC Press/Taylor & Francis, 2012).


U.S. Government Work


Evapotranspiration (ET) is an important component of the hydrologic budget because it reflects the exchange of mass and energy between the soil–water–vegetation system and the atmosphere. Prevailing weather conditions influence potential or reference ET through variables such as radiation, temperature, wind, and relativity humidity. In addition to these weather variables, actual ET (ETa) is also affected by land cover type and condition, as well as soil moisture. The dependence of ETa on land cover and soil moisture, and its direct relationship with carbon dioxide assimilation in plants, makes it an important variable for monitoring drought, crop yield, and biomass—a critical capability for decision makers interested in food security, grain markets, water allocation, and carbon sequestration (Bastiaanssen et al., 2005).

Because ET can be difficult to measure accurately, especially at large spatial scales, several different hydrologic modeling techniques have been developed to estimate ETa using satellite remote sensing. In general, the ET modeling techniques can be grouped into two broad classes that include models based on surface energy balance (e.g., Bastiaanssen et al., 1998; Su et al., 2005; Allen et al., 2007; Anderson et al., 2007; Senay et al., 2007) and water balance (e.g., Allen et al., 1998, Senay, 2008) principles. While water balance models focus on tracking the pathways and magnitude of rainfall in the soil–vegetation system, most remote sensing energy balance models use land surface temperature (LST) as a primary constraint in partitioning radiant energy available at the surface between heat and water fluxes.