Written for presentation at the

Abstract. The physical processes of evaporation and transpiration, collectively termed evapotranspiration, are discussed with respect to the unique conditions specific to forested environments. Forests have significant variations in ET rates due to 1) diurnal, seasonal, and annual climatic fluctuations; 2) spatiotemporal differences in vegetation; 3) evaporation of precipitation intercepted by vegetation, litter, and soil; 4) evaporation from water bodies; and 5) physiographic differences. The earliest methods for computing ET relied on empirical relations between climatic variables and consumptive water use by crops. Later formulations derived potential evaporation by relating solar radiation and temperature to the physical process of latent and sensible heat flux. To generalize Penman’s equation for crops that were water-stressed, Monteith incorporated a canopy resistance term to describe the effect that partially closed stomates have on evapotranspiration. Later researchers have modified these equations to account for variable crop density, rainfall interception, bare-soil evaporation, and multiple canopy layers. WEPP primarily uses a modification of Ritchie’s method to compute ET. Although WEPP gives the user the option to use either Penman’s, Priestly-Taylor’s, Hargraves’, or Penman-Monteith’s equations for calculating ET, the The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessaril