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Modification of the Evapotranspiration Routines in the WEPP Model: Part I

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Paper number  032293,  2003 ASAE Annual Meeting . (doi: 10.13031/2013.14977) @2003
Authors:   William J. Conroy, Joan Wu, William Elliot
Keywords:   Evapotranspiration, Penman-Monteith, WEPP, Water Erosion Prediction Project

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 Penmans 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 Ritchies method to compute ET. Although WEPP gives the user the option to use either Penmans, Priestly-Taylors, Hargraves, or Penman-Monteiths equations for calculating ET, the coding for Hargraves and Penman-Monteiths equations are incomplete, and are therefore turned off. The WEPP model adequately accounts for seasonal and climatic fluctuations, spatiotemporal difference in vegetation, and physiographic differences. Recommended improvements to the WEPP models ET routine are: 1) completing the coding of the Penman-Monteith equation, 2) computing evaporation of intercepted precipitation, and 3) computing evaporation from water bodies and litter.

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