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Modification of the evapotranspiration routines in the WEPP model: part 1.
Conroy, W.J.; Wu, J.; Elliot, W. 2003.
Modification of the evapotranspiration routines in the WEPP model: part 1.
In: ASAE annual international meeting; 2003 July 27-30; Las Vegas, NV.
Paper No. 032293. St. Joseph, MI: American Society of Agricultural Engineers.
Keywords: Evapotranspiration, Penman-Monteith, WEPP, Water Erosion Prediction Project
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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
coding for Hargraves' and Penman-Monteith's 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
model's 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.
Moscow FSL publication no. 2003x
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