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United States Department of Agriculture

Agricultural Research Service

Title: Penmand and Gradient Energy Balance Formulations: a Sensitivity Analysis

Authors
item Aiken, Robert
item Flerchinger, Gerald
item Massman, William - USDA-FOREST SERVICE

Submitted to: Agronomy Journal
Publication Type: Abstract Only
Publication Acceptance Date: May 9, 1994
Publication Date: N/A

Technical Abstract: Soil-plant-atmosphere simulations can be enhanced by quantifying the accuracy of model assumptions. Penman and gradient forms of the energy balance equation are compared, using the temperatures of three evaporating sources: soil, residue and plant canopy. Both forms contain similar driving variables--net absorbed radiation, amplified by a convective transport term, and a vapor pressure gradient term. Sensitivity functions are derived with respect to atmospherice boundary conditions. Comparative analysis of predictive error, for a given parameter uncertainty level, is presented for the case of incomplete canopy and residue cover. Sensitivity to errors in absorbed radiation, convective transport and vapor pressure gradients is less for the Penman form due to a weighting term applied to absorbed radiation and convective transport, and to the form used to express gradients in vapor pressure deficits. Penman predictions deviate systematically from those of the theoretically correct gradient form-- attributed to ambigious definition of the 'proper' temperature for evaluating the slope of the saturated vapor pressure curve. A thermodynamic solution is proposed.

Last Modified: 12/20/2014
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