Analytical Estimation of Modified Penman Equation Parameters

Abstract

The mathematical expression of the modified Penman equation requires values for the adjustment factor C, the saturation vapor pressure and its slope with respect to air temperature, the extraterrestrial radiation, and the maximum sunshine duration, among others. The values of these parameters are measured, taken from tables, or estimated from analytical expressions. Using tables is cumbersome and introduces uncertainties in estimating the parameters. Various research works found in the literature refer to the analytical evaluation of these parameters, but the need to more accurately calculate them is essential since errors introduced in evapotranspiration estimates are sometimes quite significant. In this paper, mathematical expressions that describe the parameters used for the calculation of the Penman reference crop evapotranspiration are derived through a nonlinear regression procedure. Comparisons of these expressions to those found in the literature show that they undoubtedly give more reliable results.