ANALYTICAL ESTIMATION OF MODIFIED PENMAN EQUATION PARAMETERS

The mathematical expression of the modified Penman equation requires v alues for the adjustment factor C, the saturation vapor pressure and i ts slope with respect to dr temperature, the extraterrestrial radiatio n, and the maximum sunshine duration, among others. The values of thes e parameters are measured, taken from tables, or estimated from analyt ical expressions. Using tables is cumbersome and introduces uncertaint ies 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 signifi cant. In this paper, mathematical expressions that describe the parame ters used for the calculation of the Penman reference crop evapotransp iration are derived through a nonlinear regression procedure. Comparis ons of these expressions to those found in the literature show that th ey undoubtedly give more reliable results.