ERRORS IN PENMAN-WRIGHT ALFALFA REFERENCE EVAPOTRANSPIRATION ESTIMATES .1. MODEL SENSITIVITY ANALYSES

Sensitivity analyses of the Penman-Wright alfalfa reference evapotrans piration (ET(r)) model to errors in parameters and weather data using a factor perturbation simulation approach were conducted. Daily weathe r data for 1989 to 1991 collected at 20 weather stations in central Wa shington State were used in the analyses. The results generally show t he Penman-Wright model is most sensitive to combined error in maximum and minimum temperatures, followed by progressively less sensitivity t o errors in maximum temperature, solar radiation, dewpoint temperature , wind speed, and minimum temperature. Sensitivity coefficients were c omputed for each weather variable. Values obtained were in general agr eement with the literature. The percent error in daily ET(r) per unit error in a given weather variable was computed. Results may be used to estimate the potential error in Penman-Wright ET(r) estimates in area s having climates similar to the study area. The results indicate the relative sensitivity of the Penman-Wright ET model to systematic and r andom error in the weather variables. Actual weather measurement error s are needed to effectively determine expected ET errors under operati onal conditions.