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

Sensitivity analyses of the Penman-Wright alfalfa reference evapotranspiration (ET(r)) model to errors in parameters and weather data using a factor perturbation simulation approach were conducted. Daily weather data for 1989 to 1991 collected at 20 weather stations in central Washington State were used in the analyses. The results generally show the Penman-Wright model is most sensitive to combined error in maximum and minimum temperatures, followed by progressively less sensitivity to errors in maximum temperature, solar radiation, dewpoint temperature, wind speed, and minimum temperature. Sensitivity coefficients were computed for each weather variable. Values obtained were in general agreement 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 areas having climates similar to the study area. The results indicate the relative sensitivity of the Penman-Wright ET model to systematic and random error in the weather variables. Actual weather measurement errors are needed to effectively determine expected ET errors under operational conditions.