ERRORS IN PENMAN-WRIGHT ALFALFA REFERENCE EVAPOTRANSPIRATION ESTIMATES .2. EFFECTS OF WEATHER SENSOR MEASUREMENT VARIABILITY

The effects of weather measurement variability on Penman-Wright alfalf a reference evapotranspiration (ET(r)) estimates were investigated. Me asurement variability or error was estimated in two ways-(1) using man ufacturer's specifications of accuracy, and (2) by actual measurement. Results showed that evapotranspiration (ET) errors due to combined ra ndom and systematic error in weather measurements were less than 5.5% of average daily ET(r) when sensors were operating within error level envelopes equivalent to their specifications of accuracy. When sensors were operating at the limits of their specifications of accuracy, the greatest ET error was due to solar radiation measurement error, follo wed by dewpoint, maximum temperature, and wind speed measurement error s. Weather measurement variability derived from sensor evaluations res ulted in different ET error. Air temperature measurement variability w as small yielding ET errors less than 4% of average daily ET. Dewpoint temperature errors, based on relative humidity measurement error of 1 0%, resulted in ET errors of about 6 to 7% of average daily ET Solar r adiation measurement variability resulted in ET errors of nearly 8.5% of average daily