Errors in Penman-Wright Alfalfa Reference Evapotranspiration Estimates: II. Effects of Weather Sensor Measurement Variability

The effects of weather measurement variability on Penman-Wright alfalfa reference evapotranspiration (ETr) estimates were investigated. Measurement variability or error was estimated in two ways—(1) using manufacturer’s specifications of accuracy, and (2) by actual measurement. Results showed that evapotranspiration (ET) errors due to combined random and systematic error in weather measurements were less than 5.5% of average daily ETr 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, followed by dewpoint, maximum temperature, and wind speed measurement errors. Weather measurement variability derived from sensor evaluations resulted in different ET error. Air temperature measurement variability was small yielding ET errors less than 4% of average daily ET. Dewpoint temperature errors, based on relative humidity measurement error of 10%, resulted in ET errors of about 6 to 7% of average daily ET. Solar radiation measurement variability resulted in ET errors of nearly 8.5% of average daily ET.