Evaluation of estimated weather data for calculating Penman-Monteith reference crop evapotranspiration

Utilizing the weather generator ClimGen, daily solar radiation (R-s) and vapor pressure deficit (VPD) were estimated from temperature data and used to calculate evapotranspiration at five locations, representing tropical, temperate, semi-arid, and arid climates. ClimGen was calibrated for each location using the most recent 2 or 5 years of complete daily weather records. Actual and estimated values were compared on a daily and weekly (7-day running average) basis. Error indices were defined to indicate excellent to poor performance of the estimation methods. Overall in all locations, the ClimGen estimates for both daily R-s and VPD were poor to acceptable. The weekly analyses showed significant improvement in performance for both R-s and VPD estimations in arid and semi-arid locations. Daily reference crop evapotranspiration values using the FAO Penman-Monteith equation (PM ETo) were calculated using complete daily weather records. These values were compared with (1) ETo calculated with the PM model, actual temperature data, and ClimGen estimates of daily R-s, VPD, and generated wind speed (PMEst ETo), and (2) ETo calculated solely from actual daily temperature data using a calibrated version of the Hargreaves method (HG(Adj) ETo). The daily PMEst ETo results were poor to acceptable in all locations, but analyses for weekly periods showed improved performance to acceptable and good levels for arid and semi-arid locations. The performance of the HG(Adj) ETo method was also poor to acceptable for daily ET estimates in all locations, while weekly analyses showed improvement. A non-calibrated version of the Hargreaves method did not work for either daily or weekly periods. The PMEst ETo and HG(adj) ETo methods appeared suitable for weekly periods in arid and semi-arid locations provided that at least 2 years of complete weather records were available to calibrate the parameters required. There was no advantage in using 5 years of weather records for calibration.