Proper methods and its calibration for estimating reference evapotranspiration using limited climatic data in Southwestern China

Proper methods for estimating reference evapotranspiration (ET0) using limited climatic data are critical, if complete weather data are unavailable. Based on the weather data of 19 stations in Guizhou Province, China, several simple methods for ET0 estimation, including the Hargreaves, Priestley-Taylor, Irmak-Allen, McCloud, Turk, and Valiantzas methods, were involved in comparison with the standard FAO-56 Penman-Monteith (PM) method. The Turk equation performs well for estimating ET0 in humid locations. Both the Turk method and the Valiantzas method initially performed acceptably with mean root-mean-square difference (RMSD) of 0.1472 and 0.1282mm d(-1), respectively, with only requiring parameters of temperature (T), relative humidity (RH), and sunshine duration (n). The corresponding calibration formulas to Turk and Valiantzas method were suggested as the most appropriate method for ET0 estimation with the RMSD of 0.0098 and 0.0250mm d(-1), respectively. The local calibrated Hargreaves-Samani method performed well and can be applied as the substitute of FAO-56 PM method under the condition that only the daily mean, maximum, and minimum temperatures were available, and local calibrated McCloud method was acceptable if only the mean temperature was available.