Prediction of annual reference evapotranspiration using climatic data

It is important to determine how well ETo can be estimated from easily observed Epan (free water evaporation measured by a pan) measurements and the other climatic data. Our objectives are to predict annual ETo with Epan data (with a calibrated kp (=ETo/Epan)) and with a 4-variable regression function method. The significance of the trends of Epan, ETo and kp series were detected. The whole data series (ETo, Epan, mean temperature, sunlight hours, relative humidity and wind speed) were divided into the early (L-5) years for calibrating kp and coefficients of a 4-variable function and the last 5 years for predicting ETo. From the results, significance of series trends decreased when using the modified Mann-Kendall (MMK) test compared to the Mann-Kendall (MK) method. For ETo, five out of six sites showed significant trends according to the MK statistic Z, and two sites were significant in trend combining with the MMK statistic Z*(j). For Epan, two sites were significant in trends according to Z, and zero sites were significant in trends combining with Z*(j). For kp, two sites were significant in trends according to Z, and no sites were significant in trends combining with Z*(j). Thus the calibrated kp can be treated as a constant when using the Epan method. The predicted annual ETo using the Epan and the multi-variable methods showed generally good agreements with the estimated annual ETo (based on monthly PM equation) with low relative errors (RE). Mean ETo values were well predicted by both methods. When using Epan method, RE ranged from -14.7 to -3.3% for Urumqi, from 17.6 to 21.7% for Xning, from 1.8 to 10.7% for Lanzhou, from 4.7 to 17.0% for Huhehaote, from -7.4 to 9.1% for Beijing, and from -8.6 to 2.3% for Changchun. RE of predicting annual ETo with 4-variable regression function were even lower compared to Epan method. The main error source of the predictions came from the deviation between calibrated kp and the actual kp of the predicted years when using Epan method and from random fluctuations of climatic data when using the 4-varible regression function. In conclusion, the MMK test was a robust method for trend detection because it considered serial time dependence. Insignificant trend of the kp series supports the choice of a mean value as the calibrated kp and for ETo predictions. The Epan method is recommended for prediction of annual ETo.