Forecasting reference crop evapotranspiration based on temperature forecast and Hargreaves-Samani equation
Luo Yufeng1,2, Li Si1,2, Peng Shizhang1, Wang Weiguang1, Jiao Xiyun1, Jiang Yunlu1,2, Gu Hong3
1.State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing, Jiangsu 210098, China; 2.College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing, Jiangsu 210098, China; 3. Gaoyou Municipal Water Affairs Bureau, Gaoyou, Jiangsu 225600, China
Abstract:To explore a method for forecasting short-term reference crop evapotranspiration(ET0)precisely, a proposal that is based on temperature forecast and the Hargreaves-Samani(HS)equation was raised. Thus, the observed daily meteorological data from 2001 to 2011 in Nanjing Station and the daily weather forecast data for 4 day forecast horizon in 2011 were collected. Subsequently, daily ET0 was calculated by using the FAO-56 Penman-Monteith(PM)equation, the parameters in the HS equation were corrected with 2001—2010 PM ET0 values. Then the calibrated HS equation and tempe-rature forecast were used to forecast the 4 day ET0. The forecasted and estimated ET0, forecasted and calculated temperatures were compared to evaluate the accuracy in ET0 forecast and identify the causes for the errors. The results showed that the average accuracy in forecast for the minimum temperature was 81.9%, and for the maximum temperature it was 80.1%. After calibration, the accuracy of the HS equation was relatively high. The mean accuracy, MAE, RMSE and correlation coefficient were 85.7%, 1.01 mm/d, 1.42 mm/d and 0.74, respectively, for ET0. Besides, the errors were enlarged with increasing forecast horizon. The main reason for the errors is that the mean wind speed and relative humidity are not taken into account in the HS equation. In general, the approach for predicting ET0 based on temperature forecast and corrected HS equation has a better accuracy and can provide relatively accurate ET0 data for irrigation forecast and decision making.
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