Assessment of reference evapotranspiration forecasting in the Mediterranean climate of Central Chile using the ASCE standardized Penman-Monteith equation, the Hargreaves-Samani equation, and weather predictions from the Global Forecast System model.

Accurate estimation of crop evapotranspiration (ET) is essential for effective agricultural water resource planning. To determine ET it is common multiplying the reference evapotranspiration (ET) by an appropriate crop coefficient (K). Forecasting ET could be particularly beneficial for irrigation scheduling. In this work, daily and cumulative (7 days) ET predictions were estimated with the Penman-Monteith (PM) and the Hargreaves-Samani (HS) models using environmental variables obtained from the Global Forecast System (GFS). Results were compared with ET calculated with the ASCE (American Society of Civil Engineers) PM equation from measured daily values of solar radiation, air temperature, relative humidity and wind speed. Four weather stations located in Central Chile during 2020 to 2022 were used. The best results were obtained using the HS equation, for all weather stations, the statistical indicators for daily ET predictions fall within the following ranges: Accuracy (defined as the percentage of daily ET predictions with an absolute error within 1mmd) varies between 72.8% and 91.8%; the root mean square error (RMSE) ranges from 0.59mmd to 0.98mmd; the normalized root mean square error (NRMSE) is between 16.1% and 24.1%; the mean bias error (MBE) is between 0.10mmd and 0.36mmd; and the normalized mean bias error (NMBE) varies from 2.5% to 8.3%. On the other hand, the indicators for the 7-day cumulative ET forecasts were: RMSE from 2.86mm to 5.08mm; NRMSE from 11.0% to 16.3%; MBE from 0.66mm to 2.57mm; and NMBE from 2.4% to 8.3%. These results suggest that HS ET values predicted from GFS temperature forecasts could be effective for foreseeing near-future water demand and enhancing the accuracy of irrigation scheduling and management in Central Chile.