Modification of the radiation-based Abtew reference evapotranspiration model under humid climate, Northeast India

This study aimed to improve the Abtew model for reference evapotranspiration (ET 0 ) calculation in Northeast India using seven temperature-based solar radiation models. The temperature-based models require only air temperature as input data, which can be easily measured in most locations worldwide. The performance of the improved Abtew models (A1-A7), along with the Stephen Stewart model (SS), the Irmak model (Ir), and the modified Turc model (MT), was evaluated under the climatic conditions of Dibrugarh, Northeast India, using statistical indices such as mean absolute error (MAE), root mean square error (RMSE), standard error (SE), coefficient of correlation (r), coefficient of determination (R 2 ), and index of agreement (D). The results showed that the seven improved Abtew models (RMSE 0.40-0.53 mm/day; D 0.81-0.93) outperformed the four physical models (RMSE 0.43-2.77 mm/day; D 0.49-0.91) for the ET 0 estimate at Dibrugarh. The statistical analysis identified that the A6 model ranked highest for Dibrugarh. This study highlights the significant improvement in ET 0 estimation accuracy by utilizing temperature-based solar radiation models in the Abtew model. Therefore, we strongly recommend using the A6 model to estimate ET 0 , which requires only temperature data as input, for estimating ET 0 under the climatic conditions of Dibrugarh, Northeast India.