Calibration and ranking of Valiantzas reference evapotranspiration equations under the humid climate of northeast India

This study aims to compare and calibrate seven different types of Valiantzas (Valiz) ET0 equations against FAO Penman-Monteith (P-M) equation at three sites under the humid climatic condition of northeast India. In the first stage, the different Valiantzas equations were ranked based on their original coefficient (OC) at each site. In the second stage, Valiz equations were assessed based on local calibrated coefficient (LCC). In the third stage, the selected Valiz equations were assessed based on global calibrated coefficient (GCC) considering pooled data for all the selected stations. The ranking of equations was done using a summative form of global positioning index (GPI). Different Valiantzas equations showed different performances relative to the sites having similar climate class (warm and temperate). With the OC, the Valiz 7 is ranked one at Dibrugarh and Shillong, and Valiz 2 for Aizawl. After local calibration, again, Valiz 7 is best at Dibrugarh and Shillong while Valiz 6 is best at Aizawl. With global calibration coefficient, Valiz 7 ranked best at all three selected stations. The suitable equations identified for the region are Valiz 7, Valiz 6 if full data are available, and Valiz 2 under limiting data conditions.