Actual Evapotranspiration for Sugarcane Based on Bowen Ratio-Energy Balance and Soil Water Balance Models with Optimized Crop Coefficients

Evapotranspiration is an important parameter to evaluate soil water deficit and water use efficiency, especially in locations with irregularly distributed precipitation. The aim of this study was to assess the daily actual evapotranspiration (ET a ) estimated by the Thornthwaite and Mather soil water balance adapted for crops (ThM) and by the dual Kc method with the crop coefficients optimized from inverse modeling and by the adjustment suggested in the FAO-56 bulletin. The models were optimized and evaluated with actual evapotranspiration determined by the Bowen ratio – energy balance method (ET β ) for sugarcane at full canopy closure grown in Alagoas state, northeastern Brazil. The objective function of the inverse problem was defined in terms of ET β and ET a estimated by the ThM or dual Kc method by optimizing the single crop coefficient (K c ) and the basal crop coefficient K cb , respectively. Optimized K c (1.05) and K cb (1.03) were lower than those adjusted by the K c FAO56 method (K c = 1.25 or K cb = 1.20), with optimized K c slightly higher than the K c obtained experimentally (1.01 ± 0.08). ET a estimated by the ThM and dual Kc method with optimized crop coefficients had similar high precision (r 2 > 0.79) and accuracy (d m > 0.93 and RMSE < 0.30 mm d −1 ). However, using the coefficients adjusted from the FAO56 method overestimated ET a in both models.