Evapotranspiration is an important parameter to evaluate soil water deficit and water use efficiency, especially at places with irregularly distributed precipitation. The aim of this study was to assess the daily actual evapotranspiration (ETa) estimated by the Thornthwaite and Mather soil water balance method adapted for crops (ThM) and by the dual Kc approach with the crop coefficients optimized from inversing modeling and by the adjustment procedure suggested in FAO-56. The models comparison and optimization were performed with actual evapotranspiration determined by the Bowen ratio -energy balance method (ETβ) for sugarcane at full canopy closure grown in Alagoas State, Northeastern Brazil. The objective function of the inverse problem was defined in terms of ETβ and ETa estimated by the ThM and dual Kc method by optimizing single crop coefficient (Kc) and the basal coefficient Kcb, respectively. Both optimized Kc and Kcb were lower than the adjusted Kc FAO56, with optimized Kc only 3% less than the Kc obtained experimentally. ETa estimated by ThM and dual Kc models with optimized crop coefficients (Kc = 1.05 or Kcb = 1.00) had similar high precision (r² > 0.79) and accuracy (dm > 0.93 and RMSE < 0.30 mm d -1 ), whereas using the coefficients derived from FAO 56 overestimated ETa in both models.
Evapotranspiration is an important parameter to evaluate soil water deficit and water use efficiency, especially at places with irregularly distributed precipitation.The aim of this study was to assess the daily actual evapotranspiration (ETa) estimated by the Thornthwaite and Mather soil water balance method adapted for crops (ThM) and by the dual Kc approach with the crop coefficients optimized from inversing modeling and by the adjustment procedure suggested in FAO-56. The models comparison and optimization were performed with actual evapotranspiration determined by the Bowen ratio – energy balance method (ETβ) for sugarcane at full canopy closure grown in Alagoas State, Northeastern Brazil. The objective function of the inverse problem was defined in terms of ETβ and ETa estimated by the ThM and dual Kc method by optimizing single crop coefficient (Kc) and the basal coefficient Kcb, respectively. Both optimized Kcand Kcbwere lower than the adjusted KcFAO56, with optimized Kconly 3% less than the Kc obtained experimentally. ETa estimated by ThM and dual Kc models with optimized crop coefficients (Kc = 1.05 or Kcb = 1.00) had similar high precision (r² >0.79) and accuracy (dm>0.93 and RMSE < 0.30 mm d-1), whereas using the coefficients derived from FAO 56 overestimated ETa in both models.
Determining the real water requirement for pastures is essential for the rational use of irrigation. The aim of this work was to assess the crop coefficient and performance of the Thornthwaite and Mather soil water balance (ThM) adapted to estimate the daily actual evapotranspiration (ETa) of a pasture in relation to the Bowen ratio - energy balance method (BREB). The experiment was carried out from July 2018 to June 2019 in Cachoeiras de Macacu, Rio de Janeiro State (RJ) (22º 27’S; 42º 45’W and 30 m altitude). Micrometeorological and meteorological measurements were conducted in a micrometeorological tower installed in the pasture and also in an automatic weather station, located 1 km from the pasture area. The ThM model was evaluated using linear regression between ETa determinate from BREB and the estimates from ThM using its coefficient of determination (R²) and the modified Willmott agreement index (dm). The ThM model underestimated (between 11 and 16%) the ETa for all seasons, except for spring, which overestimated by 1%. The highest precision and accuracy of the estimates were observed in autumn (R² = 0.84 and dm = 0.68) and spring (R² = 0.83 and dm = 0.82). In summer (R² = 0.56 and dm = 0.73) and winter (R² = 0.43 and dm = 0.66), the lower performance was caused by the inability of the model to represent water extraction from the soil in dry periods.
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