Thornthwaite and mather soil water balance model adapted for estimation of real evapotranspiration of the pasture

Knowing the real pastures demand is essential for a more rational use of irrigation. The present work deals with the performance of the Thornthwaite and Mather water balance method (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, RJ (22o 27 'S; 42o 45' W and 30 m altitude). Micrometeorological and meteorological measurements were conducted in a micrometeorological mast installed in the pasture and also in an automatic weather station. The ThM model was evaluated by simple linear regression between ETa BERB and the estimates from ThM, by its determination coefficient (r²) and the modified Willmott agreement index (dm). The ThM model underestimated (between 11 and 16%) ETa for all seasons, with the exception of spring, which overestimated 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 due to the model inability to represent water extraction from the soil in absence periods of rainfall.