Meteorological and biophysical controls of evapotranspiration in tropical grazed pasture under rainfed conditions

Evapotranspiration (ET) is the main driver of the energy balance partitioning and influences hydrological and carbon cycles at global, regional and local scales. Furthermore, it is the main requirement for developing strategies to improve water use in agriculture. It is known that there is a close relationship between ET and rainfall, especially in tropical environments. Thus, the main goal of this article was to evaluate how ET and its controls (surface conductance - Gs; decoupling coefficient - Ω; Priestley-Taylor parameter - α) respond to the seasonal variability of meteorological forcing in tropical grazed pastures under the climatic conditions of Northeast Brazil. ET was measured using an Eddy covariance (EC) system and analyzed based on data from two agricultural years (2015–2016 and 2016–2017) with negative (-59 mm) and positive (356 mm) rainfall anomaly, respectively. ET exhibited pronounced seasonality, closely aligned with the seasonality of rainfall. Lower daily averages were observed during the dry season in both agricultural years (1.01 ± 0.60 and 0.89 ± 0.44 mm, respectively). On the other hand, higher daily averages were observed during the rainy season (2.44 ± 0.75 and 4.83 ± 0.96 mm, respectively). The Gs patterns and the significant correlation between Gs, Ω, and α (p < 0.01) indicate that surface control prevails over atmospheric control on an annual scale. This finding is confirmed by the annual mean values of Ω (0.27, 2015–2016) and α (0.38, 2016–2017). This relationship is likely induced by a vegetative stomatal control mechanism, which protects the vegetation against excessive water loss during periods of high temperatures and low humidity levels. These findings are crucial for understanding how droughts modulate components of the energy balance and water fluxes in pastures, especially given the intensification of these events. This has implications for the implementation of climate change mitigation policies and soil management.