Energy balance and Evapotranspiration response to environmental variables in the semi-arid Caatinga biome

Abstract

Semi-arid regions worldwide face critical challenges related to prolonged droughts and limited water availability, which significantly impact ecosystem dynamics and plant productivity. This study investigates the energy balance and the phenomenon of hysteresis between actual evapotranspiration (ET) and environmental variables in areas of Dense Caatinga (DC) and Sparse Caatinga (SC) using micrometeorological data collected between 2013 and 2015. It was observed that sensible heat flux (H) was the main component of the energy balance, representing more than 50% of the available energy. In comparison, latent heat flux (LE) varied between 20% annually and 40% during the rainy season. The energy balance closure was satisfactory, with closure coefficients ranging from 65% to 88% depending on the season and vegetation type. In SC, with a more exposed surface, the conversion of net radiation (Rn) at the surface into H is increased. In both regions, during dry periods, LE decreases mirroring the scarcity of water availability. The hysteresis analysis revealed complex cycles of ET in relation to net radiation (Rn), air temperature (Temp), and vapor pressure deficit (VPD), with significant interannual variations. The correlation between ET and gross primary productivity (GPP) indicated counterclockwise hysteresis patterns in DC, suggesting that changes in GPP precede and influence ET, reflecting a rapid stomatal response to increased photosynthetic activity. In SC, the hysteresis cycles between ET and GPP were more variable, displaying both counterclockwise and clockwise loops, especially during 2015, strongly influenced by the El Niño event, indicating lower efficiency in the ET response to GPP due to water constraints. Canopy conductance (Gc) and the decoupling coefficient (Ω) also showed seasonal variations, reflecting the ecophysiological responses to environmental conditions. The correlation between these two variables suggests that Gc in SC was more influenced by external factors, like radiation and water availability. These results demonstrate that the dynamics of the Caatinga Biome are strongly tied to seasonal rainfall patterns, as the ecosystem's physiological and metabolic traits are primarily regulated by water availability. The variations observed across years and vegetation types underscore the importance of considering interannual variability and the distinctive characteristics of the ecosystem when analyzing climate relationships.