Evapotranspiration partitioning and water-use efficiency in natural grasslands of the Brazilian Pampa biome

• Water fluxes were characterized across three grassland ecosystems in the Pampa biome. • ET and T had seasonal patterns driven by R g , Temp , and VPD . • ET partitioning was controlled by T even in dry periods. • WUE remained stable across seasons. Understanding the relationship between the evapotranspiration ( ET ) components (transpiration, T , and evaporation, E ) and carbon exchanges, such as gross primary production ( GPP ), allows identifying vegetation responses to weather and climate variations, as well as to imminent climate change scenarios. Here, these relationships were studied over a natural grassland area of the Brazilian Pampa biome, which is mainly used for livestock farming. The relationships between ET components and water use efficiency ( WUE ) together with environmental variables (e.g. incoming shortwave radiation – R g , air temperature – Temp , vapor pressure deficit – VPD , soil water content – SWC , and precipitation – Prec ) were characterized at three eddy covariance sites using energy and CO 2 flux data obtained from 2015 to 2023. The results showed that the seasonal patterns of sensible heat flux ( H ), latent heat flux ( LE ), and GPP were similar at the three sites analyzed, evidencing highest values in spring/summer and lowest ones in autumn/winter. ET and T had a well-defined seasonal pattern, primarily controlled by the environmental variables of R g , VPD , and Temp . However, the dynamic of the fluxes changed in years classified as dry, resulting in a reduction of up to 7% in GPP , and an increase of up to 31% in H . This highlights the strong impact of hydric conditions on WUE , while atmospheric conditions were dominant in ET partitioning. The main process driving ET was T , even when SWC seemed to be critical for plant growth. No seasonal or drought-related changes in WUE were observed. In summary, this study stresses the impact of water availability in the Brazilian Pampa Biome, underscoring the importance for developing adaptive and mitigation strategies to address the effects of climate change on pasturelands.