Climatic and biotic controls of evapotranspiration across grassland ecosystems on the Tibetan Plateau

The Tibetan Plateau in China, the Third Pole of the world. Accurate descriptions of spatial variation in evapotranspiration (ET) and its components on the Tibetan Plateau (TP) are crucial for understanding the dynamics of the regional water cycle and energy balance. However, how various environmental factors affect ET and its components remains unclear. This study addresses this gap using observational data from 14 sites and employing the Shuttleworth-Wallace-Hu (SWH) model to examine the spatial relationships between ET, its components, and underlying drivers. The SWH model parameters were optimized using the Monte Carlo method, achieving a satisfactory model performance with an average R² of 0.75. The ratio of transpiration to ET was generally higher in alpine meadows (0.55) than alpine steppes (0.39). Across the TP, precipitation and the leaf area index together shaped spatial variation in ET and transpiration. Precipitation emerged as the primary driver of interannual variation in ET, while the leaf area index strongly regulated the proportion of ET accounted for by transpiration. Furthermore, above aridity index values of 0.73, grassland ecosystems transitioned from water to energy. This finding is significant as it challenges the common application of an aridity index value of 0.65 to distinguish arid from humid regions. It suggests that TP grasslands, with an AI of 0.65–0.73, remain primarily water-limited. This insight is crucial for understanding water-energy dynamics in similar ecosystems, calling into question the conventional binary categorization of arid versus humid areas. These findings advance our understanding of the complex land-atmosphere interactions within the global water cycle. This study used observational data from 14 sites and employed the Shuttleworth-Wallace-Hu (SWH) model to examine the spatial relationships between evapotranspiration (ET), its components, and underlying drivers on the Tibetan Plateau (TP). Across the TP, precipitation emerged as the primary driver of interannual variation in ET, while the leaf area index strongly regulated the proportion of ET accounted for by transpiration. Furthermore, above aridity index values of 0.73, grassland ecosystems transitioned from water to energy. This finding is significant as it challenges the common application of an aridity index value of 0.65 to distinguish arid from humid regions. It suggests that TP grasslands, with an AI of 0.65–0.73, remain primarily water-limited. This insight is crucial for understanding water-energy dynamics in similar ecosystems, calling into question the conventional binary categorization of arid versus humid areas. • Both precipitation and LAI significantly influence evapotranspiration (ET), with LAI playing a major role in the spatial variations of T /ET. • In the shift from arid to humid conditions, the correlation between ET and air temperature tends to increase, while the correlation between ET and SWC tends to decrease. • The transition value between water-limited and energy-limited conditions exceeds the widely accepted threshold of 0.65.