Functional Relationships Reveal Large Differences in Streamflow Response to eCO₂‐Vegetation in Global Water Models

Abstract Rising CO 2 levels substantially impact vegetation characteristics and potentially the water cycle. Using 14 global water models, we assess how elevated CO 2 (eCO 2 ) affects streamflow ( Q ) via vegetation dynamics in the period 1901 to 2020. Our results reveal considerable variability in model simulations, with discrepancies not only in magnitude but also in the directional trends of Q changes. To study these differences, we compared model outputs against observations from 1,116 small, unimpacted catchments. Functional relationships between climate variables, such as precipitation ( P ), aridity, and temperature ( T ), and eCO 2 ‐vegetation‐driven streamflow () changes show large deviations between models and observations, especially in response to changing leaf area index (LAI). No model successfully captured the complex non‐linear interactions between vegetation and Q visible in the observations, highlighting the challenges in accurately modeling eCO 2 ‐vegetation effects on the water cycle, particularly in quantifying impacts of changes in LAI and stomatal behavior on Q .