Water Ages Explain Tradeoffs Between Long‐Term Evapotranspiration and Ecosystem Drought Resilience

The partitioning of terrestrial precipitation into evapotranspiration and river flow shapes ecosystems and water resources. Using observed precipitation, discharge, and satellite‐derived soil moisture and vegetation indices for watersheds located throughout the contiguous United States, we demonstrate that ecosystems maximize long‐term average evapotranspiration when plants transpire older water (i.e., longer time since precipitation or snowmelt). However, ecosystems that transpire older water do so at the expense of decreased drought resilience and maintain higher stem water resilience than those that use younger water, suggesting a growth versus drought resilience tradeoff that is well predicted by water age. These findings indicate that the role of water age integrates processes whereby the relevance of age extends beyond its previously studied effects on biochemical and transport processes, as we reveal it shapes long‐term terrestrial evaporation rates and ecosystem plant functioning under drought stress.