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. Plain Language Summary Most terrestrial precipitation does not end up in streams but returns to the atmosphere as evapotranspiration (ET). What controls long‐term ET rates and dictates the effects of drought on transpiration and vegetation dynamics remains hard to generalize. Here we link the concept of ET water age to these variables to show that ecosystems that transpire older water have higher ET rates but do so at the expense of decreased drought resilience than those that use younger water. This indicates that water age is critical to the terrestrial water balance and plant functioning. Key Points We modeled evapotranspiration (ET) water ages for 472 watersheds across contiguous United States Older ET water ages are associated with ET rates closer to the upper physical limit, but lower vegetation resilience ET water ages explain tradeoffs between long‐term evapotranspiration and ecosystem drought resilience