Hydroclimatic Variability Shapes Long‐Term Water Balance

Abstract Long‐term water availability is commonly interpreted through mean precipitation () and potential evapotranspiration (), yet hydroclimatic variability spans seasonal to event scales and may influence water partitioning beyond what mean climate alone captures. Although the aridity index () explains much of the spatial variability in evaporative fraction (), systematic departures from classical relationships suggest that additional modes of variability shape long‐term . Here, using large‐sample observations from catchments across the United States, Australia, and Brazil, we show that seasonal covariance, monthly variance, and event‐scale storm structure leave persistent, organized imprints on long‐term water balance. We introduce a hypothesis‐driven expansion of the aridity framework that incorporates these variability terms and demonstrates that sub‐annual climate organization can be explicitly embedded within long‐term water‐balance theory. This variability‐aware perspective provides a mechanistic pathway for assessing how shifts in climate variability, even under approximately stationary long‐term means, may propagate into future water yields.