Plant water stress in response to rainfall variability is mediated by subsurface water storage, yet the controls on stored plant-available water remain poorly understood. Here we develop a probabilistic water balance model for Mediterranean climates that relates the amount of water stored over the wet season to annual rainfall statistics and subsurface storage capacity in soil and weathered bedrock. This model predicts that low storage capacity-relative to winter rainfall-results in similar year-to-year summer water availability, as both relatively wet and dry winters replenish storage. Observed water balances in seven catchments in the Northern California Coast Ranges exhibited this dynamic. We hypothesized that plants would be decoupled from precipitation variability at these storage-capacity-limited sites and observed that summer productivity and water use (inferred from the enhanced vegetation index) were independent of winter rainfall totals. These areas emerged largely unscathed from recent extreme drought, despite widespread plant mortality elsewhere.