Shaping Subsurface Water Storage Dynamics: Climatic, Topographic, and Geologic Controls in Forested Headwater Catchments

Understanding how catchments regulate water storage dynamics is crucial for predicting hydrological responses to environmental change. These dynamics unfold within the critical zone, the near-surface layer extending from the canopy to fresh bedrock, where water mediates coupling among climate, vegetation, and geology. We analyzed water storage dynamics in 33 forested headwater catchments spanning a broad range of climatic and geomorphic settings in Japan and the United States. A modified recession analysis incorporating evapotranspiration was applied to quantify storage–discharge relationships and the storage sensitivity of streamflow (εS). We then evaluated the influence of climatic, topographic, and geologic attributes (e.g., annual precipitation, snow ratio, slope, drainage density, lithology, and weathering depth) on εS. Values of εS varied substantially among catchments (0.006–0.215 mm−1). Multivariate analyses showed that subsurface and geomorphic attributes predominantly controlled εS, with lithology and weathering depth emerging as the best predictors. Weathering depth correlated negatively with εS (R = −0.518; p = 0.002), indicating enhanced deep infiltration and prolonged drainage in deeply weathered bedrock. Budyko-based water-balance analysis further showed that deeply weathered catchments with low εS align more closely with the theoretical curve, suggesting larger effective subsurface storage and more sustained baseflow under variable climatic conditions. Our findings revealed that weathering depth, which integrates subsurface critical-zone development through long-term interactions among climate, topography, and geology, to be a primary control on catchment-scale storage dynamics. These insights highlight the importance of subsurface critical-zone structure for water availability, flood potential, and hydrological resilience.