Weathering Increases Bedrock Water Storage Capacity but Not Necessarily Its Temporal Persistence in Humid Karst Hillslopes

Abstract. Bedrock moisture plays a crucial role in buffering drought, sustaining vegetation, and recharging groundwater in mountainous landscapes with shallow soils. Stronger weathering is generally assumed to enhance these functions by increasing bedrock water availability. However, our understanding of how weathering regulates the storage, retention, and release of rock moisture is still limited. In this study, using two years of high-frequency moisture observations, we quantified relative water storage, temporal stability, and storage-deficit dynamics across the soil–bedrock continuums of two adjacent hillslopes with contrasting weathering intensities in a representative humid karst catchment. Despite having 21% greater maximum dynamic storage capacity than the weakly weathered hillslope (WWH), the strongly weathered hillslope (SWH) realized only 91.65% of the water storage observed at WWH, indicating that greater storage space did not translate into greater realized storage. Strong weathering promoted rapid infiltration and drainage, thereby reducing bedrock water-storage persistence, as indicated by the faster groundwater recession and less persistent shallow groundwater at the SWH. More importantly, these hydrological contrasts also shaped the responses of the two hillslopes to meteorological drought, with the SWH experiencing a greater decline in water storage than the WWH when annual precipitation notably reduced. This greater sensitivity to reduced rainfall may partly explain the shorter trees and smaller stem diameters observed at the SWH. Accordingly, these results challenge the assumption that stronger weathering could increase water availability, providing key insights into drought resilience, groundwater recharge, and runoff generation in karst critical zones.