Harnessing Subsurface Flow at the Soil‐Bedrock Interface as a Hidden Water Resource for Rainwater Harvesting: Insights From Long‐Term Hydrological Monitoring on a Humid Karst Hillslope

Rainwater harvesting (RWH) systems are crucial for mitigating water scarcity in karst landscapes; however, their efficiency remains low even in high‐rainfall areas due to rapid infiltration and limited surface water retention. This study explores the potential of subsurface flow (SSF) at the soil‐bedrock interface as an underutilized water resource to enhance the efficiency of traditional surface flow (SF)‐based RWH systems. Over three hydrological years (2019–2022), we monitored 12 experimental plots on a humid karst hillslope across 159 rainfall events, comparing SSF and SF contributions to total runoff and their implications for RWH efficiency. Our results show that SSF significantly outperforms SF, especially during the rainy seasons. Incorporating SSF into RWH systems increased water collection efficiency from 2.2% in traditional SF‐based systems to 13.1%. Key structural factors influencing SSF generation include soil thickness and bedrock weathering degree, with shallow soils (<50 cm) and weakly weathered bedrock serving as hotspots for SSF. Rainfall intensity and antecedent rainfall events were the key meteorological drivers of SSF. This study highlights the need for SSF‐focused RWH designs in karst landscapes, offering a practical solution to enhance water availability where conventional cisterns fail. The findings have broader implications for water resource management in similar geological settings.