Topography and unsaturated zone thickness influence groundwater drought response and recovery in karst critical zone

Understanding groundwater response to and recovery from drought is essential under intensifying drought, yet the roles of topography and soil-bedrock structure in regulating groundwater dynamics remain poorly understood in karst aquifers. This study examined seven monitoring boreholes across soil-covered hillslope (SH) and rock-exposed hillslope (RH), including upslope, midslope, downslope, and depression. Hourly groundwater levels and weekly δ 2 H and δ 18 O data from 2021 to 2024 were used to quantify rainfall response thresholds, recharge proportions, and mean transit time (MTT) during predrought, drought, and recovery periods. The results indicated that drought reduced groundwater level kurtosis and skewness to 0.14 and 0.87, respectively, and shifted groundwater isotopes closer to the multi-year meteoric water line. Specifically, the SH showed a larger increase in weekly rainfall response threshold (13.0 mm), but smaller reductions in recharge proportion (6.4%) and MTT prolongation (41 d) during drought period. Along the SH, groundwater level variability decreased downslope, whereas low-water maintenance capacity increased. During recovery period, groundwater levels and low-water maintenance returned to predrought levels on the SH, indicating stronger drought regulation by thicker soil and weathered bedrock. In contrast, the RH responded more rapidly to drought (76 d), but showed a longer impact duration (279 d) with isotope evidence of stronger recent precipitation input and greater spatial heterogeneity caused by fractures and conduits. The PLS-SEM explained 79% of groundwater level variation and showed that slope and outcrop rate were the main structural factors associated with drought response. These findings indicated that steeper and more rock-exposed hillslopes were more drought-sensitive and had weaker recovery capacity, providing a basis for drought assessment and critical zone management.