Event- and annual-scale precipitation extremes enhance groundwater recharge at the ecological restoration catchment of hilly and gully region

Precipitation extremes and ecological restoration projects significantly influenced hydrological processes by mitigating or aggravating groundwater depletion within the Earth's critical zone. However, scientific evidence remains limited due to the strong dependence of deep recharge on both unsaturated zone thickness and precipitation event magnitude. Here, we analyzed seven-year field datasets encompassing precipitation, soil water content (SWC), surface water (reservoir water), and groundwater from an ecological restoration catchment on the Chinese Loess Plateau. Precipitation extremes triggered deep hydraulic connectivity between the unsaturated and saturated zones mainly through preferential flow pathways, as evidenced by depleted of δ¹⁸O, increased of SWC profiles, and rising water table. Temporal-spatial patterns of SWC (0–4000 cm profile) revealed preferential flow pathways on a sunny slope with recharge efficiency regulated by precipitation patterns and topography. In the gully, water tables showed a positive correlation with precipitation amount, duration, and initial SWC. These hydrological drivers induced significant differences in water table changes among precipitation years. In contrast, the slope (0–500 cm profile) maintained persistent water deficits with limited recharge response, despite precipitation inputs (except for a 167.7-mm event). Furthermore, precipitation variability coupled with plant root uptake altered the vertical soil water gradient, with the 200–300 cm layer functioning as a hydraulic buffer. However, prolonged drought triggered an accelerated water table recession at 60.8 mm/year, while creating carry-over soil water deficits that extended beyond dry years, sustaining 15.4 mm/year depletion even in the following normal year. When considering the saturated zone contribution, total recharge rates demonstrated the positive hydrological feedback of ecological restoration projects to annual precipitation, accounting for 45 % of precipitation (327.8 mm/year) during a wet year. Therefore, it can be inferred that event- and annual-scale precipitation extremes enhance groundwater recharge at the ecological restoration catchment. These findings provide critical scientific support for maintaining groundwater sustainability in such systems.