Groundwater recharge responses to vadose zone thickness variation and rainfall patterns in arid Mongolian pine plantations

In arid and semi-arid regions, vadose zone thickness strongly influences precipitation infiltration and groundwater recharge, both increasingly affected by climate-driven shifts in rainfall patterns. However, its influence on soil water dynamics and groundwater recharge in plantation ecosystems remains inadequately understood. This study, conducted in the Mu Us Sandy Land of China, monitored stable isotopes, soil water, and groundwater levels over two years across three Mongolian pine plantations. These sites had different initial groundwater depths: 4 m (downland), 9 m (midland), and 13 m (upland). Key findings include: (1) Downland exhibited significantly greater mean soil water content (SWC) in the middle and deep soil layers than in the midland and upland (p < 0.05). (2) The soil desiccation index was higher in the midland and upland than in the downland. SWC increased in the upper and middle layers from the dry to the rainy season, but deep SWC decreased in the upland. (3) Rainfall events ≥20 mm day⁻¹ replenish deep SWC in the downland, whereas ≥30 mm day⁻¹ was required in the midland and upland. (4) In 2022, precipitation was more concentrated in ≥10 mm·day⁻¹ events, totaling 311 mm (74.5 % of the annual total), and produced greater groundwater recharge than in 2023, when such events accounted for only 190 mm (56.5 %). These findings demonstrate that vadose zone thickness and rainfall intensity thresholds critically control groundwater recharge—a process further modulated by Mongolian pine plantations. Therefore, integrating these factors into management strategies is essential for sustainable groundwater conservation in arid regions.