Spatio-temporal variations and driving mechanisms of groundwater recharge in the Inner Mongolia Reaches of the Yellow River Basin, China: an approach combining isotope hydrology and statistics

Unraveling the groundwater recharge process is crucial for water resource sustainability. However, systematic studies of groundwater recharge in large river basins are still limited due to the strong anthropogenic impacts and complex hydrogeologic conditions. This study investigated the spatio-temporal variations and driving mechanisms of groundwater recharge in the Inner Mongolia Reaches of the Yellow River Basin (IMR-YRB) based on 396 shallow groundwater samples collected from 132 sites during the wet, dry, and normal seasons, and combined with hydrogen (δD) and oxygen (δ 18 O) isotopes and statistical methods. The backward wind trajectory analysis indicated that the water vapor firstly reached the high elevation area, with over 70% of wet season precipitation originating from westerly air masses and 24% from southeast Pacific moisture, consistent with precipitation infiltration being an important groundwater recharge pathway. However, significant spatio-temporal variation in the isotopic composition of groundwater was observed, suggesting the influence of additional recharge processes. Statistically, most of the study area (> 50%) showed δ 18 O enrichment and d-excess depletion in groundwater, indicating that evaporation exerted a strong influence on the groundwater recharge process. Specifically, the three principal components extracted by principal component analysis (PCA) explained 73.108% (dry − wet) and 80.756% (normal − dry) of the total variance, suggesting the primary groundwater recharge mechanisms: (1) a potential contribution from deep groundwater, showing a spreading trend from higher to lower elevation zones; (2) lateral runoff recharge affected by evaporation, which was more intensive in the lower and middle reaches than in the upper reaches; and (3) vertical recharge from irrigation return flow, which was predominant in the irrigation districts during the intensive irrigation periods. This study provides new insights into the groundwater recharge in the IMR-YRB and other large river basins, and offers a coupled isotope-statistics analytical framework for basins with similar semi-arid and irrigation-intensive characteristics.