Deciphering spatio-temporal variation of groundwater recharge patterns in the large irrigation district.

Global warming and human activities have an inconvenient impact on groundwater systems, frequent agricultural irrigation may alter the groundwater recharge and circulation. However, research on the spatio-temporal variations of groundwater recharge patterns in large irrigation districts remains insufficient. Focusing on the Yellow River South Bank Irrigation District in Inner Mongolia, this study analyzed the hydrochemical and stable isotopic compositions of canal water, river water, and groundwater during April, July and November to explored the spatio-temporal evolution of groundwater recharge pattern. The δ2H and δ18O analysis indicates that the main recharge sources of groundwater are lateral runoff, canal water, and river water. The spatial variation of isotopic compositions in three water bodies along the flow path exhibit seasonal variation. The primary recharge from upstream to downstream changes from lateral runoff (> 50%) to canal infiltration (> 50%), with a most contribution of river infiltration (> 40%) in the midstream in April (spring). Groundwater recharge is dominated by lateral runoff and river infiltration in July (summer), which is affected by precipitation (> 40%) in the downstream. Canal infiltration (> 35%) and lateral runoff (> 35%) mainly replenishes groundwater in November (autumn). Reverse hydrogeochemical modeling further validated the groundwater recharge sources. Groundwater hydrochemistry is predominantly governed by multiple mineral dissolution/precipitation, while also being influenced by seasonal variations in recharge sources. This study provides important insights for protecting vulnerable riverbank agricultural groundwater systems.