Quantification of delayed recharge by soil surface and riverbed infiltration in a deep groundwater depression zone in the North China Plain

Agriculture on the North China Plain (NCP), hometo over 300 million people, heavily relies on groundwater extraction to feed its irrigation systems. This has created a largegroundwater depression zone up to 80 m deep, severely limiting sustainable groundwater extraction and crop production.Effective recharge is essential to restore this depleted zoneand secure future sustainability. Few studies have quantifiedrecharge delays and efficiencies in deep vadose zones withcomplex lithology. Here we simulated infiltration times andpercolation velocities in the Ningbailong groundwater depression zone, a typical overexploited site in the NCP usingHYDRUS-1D with measured borehole lithology and hydrometeorological data. Two infiltration modes, precipitationfed soil infiltration and riverbed infiltration, were considered.Spatial distributions of infiltration times and percolation velocities were obtained, and recharge efficiencies were compared between these two infiltration modes. Results showedthat times for precipitation-fed recharge averaged 446 d andvaried with lithology and thickness, from 10 d in western Taihang foothills (dominated by coarse sands) to 1395 d in central/eastern plains (finer clays and loams). Riverbed rechargewas markedly faster, averaging 91 d, indicating higher infiltration efficiency than precipitation under equivalent lithological conditions. Regression equations were derived to predict percolation velocities using vadose zone thickness, sandfraction, and clay fraction as key predictors. These findingselucidate how vadose zone thickness and lithology amplifyrecharge lags and control recharge efficiency. They also highlight the potential for managed aquifer recharge strategies,such as constructing infiltration basins for flood capture, offering strategies to reduce groundwater over-exploitation insimilar depression zones.