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

Abstract. Agriculture on the North China Plain (NCP), home to over 300 million people, heavily relies on groundwater extraction to feed its irrigation systems. This has created a large groundwater depression zone up to 80 m deep, severely limiting sustainable groundwater extraction and crop-production. Effective recharge is essential to restore this depleted zone and secure future sustainability. Few studies, however, have quantified recharge delays and efficiencies in deep vadose zones with complex lithology. Here we simulated infiltration times and percolation velocities in the Ningbailong groundwater depression zone, a typical overexploited site in the NCP using HYDRUS-1D with measured borehole lithology and hydro-meteorological data. Two infiltration modes were considered: precipitation-fed and riverbed infiltration. Spatial distributions of infiltration times and percolation velocities were obtained, and recharge efficiencies were compared between these two infiltrations. Results showed that times for precipitation-fed recharge averaged 446 days and varied with lithology and thickness, from 10 days in western Taihang foothills (dominated by coarse sands) to 1,395 days in central/eastern plains (finer clays and loams). Riverbed recharge was markedly faster, averaging 91 days, indicating higher efficiency than precipitation under equivalent lithological conditions. Regression equations were derived to predict percolation velocities from vadose zone thickness and soil particle fractions. These findings elucidate how vadose zone thickness and lithology amplify recharge 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 in similar depression zones.