Abstract:

The question of whether rainfall can penetrate a thick unsaturated zone to reach the water table has long been a contentious issue in the field of groundwater recharge. The extreme rainfall event that occurred in the North China Plain from July 23rd to 27th, 2023, equivalent to a large-scale infiltration experiment, provided a valuable opportunity to analyze how groundwater recharge is influenced by the depth to the water table. This study focuses on the Baiyangdian region in the Xiong’an New Area, examining the response patterns of groundwater at different depths during this extreme rainfall event, which delivered a cumulative rainfall of 289.2 mm over three days. Data were collected from four automated monitoring wells in the shallow water table area around Baiyangdian and six automated monitoring wells in the deep water table area within Rongcheng County. In the shallow water table area, vertical infiltration reached the water table approximately 16 hours after the onset of the storm, causing the water table to rise by 1.36 to 1.79 meters. In the deep water table area, river water levels rose rapidly after the storm, leading to leakage that formed a groundwater mound. Due to the backwater effect, groundwater levels within 6 km of the Nanjuhe River rose rapidly by 1.38 to 3.67 meters. This study demonstrates that vertical infiltration through the unsaturated zone is the primary source of groundwater recharge in shallow water table areas. In contrast, in deep water table areas near river channels, post-storm river infiltration and the resulting backwater effect are the main sources of groundwater recharge. These findings improve the understanding of groundwater recharge mechanisms at varying depths and provide valuable insights for analyzing water table recovery processes and the effects of ecological water replenishment in the North China Plain and other regions facing groundwater overexploitation.

Key words: groundwater recharge, phreatic backwater, ecological water replenishment, thick vadose zone, Baiyangdian Lake