Observation-constrained long-term simulations of water dynamics and groundwater recharge under intensive agriculture in the North China Plain

Groundwater recharge in intensively farmed regions is increasingly influenced by climate change and human activities. In the North China Plain (NCP), decades of intensive agriculture have driven persistent overextraction, with the groundwater table declining by ∼85 cm/yr to depths of 20–50 m. Quantifying vertical recharge through the thick vadose zone is essential for sustainable groundwater management. Using in-situ observations of volumetric water content and matric potential from a 48-m caisson observatory, we calibrated and validated Hydrus-1D model, and then produced observation-constrained long-term simulations (1990–2023) for a 30-m thick vadose zone. The model reproduces depth-progressive responses to surface water inputs: flux variability is largest in the root zone, whereas deeper layers respond more steadily, especially following extreme rainfall sequences. A persistent zero-flux plane occurs near 4 m depth. Over the 34-year reconstruction, the mean annual potential recharge is 202 mm/yr, equivalent to an average deep infiltration rate of 0.51 mm/d. Groundwater recharge in the NCP predominantly occurs during summer rainfall, with extreme precipitation events enhancing vertical water fluxes by approximately 4.6–7.6 % relative to scenarios with evenly distributed rainfall. These results clarify vertical water movement in thick vadose zones under intensive agriculture and provide practical guidance for regional recharge estimation and sustainable groundwater management.