Continuous shallow groundwater decline and accidental extreme precipitation control the soil nitrate leaching of a well-irrigated area in the North China Plain

Study Region: A well-irrigated area along the Yellow River in the North China Plain. Study Focus: High nitrate concentrations in groundwater induced by soil nitrate leaching are found worldwide. In this research, we examined the main processes and mechanisms of nitrate leaching with 16 years of long-term observational data to evaluate the potential threat of nitrate pollution to the groundwater in a well-irrigated area. New Hydrological Insights for the Region: This study uncovered significant differences in the primary mechanisms of nitrate leaching in the region at different stages in response to the continuous groundwater table decline. In the first stage, groundwater tables were less than 5 m, nitrate leaching was mainly driven by irrigation or precipitation with average annual percolation of 116.8 mm. In the second stage, groundwater overexploitation led to a decline in the groundwater table, inducing the internal drainage and nitrate leaching of the lower root zone. In the third stage, a further decline of groundwater table almost cut off the hydrological connection of root zone to the groundwater, which hampered water percolation and thus nitrate leaching from the root zone during next irrigation or precipitation. However, one extreme precipitation as much as in 2021 washed out almost all the accumulated nitrate in the root zone. The findings provide a reference for nitrogen management and groundwater protection in similar regions.