To explore the influence mechanism of tides on the intrusion of saline water into adjacent aquifers in estuaries, a two-dimensional transient fully coupled numerical model of river-aquifer parallel to the river was established based on the consideration of the variation of water level and salinity along the river under the action of tides. The flow field and salinity distribution in the river and aquifer under the action of tides were analyzed and compared with those under the condition without tides. The results show that tides promote the rapid intrusion of seawater into the upstream of the river through the estuary and then infiltrate into the adjacent shallow aquifer, forming salt fingers and local circulation, resulting in a large-scale salinization of the adjacent shallow aquifer. Compared with the seawater intrusion in coastal aquifers and the corresponding conditions without tides, the salinization degree and range are significantly increased. Within a single tidal cycle, the degree and range of saline water intrusion in the aquifer fluctuate with the tides, reaching the strongest in the middle of the ebb tide, with a lag compared to the tidal water level changes. Part of the infiltrated salt accumulates in the adjacent shallow aquifer and diffuses to the deep aquifer, and part is discharged to the river with inland groundwater and then flushed to the ocean by inland river water.

Guided by the concepts of spatial control and target control in the Regulations on Groundwater Management, this study classifies the current research progress and status of groundwater overexploitation control and dual control of groundwater quantity and water level systems. Summary are that the research on groundwater overexploitation control system is increasingly closely integrated with China’s specific national conditions, but the relevant management systems and norms still need to be improved urgently. The groundwater overexploitation control system clearly defines the division standards for areas that have already experienced overexploitation, but lacks a unified and comprehensive groundwater resource control zoning standard. The dual control system of groundwater quantity and water level overcomes the limitations of unilateral control of water level or water quantity, failure to consider the inherent relationship between groundwater level and water quantity, and difficulty in effectively alleviating ecological and environmental problems caused by changes in water level or water quantity. The difficulty in the study of water volume and water level dual control system lies in the fact that the water level in the point distribution can be monitored to control the mining output of the areal distribution. It is suggested to control the groundwater exploitation in a certain period by managing the water level in a specific time, or control the point water level by controlling the areal mining output.

To explore water-saving irrigation strategies for crops in the North China Plain under the joint control of groundwater level, irrigation water use, and crop yield, a multi-objective optimization simulation framework for water-saving irrigation, named irrigmoo, was developed. This framework integrates a groundwater numerical model based on FloPy, a crop water requirement model, and the NSGA-Ⅱ multi-objective optimization model. Using Cangzhou City in the central-eastern part of the North China Plain as the study area, field data were collected to calibrate the groundwater numerical model, construct the crop water requirement model, and configure the NSGA-Ⅱ multi-objective optimization model. A set of Pareto non-dominated solutions for irrigation schemes under limited water conditions was obtained, and representative groundwater extraction reduction and alternative water source substitution schemes were selected. The results indicate that reducing the proportion of groundwater in irrigation water by 42. 13% and replacing 15. 28% of groundwater with alternative water sources could lead to a 2. 36 m rise in deep groundwater levels, a 26. 86% reduction in irrigation water use, and a 3. 94% decrease in winter wheat yield. If groundwater extraction for irrigation is completely banned and 37% of the current extraction is replaced with alternative water sources, the deep groundwater level could rise by 8. 95 m, irrigation water use could be reduced by 62. 87%, and winter wheat yield would be decreased by 12. 7%, while still remaining within yield constraints.