According to the definition of the barrier effect of underground structures, the concept of equivalent barrier depth was introduced. Analytical solutions for the groundwater head, groundwater retention time distribution, and average retention time in the urban confined aquifer under three boundary conditions of constant flow rate, constant head, and diffuse recharge were derived. Numerical experiments were set up to verify the results, and the results show that the analytical solution is in good agreement with the numerical solution. The results demonstrate that the analytical solution is in strong agreement with the numerical solution. However, the fitting performance deteriorates due to hydrodynamic dispersion when the dimensions of the underground structure are relatively large. Moreover, the analytical solutions of the average retention time agree well with the numerical results in various simulation cases (with a relative error of no more than 5.94%). Even in the three-dimensional flow field, the derived analytical solutions can be used to estimate the average retention time of confined aquifers. Finally, the analytical solution is applied to the confined aquifer flow field before and after the construction of four subway stations in Chengdu Metro Line 2. The calculation results show that compared to the head, the retention time is more sensitive to the parameters of underground structures.

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.