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Shenzhen is the major financial and high-tech center in southern China. The megacity has grown rapidly in the last 40 years with the population increasing from about 30,000 in 1979 to 20 million in 2016. The study area (2,015 km 2) is about 42% urban and 58% undeveloped land. The rapid development of the megacity has resulted in severe degradation of the groundwater and surface-water resources and has created a nearly insatiable demand for water, with an average consumption of 2000 × 10 6 m 3 / year. Groundwater is an important component of the baseflow of the many streams in the area and is used for potable water supply and irrigation in some of the rural parts of the municipality. This study develops a conceptual model and quantitative framework for assessing the groundwater resources of Shenzhen. The groundwater system consists of shallow aquifers of alluvium and weathered bedrock overlying low permeability igneous and sedimentary rocks. The complex geologic setting was conceptualized as a block structure with blocks bounded by high-angle faults. The water budget in Shenzhen was quantified. The estimated average groundwater discharge is about 12% of annual precipitation. The study provides a starting point to investigate how a megacity such as Shenzhen should manage and protect its groundwater as a strategic resource and environmental asset. It is also a basic management tool for analyzing and contributing to urban drainage concepts such as the Bsponge city^.

Numerical modeling of water movement in both unsaturated soils and saturated groundwater aquifers is important for water resource management simulations. The development of efficient numerical algorithms for coupling unsaturated and saturated flow has been a long-lasting challenge in hydrologic modeling, especially for regional-scale modeling. In this study, a new method coupling the Finite Element Method (FEM) and Finite Difference Method (FDM), FE-FDM, is developed to solve Richards equation for simulating unsaturated–saturated water flow. The FEM is adopted to discretize the governing equation in the horizontal direction, while the FDM is used in the vertical direction. This method combines the advantages of FEM in domain discretization, especially for complex computational domain, and the advantages of FDM in modeling simplicity and efficiency. The validity of the new method is demonstrated with three test cases that show that the FE-FDM solutions are accurate and are applicable for regional scale problems. In the test cases, the FE-FDM solutions are compared with experimental data and numerical results calculated with common software packages including FEFLOW and COMSOL. This study verified that the FE-FDM is applicable for simulating water flow in the unsaturated–saturated zone.

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