Recharge estimation and sustainability assessment of groundwater resources in the North China Plain

Abstract

Sustainable use of groundwater resources requires a comprehensive understanding of the groundwater flow system, including recharge mechanisms and system dynamics to quantify future availability and variability of groundwater resources in response to climatic conditions (recharge) and human activities (pumping). This dissertation explores sustainability issues in the North China Plain (NCP), a region plagued by one of the worst groundwater overuse and depletion problems globally. The dissertation is organized as three closely related but self-contained papers. The first paper describes the development of a regional, three-dimensional groundwater flow model, and its application in investigating the overall flow dynamics of the groundwater system across the NCP under both predevelopment and post-development conditions. The output from the groundwater model provides estimates of groundwater depletion rates over the post-development period, which average about 4 km^3/yr. Mean annual groundwater recharge of the overall plain, as estimated through calibration of the groundwater model is ~120 mm, which is in reasonable agreement with previously reported values based on the regional water balance method. Groundwater storage depletion, as estimated from groundwater level fluctuation data and from the numerical simulation is highly correlated with variations in precipitation. The numerical model makes it possible to integrate the available hydrologic data, providing a comprehensive approach to evaluate sustainability of groundwater resources in the NCP. The results show a severe imbalance between groundwater recharge and groundwater extraction (the primary discharge). Finally, some strategies that have been conducted towards more sustainable groundwater management in the NCP are discussed. In the work discussed in the second paper, as a way of mitigating the uncertainty inherent in specifying groundwater recharge as a calibration parameter (as commonly done in regional groundwater modeling), a simplified vertical one-dimensional unsaturated zone flow model was coupled with the regional three-dimensional saturated zone flow model. This allows more realistic simulation of the recharge process at the interface between the unsaturated and saturated zones. Soil hydraulic parameters were estimated using pedotransfer functions. Simulation over 12 years (1993¨C2008) was performed across the NCP. Simulated mean annual recharge ranges from ¡Ü360 mm in the piedmont area to ¡Ü260 mm in the middle and coastal plain areas, with a mean of ~150 mm across the NCP; this figure represents 18% of the average annual precipitation plus irrigation. Variability in soil texture and hydraulic properties is primarily responsible for the large range in simulated recharge rates. Increasing thickness of the unsaturated zone with groundwater depletion was shown to have little effect on long term mean groundwater recharge. The third paper discusses the application of direct simulation of groundwater mean age using a solute transport model to help calibrate the flow model parameters, including recharge rates. The simulated age distribution and the recalibrated flow model were then used to characterize the flow regime both under natural conditions and under conditions as altered by groundwater pumping. The model results indicate that simulated groundwater age in the NCP is affected both by the paleo-hydrologic conditions and by extensive groundwater pumping. Flow path analysis, water budget calculations, and the simulated groundwater age distribution all indicate that the lateral flow to the deep aquifer zone in the NCP is limited, and that the primary water input to the deep aquifer zone is downward leakage from the shallow aquifer zone; the extensive pumping of deep groundwater in recent decades has increased this downward flow. These results confirm that regional pumping has altered the flow regime in the deep aquifer zone, and that widely distributed vertical leakage has become a dominant process shaping the flow pattern both in the shallow and deep aquifers of the NCP.