Integrated surface water and groundwater modeling of the Baoding Plain in northern China : towards groundwater sustainability through managed aquifer recharge

Abstract

The North China Plain (NCP, 140,000 km2) is one of the global groundwater depletion hotspots. Located in the northwest the NCP, the Baoding Plain (BDP, 10,516 km2), especially its more permeable piedmont area where the aqueduct of the South to North Water Diversion (SNWD) project runs through with an annual allocation of 0.70 km3 of the SNDW water, offers an opportunity to resolve the groundwater imbalance of the NCP through large-scale managed aquifer recharge (MAR). This opportunity is currently being explored by the Ministry of Water Resources in China with in-channel recharge amounting to 0.27-0.73 km3 per year in several rivers in the BDP since 2017. However, quantitative assessment is lacking regarding the long term impact of such undertaking on groundwater sustainability. Therefore, this thesis seeks to provide definitive answers to this question primarily through integrated surface water and groundwater modeling. The key findings are: Although considerable efforts were devoted for establishing a heterogenous hydrostratigraphic model, the borehole density of 0.06 per km2 in the BDP was found to be insufficient to derive a geologically meaningful lateral mean length of sand. A lateral mean length of 2.4 km was adopted holistically considering basin depositional environment to construct a heterogenous hydrostratigraphic model to compare with a homogeneous hydrostratigraphic model consisted of 3 hydrogeological units. Incorporating the heterogenous hydrostratigraphic models into an integrated surface water and groundwater model found that the groundwater storage (GWS) in the BDP declined at a rate of -1.26 ± 0.33 km3/yr (or -0.45 ± 0.1 m/yr) between 2000 and 2015, with irrigation use of 1.1 km3/yr driving this decline. In-channel recharge through 4 rivers (Nanjuma, Pu, Tang, Zhulong) augments the recharge by only 0.62 km3/yr, which is equivalent to only 20.2% of the annual GWS deficit. Among 8 scenarios investigated, only one emerges to come close to resolve the groundwater imbalance. It calls for a drastic measure of 20% irrigation reduction to provide “in-lieu” recharge of 0.21 km3/yr, transporting 0.35 km3/yr urban reclaimed water for irrigation in rural areas through agricultural MAR (Ag-MAR), plus 0.62 km3/yr of in-channel recharge using SNWD. This would restore up to 84.3% of the annual GWS deficit. The thesis also investigated where the recharge water would go, finding that 5%, 54%, 100% of the augmented recharged water through Ag-MAR in piedmont area would reach the deep aquifer after 50 years,100 years, and 300 years using the heterogenous hydrostratigraphy. In conclusion, bold and sustained efforts are needed to cure the groundwater depletion of the NCP. In addition to existing water resource allocation to ensure a large portion of SNWD to augment in-channel recharge, water demand management through significant reduction in irrigation water withdrawal and long-distance transfer infrastructure to bring urban reclaimed water for rural irrigation supply are essential.