Effectiveness of the South-to-North Water Diversion Middle Route Project on Water Delivery and Groundwater Recovery in North China Plain

The South-to-North Water Diversion Middle Route Project (MRP) was designed to transfer water from the Danjiangkou Reservoir (DR) in the southern part of Hanjiang River Basin to the water-scarce North China Plain (NCP). Since the start of its operation in December 2014, MRP-delivered water is expected to alleviate water shortage and long-term groundwater storage (GWS) depletion in NCP. However, two issues related to the effectiveness of MRP are still being debated: 1) whether DR can meet the designed water delivery target (9.5 km3/yr) without affecting downstream water supply; 2) whether MRP-delivered water can help recover the declining groundwater level in NCP. To clarify these two issues, the ensemble simulations were performed by a dynamic water diversion model to study the effectiveness of MRP in both water delivery and groundwater recovery. The observed groundwater level data from 617 wells and the estimates of water storage change from GRACE and GRACE Follow-On were used to analyze the sub-regional GWS variations in NCP. Two study periods, before (20032014) and after (20152018) the MRP operation, are identified for separate analyses. The results showed that annual downstream water demand of DR cannot exceed 23.0 km3/yr to guarantee annual water diversion volume reaching the designed target. On average in NCP, a decreasing trend of 19.15.1 mm/yr was seen in GWS based on the well observations during 20032014, but a recovery trend of +1.80.7 mm/yr was found during 20152018. The GWS recovery was most prominent in subregions where groundwater over-utilization had occurred in NCP. GRACE exhibited the capacity to detect the regional GWS depletion during 20032014, but difficult to distinguish the sub-regional GWS recovery during 20152018. The potential causes for GWS recovery were found to be complicated, not only caused by the reduction of groundwater pumping as accelerated by MRP-delivered water, but also the increasing precipitation recharge of aquifers and the enhanced management of groundwater system. The findings well demonstrated the effectiveness of MRP in meeting water delivery target and revealed that the GWS in NCP has started a gradual transition from depletion to recovery, but it remains to be observed and investigated whether the trend will be reversed again under future climate change effects.