Groundwater Storage Variation Characteristics in North China before and after the South-to-North Water Diversion Project Based on GRACE and GPS Data

The North China Plain (NCP) has been in a state of groundwater depletion for a long time, which led to a widespread vertical ground subsidence. To reveal the groundwater storage (GWS) variation characteristics in NCP in recent years, this paper uses Gravity Recovery and Climate Experiment (GRACE) and its Follow-On RL06 monthly gravity solutions to obtain the terrestrial water storage (TWS) changes in NCP (February 2011 to March 2021) and subtract the hydrological components from Global Land Data Assimilation System (GLDAS). Thus, the GWS decline rate was revealed at long-term rate of -2.94 cm/year and changes in 2013 and 2015. The groundwater depletion rate of -0.18 cm/year from 2011 to 2013, accelerated to -3.91 cm/year from 2013 to 2015, and the rate of GWS eased to -3.02 cm/year from 2015 to March 2021. The vertical displacements obtained by solving the Continuous Global Positioning System (CGPS) observations reveal a significant relief trend of surface subsidence at some stations after the Middle Route Project (MRP) completion. As well combined the special geological conditions of the basement type of CGPS stations (bedrock or sediment), and the overall vertical displacement of the east central plain (ECP) and the piedmont plain (PP) was calculated respectively using the Thiessen polygon approach. Correlation analysis between vertical displacements and GWS timeseries was performed, with Pearson correlation coefficients (PR) of 0.89 and -0.74, respectively. The GWS changes of the ECP were further correlated with the vertical displacement changes of the four sediment CGPS stations distributed in the southern part of the ECP. The results showed that the PR of three sediment stations, HECX, TJBH and TJWQ, were all around 0.85, and the vertical displacements of the three stations are consistent with groundwater depletion. All these indicate that the depletion of groundwater has affected the surface subsidence rate to some extent, and the vertical displacement changes of sediment stations can be used to infer the depletion of GWS.