Understanding the different responses from the similarity between displacement and groundwater level time series in Beijing, China

The quantitative relationship between the groundwater level (GWL) and land subsidence is essential to understand, prevent and control subsidence. This study compares the subsidence due to GWL variation at different locations and aquifer layers during the operation of the South-to-North Water Diversion Project Central Route (SNWDP-CR) in China. Sixty-nine synthetic aperture radar (SAR) images in Beijing from 2012 to 2017 are obtained to calculate the displacement using the interferometry technique. The results indicate that the subsidence with uplift or mitigation is consistent with groundwater rise or drawdown deceleration. The dynamic time warping (DTW) method is applied to compare the similarity between subsidence and GWL time series waveforms. We find that the similarity between these two activities tends to increase after the SNWDP-CR. The similarity between the subsidence and confined GWL is controlled by the hydrogeological setting. In general, it tends to increase from the upper part to the lower part of the alluvial fan. However, the similarity between the subsidence and unconfined GWL shows irregular pattern due to rainfall interference. The average similarity between the subsidence and GWL at depths of 100-180 m and that at over 250 m is 75% and 64%, respectively, prior to the SNWDP-CR. In contrast, these two values increase to 83% and 81%, respectively, after the SNWDP-CR. This indicates that the impact of deeper aquifers to the subsidence increases after the SNWDP-CR, and the depth of the contributing aquifer is 100-180 m and locally exceeds 250 m. This study provides technical support for strategic decisions on water resource management and subsidence prevention within the context of the SNWDP-CR.