## 5 Discussion
### Surface subsidence and Groundwater Level Change  
The extraction/injection of subsurface resources (e.g., groundwater, natural gas, etc.) in the ground can directly affect the surface subsidence, while the surface subsidence can also reflect the subsurface resource content. Scholars have discovered that in the areas where the extraction of underground resources is more serious, the surface subsidence is more obvious, indicating that there exists a certain relationship between the exploitation of underground resources and the subsidence of the surface [44; 45; 46]. As a key city in China, Shanghai always has a serious problem of water exploitation. According to the  
Figure 8: QX1 Comparison of Precipitation and Subsidence of National Meteorological Station.  
Figure 9: QX2 Comparison of Precipitation and Subsidence of National Meteorological Station.  
\"2018 Shanghai Water Resources Bulletin\" and \"2019 Shanghai Water Resources Bulletin\" published by Shanghai Municipal Water Bureau, Shanghai's water resources in 2018 and 2019 are shown in Table 3. For nine consecutive years, the amount of artificial recharge in Shanghai is far greater than the amount of mining, which is also the fundamental reason for the stability of subsidence in most areas of Shanghai.  
To further study the groundwater level changes and the relationship between the surface subsidence, this paper obtained the distribution in Songjiang district, Jinshan district, and Fengxian in four water level monitoring well data. Compared with the average subsidence in the surrounding 200-m area, the distribution of groundwater level monitoring wells is shown in Figure 3, and the comparison result is shown in Figure 10. The specific analysis is detailed as follows: monitoring wells A and B are located near Songjiang Erzhong, East Zhongshan Road, Songjiang District, and near Tinglin Vocational School, Songjin Road, Jinshan District; monitoring wells C and D are located near Caojing Chemical Plant, Huhang Road, Jinshan District, and near Zhujia Village, Qianqiao Town, Fengxian District.  
The gray correlation analysis method was used to compare the P1-P4 accumulated subsidence curve with the groundwater level change curve. The grey correlation degree was obtained as: 0.65, 0.92, 0.59, 0.64, thereby showing high consistency [47; 48]. When the groundwater level rises, the subsidence of the surface detection points tends to be stable or rises to some extent. Meanwhile, when the local water level decreases obviously, the subsidence of the surface detection points also occurs obviously. In July 2019, the rainy season gradually receded in the lower reaches of the Yangtze River, and the rainfall decreased significantly, thereby decreasing the groundwater level at the monitoring wells significantly. At the same time, P1-P4 experienced significant subsidence to varying  
Figure 10: Time series of surface deformation and groundwater level change at multiple observation points.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters (m3)** & **2018** & **2019** \\ \hline per capita water consumption & 313 & 314 \\ Annual surface runoff & 76.17 \(\times\) 108 & 75.96 \(\times\) 108 \\ Groundwater withdrawals & 2 \(\times\) 106 & 1 \(\times\) 106 \\ Industrial water & 34.41 \(\times\) 108 & 34.02 \(\times\) 108 \\ Domestic water & 13.26 \(\times\) 108 & 13.5 \(\times\) 108 \\ Recharge groundwater & 21.07 \(\times\) 106 & 20.14 \(\times\) 106 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Overview of water resources in Shanghai in 2018 and 2019.  
degrees, thereby verifying the correlation between the groundwater level and the surface subsidence [49; 50; 51; 52; 53].