## 4 Results: Spatial--Temporal Evolution of Subsidence
### Land Subsidence in the Urban Areas  
Zhanjiang is the largest city, the political and economic center of the LZP, with a population of 7.45 million until 2009. It has a long history of land subsidence, but only a few leveling measurements in the LZP from 1984 to 2001, which can help validate the InSAR measurement. Hence, we selected the area (latitude: 21.105\({}^{\circ}\)-21.330\({}^{\circ}\), longitude: 110.281\({}^{\circ}\)-110.435\({}^{\circ}\)) with historic leveling measurements as the experiment area. The surface deformation of the chosen area is shown in Figure 7a-c. We chose nine representative subsidence bowls with a segmentation based on a threshold of mean velocity (0.9 mm/year), marked by red dashed lines in Figure 7, to demonstrate the spatial-temporal features of land subsidence. The land use of this city is showed by the inset in Figure 2c, where we can see that six bowls (bowl 1, 2, 5, 6, 7, and 8) are in the urban areas, and two bowls (bowl 3 and 4) are in the nearby agricultural area of Xiashan and Mazhang. Bowl 9 appeared in the suburb area. The subsidence areas of these nine bowls are calculated and shown in Table 1. \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline \multirow{2}{*}{**Data**} & **B1 (U)** & **B2 (U)** & **B3 (A)** & **B4 (A)** & **B5 (U)** & **B6 (U)** & **B7 (U)** & **B8 (U)** & **B9 (S)** \\ \cline{2-10}  & \multicolumn{6}{c}{**Areas: km\({}^{2}\) (Bowl: B, Urban area: U, Agricultural area: A, Suburban area; S)**} \\ \hline JERS & 3.3 & 1.9 & 1.8 & 1.6 & 2.3 & 9.7 & 1.4 & 2.1 & 0.9 \\ ENVISAT & - & - & - & - & - & 10.7 & 2.8 & 10.8 & 4.3 \\ ALOS1 & 3.0 & 0.8 & 4.8 & 6.1 & 2.0 & 6.1 & 2.7 & 6.1 & 12.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Area of subsidence bowls in Figure 7.  
Figure 6: Relationship between soft soil thickness and mean vertical velocity. The white bars represent the number of pixels and the red diamonds represent the calculated mean velocity for each soft soil thickness interval. In addition to the average subsidence rate, we used SBAS-InSAR to generate the deformation time-series of these bowls. Four bowls with large subsidence rates, i.e., bowls 1, 4, 8, and 9, were selected to analyze the development of subsidence in the city, see Figure 8. Pixels located at or near the central of the bowl observed by each satellite were selected and plotted in Figure 7. In urban areas, such as bowl 1 and bowl 8, the deformation time-series approximates to linear, as observed in other areas [10]. The continual groundwater extraction will change the stress between confined aquifers and lead to the compaction of the compressible sediments, resulting finally in the land subsidence. However, the magnitudes of subsidence rate in these two bowls are different. Bowl 1 experienced two major changes, the subsidence rate first slowed down from 13 (Figure 8e) to 4 mm/year (Figure 8i) and then increased from 4 (Figure 8e) to 29 mm/year (Figure 8m), whereas bowl 8 increased during the periods (Figure 8g,k,o). Unlike the results of bowls 1 and 8, the subsidence of bowl 9 is complicated. In the first stage (Figure 8h), the deformation time-series trend is nonlinear, which is mainly caused by the seasonal groundwater pumping for irrigation or aquaculture along the coastline. For example, the uplift trends occur at summer (May 1995, July 1995, and August 1998) while the fall trends occur at spring (February 1997 and March 1998). The fall trends occurring at non-irrigation periods in this case may be caused by the delay from aquifer compaction to deformation. During the second stage (Figure 8l,p), the deformation time-series is close to linear. The main reason for this phenomenon is the exploitation of groundwater from the deep aquifer of the new waterworks named Lindong from 1999 [31], aiming to mitigate the over-pumping groundwater in urban areas. More details about bowl 9 are depicted by the profile bb' in Figure 9b showing the continued development of subsidence rates. Similarly, bowl 4 in the agricultural area experienced a nonlinear deformation.