## 4 Results: Spatial--Temporal Evolution of Subsidence
### Land Subsidence in the Urban Areas
More importantly, the periodic deformation time-series of Figure 8 fj,n are observed, indicating the seasonal groundwater pumping for irrigation. Figure 7: The spatial-temporal evolution of land subsidence in Zhanjiang city. (**a**) Average subsidence rate of JERS from Octorberber 1992 to September 1998; (**b**) average subsidence rate of ENVISAT from October 2003 to May 2007; (**c**) average subsidence rate of ALOS1 from December 2006 to July 2010. Zhanjiang city consists of four districts, namely Chikan, Xiashan, Potou, and Mazhang. Black dashed lines indicate the district border. Areas marked with red dashed lines are land subsidence bowls; Black circles are the leveling stations and two black solid lines are selected profiles of the subsidence bowl by aa’ and bb’. We also calculated the spatial-temporal evolution of land subsidence area showed in Table 1. Five bowls (Bowl 1, 2, 3, 4, and 5) of ENVISAT data are excluded due to their unclear boundaries. The subsidence area of bowls decreased with time in the urban area, for instance it decreased from 3.3 to 3.0 km\({}^{2}\) in bowl 1, from 1.9 to 0.8 km\({}^{2}\) in bowl 2, from 10.8 to 6.1 km\({}^{2}\) in the new bowl merged from bowl 6 and bowl 8. Conversely, in suburb and agricultural areas, the subsidence area increased continuously: from 0.9 to 4.3 and then to 12.2 km\({}^{2}\) in bowl 9, from 1.8 to 4.8 km\({}^{2}\) in bowl 3 and from 1.6 to 6.1 km\({}^{2}\) in bowl 4 (see Table 1). In order to analyze the spatial evolution of bowl 6 and bowl 8, another profile aa' is plotted in Figure 9. We can see that the two separated bowls (red line in Figure 9a) merged into one bowl (green line in Figure 9a), and then became two connected bowls (blue line in Figure 9a) from a to a', which can also be visually observed in Figure 7a-c. In addition to the subsidence area, the central bowl selected for bowls 1, 4, 8, and 9 aforementioned is regarded as the movement indication in this study. The results are shown in Figure 8a-d with red arrows, we can see that the surface subsidence extends from urban areas to suburban and agricultural areas. Figure 8: Deformation time-series of central points in four selected bowls, i.e., bowl 1, 4, 8, and 9, for each platform. The first row (**a**–**d**) represents the location of central points with optical backgrounds derived from GoogleEarth. The circles, diamonds, and triangles represent the location of JERS, ENVISAT, and ALOS1 respectively. Black words around the optical images are the adjacent land classes. The red star in (**d**) is a waterworks named Lindong started in 1999; The second to fourth rows represent the deformation time-series of four points for JERS, ENVISAT, and ALOS1. The red arrows represent the movement direction of land subsidence. The dashed lines in (**e**–**p**) are the results of linear fitting of the deformation time-series. The word ‘V=’ in (**e**–**p**) is the mean velocity in the vertical direction. It is known that subsidence in Zhanjiang city is the result of over-extraction of groundwater for domestic and industrial use [4; 32]. Groundwater exploitation from middle and deep aquifers started in the1980s and accelerated in the 1990s with the development of the economy and urbanization. However, as our InSAR measurements suggest from 1992 to 2010 the subsidence rate decreased after the government prohibited pumping groundwater in the urban areas [32] (see Figure 7). Meanwhile, the subsidence rate started increasing in the suburban areas as the groundwater was extracted to supply the growing population and expanding industries in the urban areas accompanied by the moving of the waterworks. Besides, we take three small cities of the LZP into measurement, Leizhou city, Xuwen County, and Suixi County. Unlike Zhanjiang city, the subsidence in these cities is comparatively small with the average subsidence rate 0.6, 0.7, and 1.5 mm/year, respectively.