## 4 Results: Spatial--Temporal Evolution of Subsidence
### Land Subsidence in Agriculture Areas  
Besides urban areas, there is also some subsidence in agricultural areas caused by natural and anthropogenic processes. Here we only analyze the result of ALOS1 data because of the decorrelation and uncompleted coverage of ENVISAT and JERS data in those areas. From Figure 5c, we find that: (1) there is a wide range of discrete subsidence in the arable land with the rate of 10-19 mm/year, such as the junction area between Xuwen and Leizhou, the southwest of Leizhou; (2) rapid subsidence occurred along the coastline as shown by the black dashed elongated circles. The peak maximum subsidence in this area is up to 32 mm/year. In order to get more details, we chose five representative regions denoted by red rectangles (A-E) in Figure 5c to analyze the characteristics and causes of subsidence (see Figure 10). Figure 9: Two bowl subsidence profiles (aa’ and bb’) in Figure 7. (**a**) aa’ profile going through subsidence bowl 6 and bowl 8 in the urban area. Two separated bowls, i.e., Bowl 6 and Bowl 8, represented by red merged into one bowl represented by green, and then became two connected bowls represented by blue; (**b**) bb’ profile going through subsidence bowl 9 in the suburban area, the mean velocity grows with increase of time. The red, green and blue solid lines represent the mean velocity of JERS, ENVISAT and ALOS1 datasets. Subsiding areas shown in Figure 10a are farmlands characterized with highly compressible deposits, consisted of clay, silt, and silty sand (see Figure 2a), which is vulnerable to land subsidence due to sediment loading [8; 33; 34]. Shallow aquifers are the main supplier of these agricultural areas, which can be recharged seasonally from surface runoff, such as reservoirs and canals. However, during the observation period, the LZP experienced lots of droughts, resulting in the continuous declining of the shallow aquifers' level [35]. Moreover, unguided drilling, such as mixed exploitation of water from shallow- and deep-aquifers [35], and illegal drilling both accelerated the shrinking of groundwater level and caused a wide range of land subsidence. The bowl in Figure 10a (marked with a red solid line) has an average velocity of 13 mm/year over an area of 2.2 km\({}^{2}\). Coincidently, a collapse happened  
Figure 10: Average deformation rate of the five representative coastal areas for ALOS1 dataset from December 2006 to July 2010. (**a–e**) are enlarged views of the red rectangles in Figure 4c. (**a**) is an area covered with large-scale farmlands, the maximum subsidence rate is 18 mm/year; (**b**) is located at the south of LZP with a maximum subsidence rate of 13 mm/year and a uplift rate of 3 mm/year; (**c–e**) are areas with large-scale aquaculture, the maximum subsidence rates are 23, 20, and 23 mm/year, respectively. in this area in March 2008 [35]. Figure 11a,b,d show the differential subsidence of a building and a pumping facility resulting from over-exploitation of groundwater in this area. A wide range of crops were soaking in the flood because of the rapid subsidence in the farmland (see Figure 11c). In addition, the bowl 4 in Figure 7c of ALOS1 is also located in the agricultural area, see the optical image of Figure 8b, whose deformation time-series (Figure 8n) trend presents an obvious seasonality. The seasonal deformation is mainly caused by irrigation for agricultural use. Some uplift areas on the southwest coast of the LZP are shown in Figure 10b. These uplifts may be caused by the intermittent tectonic movement. The uplift rate is about 3 mm/year, bigger than the results (\(\sim\)2 mm/year) obtained from the tide station [23]. This uncertain level is consistent with the uncertainty of ALOS1 measurement in this study (\(\sim\)2.0 mm/year). Figure 10c-e represent land subsidence areas, such as tidal flats and marshes, distributed along the coastline of the peninsula which are covered with compressible deposits of the Quaternary age (see Figure 2a).