## 6 Conclusions  
We obtained the average subsidence rate map of the LZP from 1992 to 2010 with a modified stacking technology. A relationship between land subsidence and thickness of soft soil of the Holocene age was found. The main cause of land subsidence in this peninsula is the overexploitation of groundwater for different usages, domestic and industrial use in urban areas and agricultural use in arable and coastal areas. Land subsidence with a velocity up to 32 mm/year in Zhanjiang city is propagating from urban areas to suburban areas which is coincident with the variation of groundwater level. The subsidence velocity of a wide range of inland arable lands is mainly between 10 and 19 mm/year. In addition, continuous increasing land subsidence is observed in coastal areas between the Leizhou Bay and the Zhanjiang Bay with an average velocity over 19 mm/year, caused by rapid exploitation of groundwater for aquaculture. If not addressed, the subsidence will spread and evolve into more serious disasters such as saltwater intrusion. Our work can help guide the rational exploitation of groundwater, and urban planning as well as offer technical suggestions for government decision making.  
The following are available online at www.mdpi.com/2076-3417/7/5/466/s1, Figure S1: The atmospheric phase of ALOS1, track466 and frame400 in SAR coordination. The coverage of these images is represented by a black rectangle on the right-hand side in Figure 1. The wrapped phase of the differential interferogram whose master and slave images are the same SAR image with serious atmospheric delay. For example, 20070905, 20080723, 20090910 and 2010313 for (**a-d**) respectively. The red dashed lines in (**a-d**) represent the obvious atmospheric delay of each SAR image, Figure S2: Perpendicular baselines and temporal intervals of the selected InSAR image pairs from three different sensors, JERS (**a**), ENVISAT (**b**) and ALOS-1 (**c**). The black dots and diamond represent SAR image contaminated with serious atmospheric delay used as a master and a slave, respectively, Figure S3: 1-D exponential covariance function of residual errors between the estimated displacement phase, calculated by the multiplication between velocity and time, and the single differential interferometric phase for each platform. The primary component in residual errors is caused by atmosphere, which can be estimated with 1-D exponential covariance function. **a-c** Show statistical results of the frame covered Zhanjiang city with JERS, ENVISAT and ALOS1, respectively. Every black curve in (**a-c**) represents a statistical result of residual error for a selected pair, Table S1: The parameters of the ascending ALOS-1 images selected, Table S2: The parameters of the descending JERS images selected, Table S3: The parameters of the descending ENVISAT images selected.  
The ENVISAT data were obtained through the ESA project (No. C1P.14838 and C1P.16734). The ALOS data and JERS data were provided by the Japan Aerospace Exploration Agency (JAXA) through projects P1229002 and P1390002. This work was supported by the National Natural Science Foundation of China [41574005], [41474007] and [PERSON] fund of Central South University and The National Research Foundation for the Doctoral Program of Higher Education of China under Grant No. 20130162110015.  
[PERSON] and [PERSON] performed the experiments and produced the results. [PERSON] drafted the manuscript. [PERSON] and [PERSON] contributed to the discussion of the results. All authors conceived the study and reviewed and approved the manuscript.  
The authors declare no conflict of interest.