## 4 Results and Analysis  
### Long-Term Deformation and Time Series Deformation in the Northern QTP  
The coherence weighted least squares method with 66 scenes of ALOS data (2007-2009), 73 scenes of ALOS-2 data (2015-2020) and 284 scenes of Sentinel-1 data (2017-2021) was used to retrieve the permafrost deformation in the northern QTP. The results illuminate on a large scale the permafrost deformation in the northern QTP from 2007 to 2021. The ground deformation results are shown in Figure 3.  
In Figure 3, the red color indicates the subsidence in the LOS direction, and the blue color is the uplift toward the satellite. There are a large number of lakes (deep blue color) in the western part of the study area. From the three different deformation maps obtained by using three different SAR sensors, we find that the long-term deformation velocity in the northern QTP ranges from \(-20\) to \(+20\) mm/year (ALOS, 2007-2009), \(-30\) to \(15\) mm/year (ALOS-2, 2015-2020), and \(-25\) to \(30\) mm/year (Sentinel-1, 2017-2021). Further study shows that the expansion areas of permafrost subsidence are concentrated on braided stream plains and thermokarst lakes. In these areas, due to glacial erosion, surface runoff and river alluvium, the contents of water and ground ice are sufficient, which could accelerate permafrost subsidence. In Figure 4a, we selected two typical areas, Salt Lake and a braided stream plain. Salt Lake (\(35.532^{\circ}\)N, \(93.409^{\circ}\)E) is located in the northeastern section of Hoh Xil (Kekexili) in the QTP. The water of Salt Lake mainly comes from seasonal rivers. On 14 September 2011, Zonag Lake burst its banks, which caused the area of Salt Lake to expand significantly. The outburst of Zonag Lake could affect the freeze-thaw behavior of the surrounding permafrost. We used Google Earth images to show the historical expansion of Salt Lake (Figure 4b,d). From the different period images, we find that the Salt Lake experienced significant expansion and that the distance from Salt Lake to the QTR is decreasing. Figure 4c shows the result processed from stacks of ascending ALOS datasets from 2007 to 2009. The red color indicates ground deformation away from the satellite. We find that the degradation of the permafrost mainly occurred in the northeast direction. This may be due to the existence of glaciers and braided stream plains in northeast of Salt Lake. Glaciers have created a large-scale glacial erosion of mountains, resulting in a large number of deposits. Figure 3: Long-term deformation results of the study area. Background is the DEM. (**a**) ALOS result (2007–2009). (**b**) ALOS-2 result (2015–2020). (**c**) Sentinel-1 result (2017–2021). In addition, with sufficient moisture, the effect of thawing processes is stronger than that of freezing processes. This could increase the thickness of the active layer. Figure 4d shows the result processed from stacks of descending Sentinel-1 datasets from 2017 to 2021. Similarly, the red color represents the subsidence along the LOS direction. Compared with the 2007-2009 result and 2017-2021 result, the area of permafrost degradation is expanding. We used ArcMap software to analyze the temporal and spatial changes in the permafrost within a 30-km rectangle around Salt Lake. The calculation result shows an increase of 51.7 km\({}^{2}\) areas (deformation rate \(>\) 10 mm/year) within the analysis rectangle after the outburst of Zonag Lake in 2011. The second landscape example is a braided stream plain (34.8791\({}^{\circ}\)N, 93.1845\({}^{\circ}\)E). We used Google Earth image to show the spatial properties of the braided stream plain (Figure 4f). This braided stream plain is surrounded by glaciers in the north, south and east. The terrain in the west is relatively low. There are many lake tributaries in this area. The surrounding glaciers provide many water resources for the braided stream plain. With sufficient moisture, the permafrost may undergo degradation.