Ice Thickness Monitoring Of Gahai Lake Based On Airborne Radar And Sentinel-1 SAR

As an important component of the terrestrial hydrosphere,lakes are involved in the water cycle and are important information carriers for global climate change and regional responses,as well as a link between the earth's spheres.As the seasonal ice body on the surface of lakes during the cold season,lake ice has an important impact on the regional heat and energy balance,and its production and extinction changes,especially ice thickness dynamics,is regarded as indicators of regional climate change.The Gahai Lake in Haibei Tibetan Autonomous Prefecture of Qinghai Province was taken as the research object of this paper,unmanned ship sonar and airborne radar were used to measure water depth and ice thickness in Gahai Lake.The relationship between the Sentinel SAR backscatter coefficient and the measured lake ice thickness was investigated,and the accuracy of the ice thickness simulation was also evaluated.The results showed that:The unmanned ship vehicle with sonar equipment was used to carry out water depth measurements of the Gahai Lake from 17 to 22 October 2020.Compared with the actual water depth measured using a measuring rope,the water depth measured by sonar was slightly smaller,with a RMSE of 0.06 m.This is mainly related to the higher salinity and greater spatial variability of the Gahai Lake.Combined with the Sentinel-2MSI remote sensing images and RTK measurements,the water area of Gahai Lake in2020 was 49.35 km~2,the water surface elevation was 3152.01 m,the maximum water depth was 10.33 m,the average water depth was 5.41 m,and the reservoir capacity was3.37×10~8 m~3.The Gahai Lake basin has a long and flat'U'shaped structure,the water depth on the western shore of the lake varies slowly and has a large area of shallow water,while the water depth on the eastern shore varies rapidly.Compared with 2004,the area of Gahai Lake increased by 4.46 km~2,the water surface elevation rose by 0.76m and the volume of water increased by 0.41×10~8 m~3,with similar trends to that of the adjacent Qinghai Lake.From December 31,2020 to January 6,2021,airborne ice detection radar was used to measure the ice thickness of Gahai Lake,and the mean value of permittivity was determined to be 4.6,which is higher than that of freshwater ice(3.2).The propagation speed of radar waves in the ice was 0.135-0.146 m/ns,with an average speed of 0.140m/ns The MAE of the ice thickness was 0.009 m,the RMSE was 0.010 m,the standard deviation of the radar thickness and the borehole ice thickness was 1.1%,and the uncertainty of the ratio measurement was 2.2%,which meets the requirements of the ice observation specification.The ice thickness of Gahai Lake showed a spatial pattern of being thicker in the north and west,whereas being thinner in the south and east.The maximum ice thickness was located in the northern part of the lake(0.441 m)and the minimum ice thickness was located in the southern part(0.082 m),and the ice thickness growth rate was 0.2?0.3 cm/d.The ice thickness and water depth showed a negative correlation with a correlation coefficient of 0.59.The backscatter from Sentinel-1 SAR VV and VH polarization differs,with VV polarization obtaining stronger backscatter than VH polarization.The correlation coefficient for VV polarization backscatter simulated ice thickness was 0.56,VH polarization backscatter simulated ice thickness was less correlated,and the correlation coefficient for VV/VH polarization ratio simulated ice thickness was 0.53.Using VV polarization backscatter simulated lake area the maximum ice thickness was 0.49 m,the minimum ice thickness was 0.23 m,and the mean ice thickness was 0.29 m.The RMSE was 0.075 m and the MAE was 0.061 m.The simulated ice thickness values overestimated the thin and thick ice and underestimated the ice in the 0.25-0.35 m ice thickness range.The intensity of SAR backscatter is influenced by the permittivity of the target feature,the radar incidence angle and the roughness.The freeze-thaw changes in the surface of the lake ice lead to changes in the water content,which affects its backscatter.The large area to the west of the lake covered by sand and dust also has some influence on the backscatter;the central incidence angle of the radar waves at the lake area is42.2°and the backscatter is mainly influenced by the roughness of the lake ice.The thickness of the lake ice in some areas has increased significantly,which is related to the process of ice formation in which the ice body breaks up and forms a pile-up,and an increase in the roughness of the lake ice increases the simulated ice thickness.