The Inversion Of Frozen Soil Based On Remote Sensing And The Study On The Impact Of Frozen Soil Changes On Runoff In The Source Region Of The Yellow River

Due to the impact of global warming,the degradation of frozen soil in high altitude and high latitude areas is accelerating.The Qinghai-Tibet Plateau is the"Asian Water Tower".The impact of the degradation of frozen soil on the hydrological processes and engineering safety in the Qinghai-Tibet Plateau have attracted worldwide attentions.The study of frozen soil change and its impact on runoff in the source area of the Yellow River,which is one of the typical watersheds on the Qinghai-Tibet Plateau,is important and has practical values for water resources management in the Yellow River Basin.Compared with the traditional ground observations,the remote sensing method can better describe the spatial variability of frozen soil and has great advantages in investigating frozen soil changes.In this paper,using the MODIS(Moderate-Resolution Imaging Spectroradiometer)land surface temperature(LST)product,the Temperature at the Top of Permafrost(TTOP)model and the Stefan formula are used to develop a remoste sensing model for estimating the distribution and changes in frozen soil in the source area of the Yellow River.This model is validated based on actual observational data.Based on the the remote sensing model and meteorological data,the temporal and spatial changes of the frozen soil in the source area of the Yellow River were analyzed and compared with previous similar results.In addition,this paper analyzed the sensitivity of different soil thermal conductivity calculation methods and the influence of uncertainties of the coefficients in the regression equation for estimating the daily averaged LST on the results of frozen soil estimation.Finally,this paper analyzed the changes in the runoff and the impact of permafrost changes on streamflow and river ecology in different sub-catchments in the source area of the Yellow River.The results showed that the remote sensing model can more accurately estimate the distribution of frozen soil,the maximum thickness of the seasonally-frozen ground(MTSFG)and the active layer thickness(ALT)in the source area of the Yellow River.Different calculation methods of soil thermal conductivity and the uncertainties of the coefficients in the regression equation for estimating the averaged daily LST have significant impacts on the accuracy of frozen soil simulation.The results in this paper show that the area of permafrost in the source area of the Yellow River decreased significantly in the past 40 years,it decreased from63.74×10~3km~2 in 1981-1990 to 34.25×10~3km~2 in 2011-2019,with a rate of 11×10~3km~2/10a.The ALT has increased significantly at a rate of 17.3 cm/10a,and the MTSFG has decreased significantly at a rate of 6.5cm/10a.From 1961 to 2015,the runoff in the source area of the Yellow River declined before 2002and then increased.The runoff had a step change in 2003,and the proportion of the base flow in the total runoff showed an increasing trend from 1961 to 2015.The gray correlation analysis shows that the changes in frozen soil was the major cuases for the increase in the base flow.When the permafrost coverage rate is higher than 65%,the permafrost has a significant impact on the annual distribution of runoff,and the impact of permafrost changes on runoff is more significant at longer time scales.As the percentage of permafrost coverage decreased,the recession of runoff slowed down.The degradation of frozen soil has a significant impact on the extreme flow of the Jimai station.As the percentage of permafrost area decreased,the peak runoff ratio coefficient decreased and the extreme low increased.The results of this study provide valuable references for understanding the variation of water resources and engineering management in the source region of the Yellow River under the influence of climate change.