Frozen Ground Degradation And Its Eco-Hydrological Impacts In Typical Basins In The Northeastern Qinghai-Tibet Plateau

The frozen ground has degraded at high latitudes and high elevations around the world because of global warming.Frozen ground degradation has changed hydrological and ecological processes in the cold regions,which has profound impacts on the natural ecosystems and human activities.The Qinghai-Tibet Plateau is the Asian Water Tower and the third pole of the world.It is also an important ecological security barrier and strategic resources base in China.Under the changing climate,it is crucial to understand frozen ground changes in the river source regions of the Qinghai-Tibet Plateau and the impacts on the hydrological and ecological systems,which will affect regional water resources planning,management,and decision making.In this paper,methods based on Bayesian statistics and distributed hydrological model were proposed to simulate the changes in frozen ground.The changes of the frozen ground in the Yellow River source region and the Upper Heihe River in the northeast of the Qinghai-Tibet Plateau were analyzed.The impacts of frozen ground changes on the ecohydrological processes were also evaluated.First,this study proposed a Bayesian inference model to estimate the maximum thickness of seasonally frozen ground using historical precipitation and air temperature observations.By introducing physical parameterization to the Stefan solution and using the antecedent precipitation index to quantify the unfrozen soil moisture,this study used the Markov chain Monte Carlo(MCMC)method to estimate the probability distribution of the parameters in the Stefan solution,and to evaluate the uncertainty in frozen ground depth estimations.Then this study employed a distributed model,geomorphology-based eco-hydrological model(GBEHM).The parameterization of the soil water conductivity and vegetation have been improved to have a better applicability in typical cold regions.Stefan solution based on the Bayesian methods was used for estimating the frozen ground depth in the Yellow River source region.The simulated and observed values had an average percent bias in the range of(-2.9%,9.7%),and the root-mean-square error was in a range of(0.13 m,0.35 m).The uncertainty of soil water parameters accounted for 1.4% to 21.5% of the total uncertainty in the estimation.By using the GBEHM,the results showed that the areal proportion of permafrost in the study area has decreased from 44% during 1981-1990 to 17% during 2001-2010.Regarding the hydrological effects,the increase in air temperature has caused the increase of evapotranspiration in the study area,which led to the decrease in runoff coefficient.At the interannual scale,precipitation decreased from 1981 to 2002 and increased significantly since 2002.Due to the degradation of seasonally frozen ground,surface soil water content has increased.The degradation of frozen ground and the rising temperature has enhanced the growth of vegetation in some parts of the Yellow River source region.Regarding the Heihe River Basin,the annual average air temperature has increased by 0.34°C per decade from 1960 to 2014,and the seasonally frozen ground depth has decreased by 7.4 cm per decade.Additionally,the decreasing trend in the frozen ground depth was significant at high elevations.The results of runoff composition showed that the proportion of surface runoff was decreasing and the baseflow was increasing in the Heihe River Basin.The vegetation leaf area index(LAI)in the Heihe River Basin increased by 0.045 per decade,and the start of the growing season advanced at a rate of 1.8 to 2.1 days per decade.Based on the grey correlation analysis,the results showed that the decrease in frost depth was a main reason for the advancing green-up dates and increasing LAI at the start of the growing season.Therefore,the degradation of frozen ground mainly affects the growth of vegetation in the early stage of the growing season.