| The Source Area of the Yellow River is the main water source and producing basin of the Yellow River basin,which is of decisive significance for water security,grain production,industrial development and ecological environment protection in the middle and lower reaches of the Yellow River.The Source Area is located in a special geographical location,which is sensitive to climate change.The ecological environment is very fragile and easily damaged,the drought and flood disasters occur frequently.Water security is threatened,ecological environment deteriorates,desertification intensifies,land water reserves decline and self-repair function declines with the global warming.The shortage of water resources in the Yellow River is accompanied by the decline of biological population and loss of diversity.At the Symposium on Ecological protection and High-quality Development of the Yellow River Basin,General Secretary Xi Jinping clearly pointed out that the Yellow River Basin is an important ecological barrier and economic zone in China,and an important region to win the battle against poverty in China,this task is an important guarantee for winning the battle against poverty in China.We will actively explore new roads of high-quality development with regional characteristics,improve the economic level and population carrying capacity,actively participate in the belt and Road Initiative,promote mutual benefit and win-win situation and provide political guarantee and economic support.In view of the threats to water security and deterioration of ecological environment in the Source Area of the Yellow River,the evolution of water vapor migration and coupling mechanism with soil moisture in the Source Area of the Yellow River has been carried out based on multi-source data and multi-model method in the paper.Numerical statistics and HYSPLIT backward trajectory model were used to deeply analyze the water vapor evolution law,water vapor source and rainfall change process in the Source Area of the Yellow River,then the contribution rate of water vapor and precipitation conversion efficiency were analyzed quantitatively and qualitatively.NDVI and FVC indices were used to evaluate the quality of local vegetation coverage,using land-air coupling indices ILH and ILCL it was used to diagnose the feedback mechanism between soil moisture and precipitation.It was of great strategic significance for rational development and utilization of water resources,protection of ecological environment balance and social stability and sustainable development to judge the driving factors affecting vegetation cover change and form a water cycle process integrating sky and earth based on OLS and GWR regression model.The main research results of the paper are as follows:(1)The paper qualitatively and quantitatively analyse the changes of water vapor content,water vapor flux and divergence,water vapor net budget in the Source Area of the Yellow River based on ERA-Interim reanalysis data and mathematical statistics.The overall water vapor content showed an increasing trend and the annual total water vapor content was 2.36×104 kg/m2 in the Source Area of the Yellow River.The water vapor content increased more rapidly in recent five years,and the seasonal water vapor content showed a single-peak shape,the summer water vapor accounted for 71.2%of the whole year,the water vapor content had obvious periodic characteristics.The EOF mode is used to describe the spatial distribution of water vapor content in the Source Area,the variance contribution rate of the first eigenvector(92.62%)can fully represent the spatial distribution type of water vapor content.The water vapor extends from the central part to the northwest and southeast respectively,which is caused by the difference of water vapor carried by the monsoon in different months and different regions.The precipitation(precipitation conversion efficiency)showed an increasing(decreasing)trend with an annual average of 485.6mm(23.2%)in the source area,the development potential of cloud water resources in the sky is huge.Water vapor is a sufficient condition for precipitation,but precipitation transformation is also affected by many other factors in the atmosphere.(2)In recent years,the meridional(latitudinal)water vapor flux showed an upward(downward)trend in the source area,and the annual water vapor flux was 0.52 kg·m-1·s-1 and 29.71 kg·m-1·s-1,respectively,which indicating that the meridional(latitudinal)water vapor flux was increasing(decreasing).The decrease of latitudinal water vapor flux was mainly caused by the decrease of water vapor flux in summer in the Source Area of the Yellow River.The decrease(increase)of the latitudinal(meridional)transport flux of water vapor over the plateau results in the continuous transport of water vapor over the Bay of Bengal,the Indian Ocean and the South China Sea,which is conducive to the increase of precipitation over the plateau.The convergence degree of water vapor flux divergence increased over the years,the southeast and southwest of the source area were convergence areas,while the middle and north of the source region were water vapor divergence areas.The meronal water vapor deficit,for-0.14×107 kg·s-1,the latitudinal water vapor surplus,for 1.32×107 kg·s-1.The water vapor budget in the source area is 1.19×7kg·s-1,and about 3.75×1014kg water vapor is imported into the Source Area of the Yellow River that it shows a water vapor surplus.The potential of artificial precipitation is 3.75×1012 kg,the water vapor resources are rich in the source area,which has great development potential.(3)The HYSPLIT backward trajectory model was used to track the source of summer precipitation and quantify the precipitation conversion efficiency in the Source Area of the Yellow River,there were 6254,6209 and 5989 effective water vapor tracks at 1000m,3000m and 5000m in summer,respectively.It is found that the water vapor transport paths in summer mainly come from the southwest path,the northwest path and the west-westerly path,and the contribution rates of water vapor are 21.0%,34.8%and 34.9%,respectively.The west-westerly flow is dominant.The westward water vapor path originated from the Caspian Sea and passed the south of pamir Plateau,crossed the Tarim Basin and entered the source area.The northwest path originated from Balkhash Lake in Kazakhstan,passed Tianshan Mountains and entered the source area along Kunlun Mountains.The southwest path originated from the Indian Ocean and pass the Bay of Bengal and Myanmar,through the Yarlung Zangbo River Grand Canyon over the south of the Qinghai-Tibet Plateau into the source area.A small amount of water vapor in the easterly path originated from the middle and lower reaches of the Yellow River,pass the Qinling Mountains and enter the Source Area of the Yellow River in Gannan area of Gansu Province under the action of southeast monsoon.The paper use TrajStat module to extract specific moisture and load it into the corresponding track,use CWT to calculate the average water vapor concentration corresponding to all tracks passing through the grid.It is concluded that the westerly flow under the mid-latitude westerly is the main water vapor channel in the Source Area of the Yellow River,High altitude water vapor(>1000 m)is the main water vapor source concentration area in the study area,the rainfall mainly comes from the altitude of 1000 m,but the height of the two is not consistent.(4)Soil moisture showed a declining trend in recent years with an annual average of 0.23m3/m3,the extremely high(low)values appeared in 2005 and 2010,respectively.Soil moisture changed most obviously at the beginning of the 21st century and the soil moisture fluctuated greatly.The seasonal variation showed an "M" shape,and the overall spatial pattern was high in the east and low in the west.In recent years,the drought degree in the Source Area of the Yellow River has been increasing and showed a drying trend,there is an alternating process of drying and wetting on the whole.The drought degree has gradually deepened from the southeast to the northwest.The spatial distribution of drought grade has a good similarity with the altitude,but there are differences in the seasonal drought degree.The abrupt change of temperature is earlier than that precipitation,soil moisture lags behind that precipitation according to B-G algorithm,which indicated that climate change affects the change of soil moisture before rainfall.The feedback mechanism between land and air was diagnosed by land-air coupling index ILH and ILCL.The feedback mechanism between land and air was diagnosed by land-air coupling indexes ILH and ILCL.Soil moisture and latent heat flux formed a positive feedback mechanism,water vapor and energy produced a negative feedback process between latent heat flux and lifting condensation level.It was concluded that the coupling strength is obvious between land and air,bidirectional positive feedback mechanism was formed between land and air in the Source Area of the Yellow River.The coupling strength between land and air is obvious in the Source Area of the Yellow River,a bidirectional positive feedback mechanism is formed between them.(5)The vegetation coverage quality was evaluated based on NDVI and FVC indices using MOD13A3 satellite remote sensing data in the Source Area of the Yellow River,the FVC remote sensing monitoring model,which combines the vegetation index method and pixel decomposition model,is established to estimate FVC in the source area.In recent years,the spatial pattern of vegetation coverage had been increasing—decreasing—increasing in the Source Area of the Yellow River,it indicated that the vegetation coverage had been increasing and the vegetation status had been improved with he annual mean value was 0.64.The vegetation belongs to the middle and upper distribution degree in the Source Area of the Yellow River on the whole.The vegetation coverage decreased from southeast to northwest in terms of spatial distribution with the largest in southeast and the smallest in northwest.The difference between east and west was obvious,the vegetation coverage decreased with elevation.There are few areas with medium and low vegetation coverage for many years,the higher cover vegetation is distributed in northwest,east and southwest China.The significant change trend of low vegetation and high vegetation was more obvious,the development trend of vegetation coverage in the source area was healthy.The trend of vegetation stability was basically the same in different periods in the Source Area of the Yellow River,the area of strong and weak vegetation stability was very small.Most of the vegetation was slightly degraded or slightly improved,the stability of different degrees was different.The OLS and GWR regression models were used to analyze the driving factors affecting vegetation change,the model differences and accuracy of the fitting effects of the two regression models were evaluated.The goodness of fit R2 of GWR model after adjustment was 0.72 and 0.45,the Moran’s I value of GWR residual model was lower than OLS model,it indicated that GWR model had a good fitting effect.The fitting effect of NDVI is better than FVC indicating NDVI can better represent the spatial evolution of vegetation cover in the Source Area of the Yellow River.The main influencing factors of FVC are water vapor content,temperature,rainfall,population and soil moisture in the Source Area of the Yellow River,their contributions were 26%,24%,22%,19%and-11%,respectively. |
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