Water Cycle Simulation Of Frozen Soil In Plateau Cold Region Based On WEP Model

As the birthplace of important rivers in China,the Qinghai-Tibet Plateau is the source area of the Yarlung Zangbo River,Lancang River,Nujiang River,Yangtze River and Yellow River.Glacier melt water,groundwater recharge and precipitation recharge are the main components of river runoff in this area,and different components have their own evolution laws,which leads to the complexity of river runoff variation in the source area of Southwest rivers.In recent decades,with the global warming,the phenomenon of glacier retreat and snow line rise is more obvious in this area.The water cycle process has changed significantly,the consistency of hydrological series has been destroyed,and the rule of runoff evolution has become more complex.With the development of computer technology,the numerical simulation of hydrological model has become one of the important means to study the water cycle in river basins.In order to improve the accuracy of hydrological simulation,the freezing-thawing process of seasonal frozen soil should be taken into account when constructing distributed hydrological model in Alpine mountainous areas.However,there is a lack of research results on the mechanism of frozen soil freeze-thaw process in the headwaters of Southwest rivers,and the description of soil freeze-thaw process is relatively simple in the hydrological model constructed in this region.It is necessary to strengthen the research of distributed hydrological model with physical mechanism on the basis of mechanism research.In view of the above problems,this paper carries out in-depth research and achieves the following results:(1)According to the characteristics of thin soil layer and long snow distribution time on the Qinghai-Tibet Plateau,according to the underlying surface physical structure in the cold region of the plateau,this paper creatively constructs a continuum model of snow cover-soil-loose rock layer.Based on the original WEP-COR model,the model considers the influence of snow cover and loose rock layer on the Hydrothermal Coupling simulation,and also carries out the WEP-COR model modules.The improved WEP-COR model can not only be applied to the simulation of permafrost water cycle in plateau cold region,but also be used as a module to couple with other models.(2)The Niyang River Basin,a typical tributary of the Yarlung Zangbo River,is selected as the study area.Combining with the characteristics of the study area,the simulation of soil freeze-thaw process and water cycle process of WEP-COR model is improved.Overall,the NASH efficiency coefficient of the simulated continuous body temperature ranges from 0.569 to 0.882,the average NASH efficiency coefficient is 0.794,the RMSE(mean square error)is between 1.2 and 2.1,and the average RMSE is 1.46 C.The RMSE(mean square error)of the simulated liquid water content is 0.065,the RMSE(mean square error)of the frozen soil depth is 11.2 cm,and the NASH efficiency coefficient is 0.62.The Nash efficiency coefficient is 0.810 and 0.752 for the monthly discharge of Gongbujiangda station and muddy station.Therefore,the improved model can not only better simulate the temperature,water content and freezing depth of the continuous body active layer at different depths,but also effectively simulate the water cycle process in the plateau cold region.It can be used as a simulation of water cycle in the cold region watershed.Tools.(3)In this paper,the upstream of the Yellow River basin is selected as a typical study area,and the water cycle model of the alpine mountain area constructed in the alpine mountain area is coupled with the WEP-L model of the Yellow River.The model is applied to the study of water cycle and freeze-thaw process in the source area of the Yellow River.For the frozen soil simulation,the RMSE(mean square error)average value of the measured depth monitoring period in 1965 is 12.9 cm,and the NASH efficiency coefficient is 0.95.The average RMSE(mean square error)and NASH efficiency coefficient were 16.1 cm and 0.89 respectively in 1965.In the aspect of runoff simulation,the Nash efficiency coefficient of Tangnaihai station is 0.819 and the relative error is-2.2%.The Nash efficiency coefficient of Maqu station is 0.817 and the relative error is-1.2%.The simulation results are basically reasonable and reliable.This chapter studies the evolution of frozen soil and runoff in the source area of the Yellow River under meteorological changes,and finds that both precipitation and temperature in the source area of the Yellow River fluctuate slowly,and the temperature rises rapidly after 2000.Influenced by the changes of temperature and precipitation,the frozen soil depth contour in the source area of the Yellow River will gradually move to the high altitude area,and the frozen soil depth will gradually decrease.From 1981 to 2000,the flow was mainly affected by the increase of precipitation,and the flow showed an upward trend.After 2000,the flow was mainly affected by the temperature,and the flow decreased significantly.