| Sanjiang Plain locates at the middle-high latitude cold region in China characterized as covering with mid-depth seasonal frozen soil,which is climate change sensitive and strongly human-affected.Frozen soil hydrology is the dominant processes in the Sanjiang Plain.The frozen soil acts as a solid reservoir in the river basin functioning in regulating the hydrological processes.Therefore,the runoff in the wintering period is closely related with the freezing-thawing of soil in this region.At present,hydrological studies in cold regions is still at the early stage.Meanwhile,most of previous studies focused on arctic regions and high-altitude regions.Studies on hydrological processes in middle and deep seasonal frozen soil regions are relatively lagging behind.In addition,the Sanjiang Plain is one of the most important commodity grain production base,where crop growth is closely related to soil freezing-thawing processes.In sum,investigating the runoff processes under freezing-thawing soil conditions in the Sanjiang Plain is of great theoretical and practical significance.Given this background,this dissertation is to study the evolution of runoff processes under freezing-thawing soil conditions and its responses to climate change in the Sanjiang Plain.Firstly,the change characteristics and influencing factors of frozen soil in the Sanjiang Plain were analyzed.Secondly,the relationships between runoff processes and freezing-thawing of soil were analyzed.Thirdly,a "Time-Varying Parameter" hydrological model for the Sanjiang Plain was constructed,by which,the absorption and release rates of soil water during the freezing-thawing period were calculated.Finally,the evolution trend of freezing-thawing soil and runoff processes in the Sanjiang Plain werw evaluated by using the climate change scenarios as the driver of the hydrological model.The main conclusions in the dissertation are as follows:(?)The principle of degradation of the frozen soil was revealed in terms of time nodes in the freezing-thawing processes in the Sanjiang Plain.The relationships between frozen soil degradation and the negative cumulative temperature has been quantified by regression method.Since the 1960s,the characteristic time nodes in the freezing-thawing soil of the Sanjiang Plain have deteriorated to different degrees.Compared with 1960s,the starting freezing date of mid-deep seasonal frozen soil in the Sanjiang Plain in 2000s was delayed by 1 to 10 days,the starting melting date was advanced by 4 to 12 days,and the melted date was advanced by 3 to 22 days.Correspondingly,the freezing period was shortened by 3 to 17 days,and the melting period was shortened by 2 to 14 days.In addition,the maximum depth of frozen soil in the Sanjiang Plain showed a decreasing trend,ranging from 0.7 to 28.2cm/10a.and it was advanced by 11 to 26 days.It is mainly because the reduction of accumulated negative temperature.(?)On the basis of analyzing the characteristics of runoff in the Sanjiang Plain,the dynamic relationship between runoff and freezing-thawing soil is analyzed.The the mechanism and mode of runoff generation of temperature-based various sources in the Sanjiang Plain are determined.The runoff in Sanjiang plain is characterized by having double flood peaks,among which there is a significant correlation between runoff and temperature in spring flood season,with a correlation coefficient over 0.88.In the processes of soil freezing,the correlation coefficient between runoff and temperature is greater than 0.84.During the unstable freezing period,flow patterns consist of the transient saturated runoff,the unsaturated flow and the groundwater flow.The mode of flow generation during the stable freezing period in winter includes temporary or short-term saturated runoff formed when snow melts and groundwater recharge runoff below the frozen layer.In the unstable melting stage of spring,the saturated runoff and unsaturated flow coexist.The mode of flow generation in stable melting stage includes superosmotic production flow and underground flow.The water withdrawal processes of spring flood includes three stages:snowmelt recharge runoff,mid-stream runoff and surface runoff from water recharge in frozen layer,and runoff generated by precipitation during spring flood,and the process of summer retreat includes runoff formed by precipitation and runoff from soil water and soil to replenish surface water.It has been shown that the interdecadal variation of runoff in the Sanjiang Plain has been decreasing in the past 60 years.(?)The 'Time-Varying Parameter' VIC model has been established based on the spatialization of the hydrological parameters.Spatially,base flow parameters such as Dm,Ws and Ds in VIC model are spatialized,and values of Dm,Ws and Ds in the Sanjiang Plain are 9.98?11.51,0.632?0.920 and 0.499?0.5 87,respectively.Temporally,the time-varying processes of three hydrological parameters,i.e.D2,D3 and B,were determined by the time-varying parameter method.The D2 values were 0.61,0.91,035,0.33 and 0.62,D3 values were 0.86,0.56,0.87,0.97 and 0.71,and B values were 0.41,0.34,0.36,0.42 and 1.13,respectively.The verification results showed that the Nash efficiency coefficient was basically above 0.73,the correlation coefficient was almost all greater than 0.79,and the relative deviation,mean absolute error and root mean square error were also within the allowable range.This indicating the model performed well in the study area.Based on the model,the formula of soil moisture absorption and release rate in freezing-thawing period in the Sanjiang Plain was determined based on runoff coefficient.The quantification of soil moisture absorption and release rate in freezing-thawing period was realized.From 1961 to 2014,the water absorption rate of soil in the Sanjiang Plain during the freezing period was 47%?53%,and the water release rate of soil during the thawing period of frozen soil was 59%-67%.(?)Under the three future emission scenarios of RCP2.6,RCP4.5 and RCP8.5,the frozen soil in the Sanjiang Plain is continuously degraded.In terms of flow generation,results showed the trend of decreasing the water absorption rate of the soil during freezing-thawing period and the water release rate of the soil during thawing period.Specifically,the reduction rates of the maximum freezing depth and freezing days under the three scenarios were 0.34cm/a,0.34cm/a,0.64cm/a,0.28d/a,0.31d/a,0.43d/a.The annual runoff variation showed that the runoff of 9 hydrological stations in Sanjiang Plain shows an increasing trend under different discharge scenarios in the future.The runoff distribution in a year showed equalization by which the spring flood is advanced by 5.01 days,7.71 days and 8.49days respectively,the peak flood flow decreased by 5%,8%and 10%respectively,but the total runoff in the year increased by 6%,9%and 11%respectively in the scenario of RCP2.6,RCP4.5 and RCP8.5.Meanwhile,under the three scenarios,the soil water absorption rates during freezing period were 42%to 47%,41%to 45%and 33%to 37%,respectively.The soil water release rates during thawing period were 51%to 66%,54%to 64%and 40%to 61%,respectively. |
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