| Soil infiltration is one of the important water cycle processes,and identifying its evolution characteristics plays an important role in ensuring water security and ecological security.The Qinghai-Tibet Plateau,"the top of all mountains and the source of all waters",has a vast territory,high and undulating terrain,and a large number of seasonal frozen soil resources with different physical and chemical properties are distributed in space.Driven by plateau meteorology,the random,repeated and active freeze-thaw process makes the water infiltration process in seasonal frozen soil special and complex,which needs to be clarified.At the same time,under the process of continuous global warming,the freeze-thaw and infiltration processes in seasonal frozen soil are deeply affected.Therefore,the response of soil infiltration under climate warming and its countermeasures have been paid close attention.In this thesis,taking the Nagqu River Basin on the Qinghai-Tibet Plateau as an example,typical experimental fields were selected and three main experimental contents including sensor embedding and monitoring,soil sample collection and detection,and Double loop infiltration were completed.The data of soil temperature and humidity,matrix potential,physical composition,chemical properties,infiltration amount and infiltration rate in Nagqu River Basin were harvested and sorted out.From the features of soil infiltration process and the characteristics of short-duration freeze-thaw events,combined with the internal physical and chemical properties of soil,the temporal and spatial laws and influence relationships of two important hydrological elements of infiltration and freeze-thaw under the current situation(From 2019 to 2021)of the Nagqu River Basin were explored.The Green-Ampt infiltration model was improved by introducing temperature parameters and analyzing the relationship between negative temperature and soil water holding,water potential and water conductivity.Finally,the numerical simulation of the model was used to explore the response of soil infiltration to it under climate change and the evolution of freeze-thaw events,and specific and feasible coping strategies were proposed.The main results are as follows:(1)The features of soil infiltration at two different freeze-thaw periods(the complete thawing period,the freezing process period)and four different altitudes(5015m,4732 m,4693m and 4443m),and the relationship between initial and stable infiltration rates and soil physical and chemical factors were analyzed.Under the same level of liquid moisture,the infiltration rate in the complete thawing period can be significantly greater than that in the freezing process period.Under the current situation of the complete thawing period of the Nagqu River Basin,the soil in the low-altitude area below 4500 m has a high infiltration rate and good permeability,the soil in the middle-altitude area of 4500-5000 m has a slightly lower infiltration rate and the second highest permeability,and the soil in the high-altitude area above 5000 m has a low infiltration rate and worst permeability.The initial infiltration rate of the soil has a very significant negative correlation with the initial moisture,the stable infiltration rate has a very significant negative correlation with the bulk density,and has a very significant positive correlation with the total organic matter,total nitrogen and phosphorus content and clay content.(2)The characteristics of short-duration freeze-thaw events were evaluated from five indicators: start and end time,total duration,total number of times,cycle frequency and intensity.In the intensity evaluation,the Copula function method was used to construct a comprehensive intensity index,which improves the efficiency and comprehensiveness of the evaluation.The general characteristics of freeze-thaw events in the Nagqu River Basin under the current situation are mainly affected by altitude factors and show a certain law with the altitude: Events start to occur in turn from high-altitude area to low-altitude area before the winter,and ends in turn from low-altitude area to high-altitude area before the summer in the coming year.Compared with the high-altitude and low-altitude areas,the mid-altitude area has the characteristics of longer total duration,more total number of times,and higher cycle frequency.In the Nagqu Basin,the Cuomaxiang is the area with the longest total duration,the highest total number of times and the highest frequency,while the Xiaotangula is the area with the highest intensity,and they are more significantly affected by freeze-thaw.(3)The Green-Ampt infiltration model was improved and infiltration simulation scenarios were set up to explore the response of soil infiltration under different degrees of climate warming(temperature rising)and under the evolution of different degrees of freeze-thaw event characteristics(humidity difference).From 2021 to 2050,the increments of frozen soil infiltration will be the largest(the highest degree of response)in the RCP8.5,the second in the RCP4.5,and the least in the RCP2.6.And the increments of each scenario all are the largest in the middle-altitude area(the highest response degree),followed by the low-altitude area,and the least in the high-altitude area.When the freeze humidity difference in the Nagqudaqiao decreased to the freeze humidity difference in the Cuomaxing,the infiltration increased the most.When the freeze-thaw humidity difference in Xiangmaoxiang increased to the freeze-thaw humidity difference in the Nagqudaqiao,the infiltration decreased the most.The highest degree of response occurs when the evolution of freeze-thaw characteristics occurs between the middle and low altitude areas.This study is conducive to further exploring the mechanism of the hydrological cycle in the alpine region,and provides a certain reference for the effective utilization of water resources in the alpine region,the rational development of cold region engineering construction,natural disaster prevention and ecological protection. |
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