基于优先流特征分析的长江三峡花岗岩坡地土壤水文过程研究

The major threats to the ecological security of Three Gorges Reservoir Area are soil erosion and geologic hazard, which were usually triggered by the process of rainfall and runoff. Soil hydrological processes on hillslope featured with more preferential flow affect the transformation processes of rainfall to runoff seriously, which make it the hotspot of soil physics and ecohydrology research. Three observation plots covered with coniferous forest, deciduous broadleaf forest, coniferous-broadleaf mixed forest will be designed and arranged respectively on the hillslopes located in the upper reaches of Three Gorges Dam of Yangtze River. Long-term fixed observation of soil moisture, underground water level, surface runoff, vegetation interception and isotope data will be conducted. And the characteristic parameter of terrain, vegetation, soil property, and water flow will be investigated or analyzed in lab. Through the numerical simulation of above data, the spatial and temporal characteristics of soil hydrological factors can be described accurately, which are under the integrated influence of soil horizons and vegetation. And the flow patterns of soil water and the routes of preferential flow can be constructed in four spatial scales, i.e. soil pore scale, core scale, pedon scale and slope scale. The spatial and temporal distribution of soil preferential flow, its occurred mechanism and contributed factors can be revealed. And the spatial and temporal changes of surface runoff, interflow, deep percolation and other water balance component can be quantified, including the contribution of preferential flow. The research results can enrich the content of hydropedology in mountain area, provide theoretical basis for the construction of regional hydrological models, offer scientific support to soil and water conservation and ecological restoration in the Three Gorges Reservoir Area.

水土流失和地质灾害是威胁三峡库区生态安全的主要问题，降雨汇流是其重要诱因。多优先流发育的斜坡土壤水文过程是影响山地降雨汇流的关键环节，也是土壤物理学和生态水文学研究的热点。本研究拟在长江三峡大坝上游的大老岭林区布设阔叶林、针叶林和针阔混交林覆盖的坡地观测小区，利用自然条件下长期定点监测的土壤水分、地下水位、地表径流、植被截留和同位素等数据，以及地形、植被、土壤属性和水流特征等要素的调查、分析和测定结果，结合数值模拟的方法，准确刻画土壤和植被耦合影响下的土壤水文要素时空分布特征；构建孔隙-土芯-剖面-坡面等四个尺度的土壤水流模式与优先流路径；揭示土壤优先流的时空分布特征、发生机制与驱动因素；量化地表径流、壤中流和深层渗漏等水平衡分量的时空动态，以及优先流的影响权重。研究成果可丰富山地土壤水文学的研究内容，为区域水文过程模型的构建提供理论支撑，为三峡库区的水土保持和生态恢复提供科学依据。

坡面土壤水文过程是影响流域产汇流的关键环节，三峡山地土壤水文过程研究对库区水土保持和生态恢复有重要意义。研究选择库首常绿针叶林、林灌混交林和草地等植被覆盖的花岗岩坡地和集水区，通过野外实验观测、调查采样、室内分析和数值模拟分析，在坡面和集水区尺度上对土壤水文属性、剖面水流特征、土壤水分响应、优先流过程等进行了研究，结论如下：(1)三峡山地土壤饱和导水率在8.64~2128.32cm/d之间。海拔越高，土壤渗透性能越好。不同植被覆盖的坡地相比，常绿林饱和导水率为7.80~322.81cm/d，大于落叶林地和草地。相同植被覆盖下，坡上土壤入渗率高于坡下。(2)渗透性能与优先流路径和大孔隙数量关系密切，林灌混交林和常绿林土壤剖面的水流路径和水流染色面积大于弃耕草地，坡上位置染色比例(41.5%)大于坡下位置(35.6%)。CT扫描显示，形成优先流的大孔隙数量、表面积和孔隙率均为亚高山常绿林最大，低山针叶林和茶园最小。(3)坡地土壤水分的时序变化存在空间稳定性和剖面不同深度的自相关关系。不同发生层土壤水分时间序列相似性显著(P<0.01)，常绿林不同深度土壤水分相关性大于林灌混交林和草地。相同植被覆盖坡上位置相似性大于坡下。表层水分与深层水分的小波相干显著，相干面积随时间尺度变化，月尺度(58.6%)>周尺度(42.8%)>日尺度(21.8%)。(4)坡地土壤涵养水源功能受降雨过程和植被-地形的影响。土壤水分对<8mm的降雨没有响应，降雨量>30mm时剖面水分均有增加。与草地相比，林地尤其是混交林的降雨入渗量较大，剖面蓄水量增加最多。受侧向流影响，坡下比坡上触发土壤水分响应所需的降雨量更小，储蓄水分更多。(5)利用高频土壤水分数据分析显示，三峡山地优先流的平均发生频率11%-12%左右，低于美国Shall Hill关键带。不同植被覆盖和不同坡位处相差较大，常绿针叶林坡上优先流频率为32.69%，草地坡上和坡下仅9.46%。同一集水区内，沟头和谷底发生频率达53.57%和46.43%，而沟内中间位置发生频率不足11%，两侧坡面频率25%左右。优先流的发生受降雨量的影响较明显，但需要制定合适的土壤水分响应和降雨过程判别标准。