华北土石山区典型林分坡地径流形成机制研究

Hillslope runoff is the connecting link between rainfall and streamflow. It is hot and difficult point of forestry hydrology to ascertain formation mechanism of hillslope runoff under different forest types. Compared with bare land, three typical forest stands (i.e Pinus tabulaeformis, Quercus variabilis Blume, and Vitex incisa Lam. var. heterophylla Franch) are as the research objects in rocky mountain area of northern China. Theories and methods of traditional hydrology and isotope hydrology are used to continuously monitor and sample the water, which includes rainfall, throughfall, stemflow, litter water, soil water, overland flow, and subsurface flow in four typical plots. δ2H and δ18O in water samples, combined with the soil moisture before rainfall and its isotopic content, are compared to intuitively evaluate the similarities and differences of the water source through the correlograms of δ18O versus δ2H. Based on water balance theory and the law of conservation of matter, hydrological statistical method and two water resource segmentation model are applied to expound the vertical distribution of rainfall in the typical forest lands, water resource composition and its variation law with time at different depths of soil water, overland flow, and subsurface flow. The laws of soil water movement and formation mechanism of hillslope runoff are ultimately revealed. The study will provide theoretical basis for forest vegetation construction and water resource formation in rocky mountain area of northern China.

坡地径流是连接降雨和河川径流的纽带，探明不同林分类型下坡地径流的形成机制是森林水文学研究的热点和难点。本项目以华北土石山区典型林分（油松、栓皮栎、荆条）为研究对象，以裸地为对照，运用传统水文学和同位素水文学相关理论和方法，连续监测了4个径流场降雨、穿透雨、树干茎流、枯落物持水、不同深度土壤水分及水势、地表径流和亚表层径流的动态变化，连续取样测定相应环节水分的δ2H和δ18O，结合降雨前土壤水分及其同位素含量，通过比较各环节水分δ2H-δ18O关系直观评价各环节水分来源的异同性。基于水量平衡理论和物质守恒定律，采用二水源分割模型和水文统计方法，阐明降雨在典型林地中的垂直分配规律，定量分析不同深度土壤水分的水源构成，地表径流和亚表层径流的水源构成比例及其随时间的变化规律，揭示典型林分土壤水分运动规律和坡地径流形成机制。本研究可为该区森林植被建设和水资源形成规律方面提供理论依据。

华北土石山区人均水资源量仅占全国的1/4, 随着一系列林业生态工程的实施，森林植被明显增加，正确认识林地径流产生机制可为该区植被建设和水资源管理提供科学依据。本研究以典型灌木林地、栓皮栎林地和油松林地为研究对象，运用环境同位素示踪技术在坡面尺度上研究了降雨垂直分配特征及土壤水分运移规律。结果表明：灌木林、栓皮栎林和油松林林冠层截留量分别占总降雨的82％、51%和55%，枯落物层截留量分别占总降雨的6%、22%和14%，土壤层持水量分别占总降雨的12%、27%和31%。在这场持续17小时的降雨过程中，栓皮栎林地0~30cm深度土壤水势在[-0.2, 0]范围呈上升趋势，在30~45cm土壤深度土壤水势在[-0.7,-0.5]范围内呈下降趋势，在45~60cm深度土壤水势在[-0.35,-0.25]范围内呈不变趋势，说明栓皮栎林地土壤水分由表层向深层移动，降雨后期移动速度增加，但土壤水分移动范围仅在土壤厚度45cm范围内；油松林地0~15cm深度土壤水势在[-0.2, 0]范围呈略微上升趋势，在30~45cm深度土壤水势在[-0.55,-0.45]范围内呈下降趋势，在60cm深度土壤水势在[-0.35,-0.25]范围内呈不变趋势，说明油松林地土壤水分也是由表层向深层移动，随降雨时间移动速度增加，但土壤水分移动范围也仅在厚度45cm范围内。降雨初期灌木林地表层土壤（0~10cm）水分来源中降雨、表层水和次表层水（10~20cm）各占1/3, 降雨中期次表层水贡献率达82%，降雨末期降雨比例达84%，说明灌木林地冠层和枯落物层对降水有明显的截持作用，并在降雨后期缓缓供给土壤；降雨初期栓皮栎林地表层土壤水分来源中降雨、表层水和次表层水贡献率各占5%、45%和45%，降雨中期降雨的贡献率增加至20%，表层水和次表层水的贡献率各减少至40%，降雨末期降雨贡献率增加至50%，表层水和次表层水的贡献率各减少至20%和30%，说明随降雨时间增加，降雨逐渐驱替出土壤原有的水分；油松林地降雨初期降雨的贡献率占比达64%，远大于栓皮栎，这主要因为油松林对降雨的截留较小所致，降雨中期和后期降雨贡献率均大于栓皮栎林。综上，说明灌木林地上部分截持降雨的能力最大，但同时地上部分持水供给土壤层的能力也最大。