土壤大孔隙流流速测量方法及影响机制研究

Soil macropore flow is an important process in hydrological cycle on land slope, and its velocity play a crucial role in soil infiltration process and runoff generation, to affect solute transport processes. The aims of the study are to measure the macropores flow velocity under different hydraulic gradients and analyze the velocity characteristics and effect mechanisms. The study is based on constructing of imitated soil macropores with different diameters and orientations for pores in a visualized soil container. The volume method will be used to calculate the averaged velocity of macropore flow by measuring outflow volume from the macropores in a certain period. The dye tracing method will be used to measure the peak velocity by recording the time of the front of dying tracing to reach the outlet of a macropore and the distance between the added position to the outlet. The electrolyte tracer will be also used to measure the true velocity based on the movement of the electrolyte. The relationship among the true velocity, the averaged velocities, and the peak velocities will be mathematically quantified for further determining the correction coefficient of peak velocities and averaged velocities. The calibration for the impacts of the changing process of velocity over gradient, pore size and water pressure on velocity and the mechanisms of the effect will be analyzed and modeled based on the principle of pipe flow hydraulics. The effect mechanisms of different factors, including gradient, pore size and hydraulic gradients on the velocity will be described and the response mechanism of macropores flow velocity to the related affecting factors will be analyzed. The results of this project will provide important theoretical basis to promote researches on quantification and mathematic simulation of macropore flow.

土壤大孔隙流是陆地坡面水循环的重要过程，其流速特征与土壤入渗产流密切相关，也影响溶质迁移过程。本项目采用室内可视化的土槽，构造不同孔径和取向的大孔隙通道，测量不同地表水力梯度下的大孔隙流的流速；在大孔隙出口采用确定的时段测量过水流量，用体积法计算质心流速，采用在水流流道内注入染料示踪溶液，测量染色剂前锋达到出口的时间，由加入位置间的距离计算优势流速，采用电解质示踪自动测量方法，测量水流的真实流速；揭示三种流速之间的定量关系，确定质心流速和优势流速的校正系数；采用管道水力学原理，刻画大孔隙流流速随坡度、孔径和水力梯度的变化规律，构建流速与以上影响因子间的关系模型；确定相关因素对大孔隙流流速的影响机制，揭示大孔隙流流速对相关影响因子的响应特征，为推进大孔隙导致的优先流的定量化和数学模拟研究提供理论依据。

土壤大孔隙流流速特征与土壤入渗、地表产流密切相关，也影响着溶质迁移过程。本项目构建了大孔隙流流速测量系统，优化了电解质示踪测量方法和装置，实现了室内条件下大孔隙流质心流速、优势流速及真实流速的测量。电解质示踪测量装置可以准确测量大孔隙通道内不同位置点的真实流速，染色示踪法可以测量不同工况下的优势流速，体积法可以准确测量孔径2和5mm条件下的质心流速，由于10mm条件下，大孔隙末端进入气体的影响，体积法无法测量得到该孔径条件下的质心流速。大孔隙通道内真实流速介于0.48到1.49m/s；优势流速介于0.36到1.33m/s；质心流速介于0.27到0.49 m/s。确定了质心流速和优势流速矫正系数。质心流速和优势流速的矫正系数与孔径相关。孔径2和5mm条件下，优势流速的矫正系数分别为0.77和0.74，质心流速的矫正系数分别为0.60和0.53。在10mm条件下，优势流速矫正系数为0.85显著高于孔径2和5mm条件下的优势流速矫正系数。由于孔径10mm条件下，大孔隙流速未充满孔隙，大孔隙通道内流速逐渐增加，染色示踪剂扩散迅速，测量时间变短，因此测量的大孔隙流优势流速偏大。电解质示踪法测量得到大孔隙优先流流速大于体积法和染色剂示踪法测量所得结果。考虑到大孔隙优先流水流对示踪剂稀释造成的视觉影响，电解质示踪法测量的流速具有其合理性。因此，相比于染色示踪和体积法，电解质示踪法可以更好地测量大孔隙通道内优先流流速。揭示了黄土高原基质入渗地带性差异影响机制。排除优先流影响，实测的土壤基质入渗与纬度呈显著正相关。粘粒及砂粒含量和初始含水率是影响稳定基质入渗率的主要因素。各入渗阶段粘粒含量与基质入渗率呈负相关（P<0.05）。初始含水率与基质入渗呈负相关。沙粒含量与稳定阶段入渗率呈正相关（P<0.05）。各地区平均粘粒含量随纬度增加而减少，平均沙粒含量随纬度增加而增加。即黄土高原由南向北，粘粒含量减少，含沙量增加，土壤含水率减小。因此受土壤物理性质的影响，黄土高原土壤基质入渗呈现出显著的地带性差异。