非饱和黄土中水分迁移规律的研究

Precipitation (Irrigation) is the unique source of groundwater recharge in the loess area. But observation on the natural profiles after a longtime raining and monitoring for the artificial rain tests suggest that penetration depth of water in loess is limited, which implies that the surface water is hardly gets to the groundwater level. However, monitoring of the groundwater suggest that the level fluctuates seasonally, just with some time of lagging. To solve the self-conflict scenarios, this project is gong to study the unsaturated permeability regularities between the rain soaked zone on the surface and the groundwater underneath by in-situ monitoring. A monitoring shaft on the top of a loess plateau has been dug to the depth of reaching to the groundwater level. Monitoring for the precipitation, evaporation, humidity and temperature will be conducted on the surface, water level fluctuation and spring outflow will be conducted in the shaft base and the gullies aside respectively; the moisture contents, matric suctions and temperatures changing with depth will be monitoring with the probes entombed on the wall of the shaft with definite spaces. Meanwhile, samples at each loess bed will be collected to measuring the basic physical properties and unsaturated water conductivity curves. An unsaturated permeability model will be constructed. Based on the input(precipitation) and output(evaporation and spring flows) boundary data, as well as the control parameters on the flow path (matric suction and temperature potentials), the groundwater level will be predicted with the model and testified with the monitoring results, which will contribute to discover the moving regularities of the water in loess. The study of unsaturated permeability of the loess will be the basis to renew the knowledge about some loess engineering problems, such as loess collapsibility, loess slope stability and loess underground space stability.

降雨(灌溉)是西北黄土地区地下水的仅有补给来源，但雨后天然剖面和人工模拟降雨观测都发现地表水在黄土中的入渗深度有限，似乎难以到达地下水位；而对地下水的观测则表明水位随着季节而变化，只是有不同程度的滞后。为了揭示这一自相矛盾的现象，本项目通过现场观测研究地表浸润带和地下水之间的非饱和渗流规律。在黄土塬顶开挖一深至地下水位的观测井，地表观测降雨量、蒸发量、湿度、气温等；井底观测水位；附近沟底观测泉流量；沿井壁一定间距布置探头观测含水量、基质吸力和温度。同时在各土层采样，测定土的基本物理指标和非饱和渗透曲线。建立非饱和渗流模型，根据观测获得的输入(降雨)、输出(蒸发、泉排泄)边界数据和渗流途径上的控制参数(吸力势、温度势)，预测地下水位动态变化，用实际监测结果验证理论模型，揭示黄土中水分运移规律。基于黄土非饱和渗流的研究，是重新认识黄土地区工程问题，如黄土湿陷、黄土边坡稳定和黄土硐室稳定的基础。

黄土地区地表降水或灌溉的垂直入渗是地下水的唯一补给来源。但地表水在黄土层包气带中的运移方式长期以来存在着争议，一种观点认为地表水是通过优势通道补给地下水的，以黄土中存在大量垂直节理为依据；另一种观点认为地表水是通过活塞流均匀入渗的，但又缺乏直接的观测证据。为了解决这一问题，本课题在黄土高原中部的甘肃正宁县建立了现场观测站。观测站利用当地开挖的一口深100米的民井。在地表设立了降雨量、蒸发量、气温、气压、湿度等观测设备，井壁上安装了水分计、水势计和温度计，井底安装了水位计。监测始于2015年6月1日，到目前已获得了2年半的监测数据，同时在室内做了相似的土柱入渗试验。根据观测和试验数据，得出了以下结果:黄土按含水状态在垂直方向可分为三个带，活动带、稳定带和饱和带。活动带自地表至地下2-7米，活动带的水分受降雨、蒸发蒸腾的影响随时间变化，自活动带到地下水位以上的毛细饱和带为稳定带，稳定带黄土中的水分、温度不随时间变化或变化很小，稳定带以下为饱和带。黄土中的水分在活动带为瞬态流，无论是流量还是方向都随时间变化，向下到稳定带则过渡为稳定流。由于黄土中的水分迁移是典型的非饱和流，渗透性随含水率变化，地表水在下渗过程中，随着土的含水率降低渗透性显著降低，这导致在地表水补给阶段会形成一个湿润峰。地表一旦停止补给，湿润峰以上的土层的水分则在蒸发和蒸腾作用下向大气排泄，只有一小部分水分透过湿润峰向下补给，向下补给的这部分水流逐步过渡为稳定流，该稳定流的区域就是稳定带，稳定流的流量就是地表水向地下水的补给量，在稳定带内不同高度的流量相等。由于稳定带含水率不变，无法观测到其中水的流动导致对该问题的争议。为了清楚地说明这一现象，我们提出了一个与非饱和渗流相似的水力模型，该模型由一系列串连的漏斗组成，利用该模型，无论从物理现象，还是从理论计算都可直观说明黄土中水分运移形成的三个带及其形成机制。