黄土中降雨入渗规律的现场监测研究

In-situ observation on rainfall infiltration in loess

已有许多人工降雨试验确定的黄土入渗深度有限,一般很少超过4 m,由此认为降雨难以通过正常渗流途径到达地下水位,而是通过裂隙、落水洞等通道灌入深部补给地下水的。然而调查发现,这种入水通道仅在黄土塬边的卸荷区常见,塬的中部很少。为了了解黄土地区地表水是以何种方式补给地下水的,在甘肃正宁县建立了一个监测站,通过在一深度为10 m的探井井壁上埋设土壤水分计,对天然降雨入渗条件下不同深度黄土层的体积含水率变化情况进行了为期一年的连续监测,同时采用雨量计记录其间的日降雨量。结果表明:2 m以内的浅部土层,土壤水分具有周年的背景变化趋势,该趋势和蒸发量的变化趋势吻合。当日降雨量小于18 mm/d时,水分仅在表层循环,对地表以下(>20 cm)的含水率几乎没有影响。当日降雨量大于18 mm/d时,才会引起土壤含水率骤增,降雨量越大,土壤含水率增幅越大,影响深度越大,随着深度增加,增幅减小,时间上渐有滞后。观测点黄土的浸润带约为2 m,2 m以下的非饱和黄土中,水分以非饱和渗流或水汽形式迁移,水汽迁移量很小,但不可忽视,当遇到透水性差的古土壤层时,会在其顶部富集,长期作用则可能形成软弱带,诱发黄土滑坡。

By many artificial rainfall tests, the depth of rainfall infiltration in loess area was 4 meters at most. So, it is considered that the rainfall is hard to reach groundwater through the normal leaking way, but through vertical fissures,sinkholes or some other water paths to recharge groundwater. However,according to our investigation, these water paths only occur in unloading areas which always in the edge of loess tableland, but rare in the center. In order to make clear surface water recharge groundwater in loess area, a monitoring site was set up in Zhengning county, Gansu Province. By using the soil moisture meters which inserted into the wall of a 10-meter-deep exploratory well, the changes of volumetric moisture content of soil layers with different depth were observed continuously for one year under the condition of natural rainfall, the daily precipitation during the monitoring period was also recorded by rain gauge. The results show that the moisture content of the shallow layers within 2 meters changed in annual cycle, and the trend is consistent with the change of evaporation. When the precipitation is less than 18 mm/d,the circulation of soil moisture occurs mainly in surface layers, and has little effect on the moisture content below 20 centimeters. But when the precipitation is more than 18 mm/d, the moisture content may have a sudden increase. The greater the rainfall, the higher the growth of moisture content and the deeper the affected range will be. With the increasing depth, the growth decreases and the change of moisture content lags behind the depth. The infiltration zone of the loess is about 2 meters. In the unsaturated soil below 2 meters, water moves mainly in the form of unsaturated seepage or vapor form, the motion of vapor is very small but cannot be ignored, the water would gather on the surface as encountering paleosol layer with low permeability,weak zone will be formed after long-term accumulation,inducing loess landslides eventually.