渭河咸阳段非饱和层状沉积物中水分分布特征

Water content distribution of unsaturated layered sediments of Weihe River in Xianyang section

层状结构是河流相松散沉积物中最常见的沉积结构,通常表现出一些特有的水力学性质,对水分运动过程及沉积物的持水性能产生影响。本文通过对渭河陕西咸阳段河漫滩一个层状沉积物剖面的含水率分布特征进行原位观测,讨论层状沉积物中含水率的分布特征,及其与粒度和毛细壁垒效应的关系。结果表明:在以水平层理和交错层理为主的粗细相间的层状河流沉积物中,天然含水率也呈干润相间的层状分布特征,各层含水率的大小主要与粒度成负相关;沉积物粗细相间的层状分布为毛细壁垒的形成提供条件,在降雨入渗和土壤水蒸发过程中,毛细壁垒效应对水分运动的控制会导致细粒层中含水率高于粗粒层。层状松散沉积物中水分的这种分布规律,为理解土壤持水性能提供了新的思路,对研究土壤含水率与降雨入渗、蒸发等水文过程的关系具有重要意义。

[Background] Layer structure is the most common sedimentary structure in alluvial unconsolidated sediments,which usually exhibits special hydraulic properties to influence water flow and water retention ability.In this paper,a situ investigation on the water content distribution,grain-size and bulk density for a layered sediment profile was conducted at the floodplain of Weihe River in Xianyang,in the center of Shaanxi,then the effects of the grain-size and the capillary barriers on natural water content were discussed.[Methods]The experiment site outcrop was mapped into 11 macro layers having different texture and structures.With each layer,well developed laminations were observed.We investigated the characteristics of water content and its corresponding influence factors including soil bulk density,grain-size and matric suction in each layer,based on sampling analysis and the measurement of EC-5 Small Soil Moisture Sensor and MPS-6 Calibrated Water Potential Sensor.Firstly,we analyzed the relationship between grain-size and water content in each layer,based on this,the effect of capillary barriers on water content was discussed.Then,we analyzed the influence of capillary barriers in the process of precipitation infiltration and soil water evaporation.[Results] The results illustrated that inthe layered alluvial sediments with alternating layers of coarse and fine sands,natural water content exhibited in a layered distribution on a form of interbedding high and low water content.The magnitude of the water content was negatively correlated with the grain-size,and was positively correlated with the bulk density.The water content of site outcrop was between 0.010 cm^3/cm^3-0.148 cm^3/cm^3,the median size ranged from 0.16 mm to 0.39 mm and the bulk density ranged from 1.32 g/cm^3 to 1.53 g/cm^3.The results also demonstrated that the layered sediments with alternating layers of coarse and fine sands formed the capillary barriers in the process of water flow,therefore by which the water content distribution was controlled.In both process of precipitation infiltration and soil water evaporation,the capillary barriers effects will lead to the water content in fine layer obviously higher than that in coarse layer.[Conculsions] In conclusion,capillary barriers become a significant factor to control water flow in unsaturated soils.At the process of precipitation infiltration,capillary barriers have the effect on permeability reduction,which can be used in designing landfill anti-seepage system,optimized drainage layers and so on.Additionally,insights are potentially useful for designing mulching strategies and capillary barriers aimed at reducing evaporative losses,increasing water retention ability of soil,which is significantly important in arid and semi-arid regions.