不同类型层状土壤持水能力的研究

为了了解不同类型层状土柱持水能力,利用砂土和砂黄土2种土壤,设置3种不同厚度分层土柱(11.25、22.5、45 cm)和2种匀质对照土柱,测定了土柱自初始饱和条件下的排水过程;同时利用匀质土柱测定结果标定2种土壤水力参数,通过Hydrus-1D模型对不同类型层状土柱排水过程进行模拟分析,获得了不同类型层状土柱的田间持水量。结果表明,层状土柱持水能力随着分层厚度的减小而增加,当分层厚度减小到一定程度时土柱持水能力不再随着分层厚度的减小而增加,该临界厚度取决于下层粗质土壤对上层细质土的吸力与上层细质土壤进气吸力之间的相对大小。本试验所用2种土壤分层临界厚度大约在5 cm左右;土柱失水主要来自下层较粗质地土壤,由饱和时的0.385 cm~3/cm~3减小到0.04 cm~3/cm~3。上层细质土壤含水量随着分层厚度的减小而增加。研究结果可为干旱半干旱地区矿区恢复和污染物填埋提供理论指导。

Study on water retention capacity for drained soils with different textural layering

Abstract: The impact of textural layering on water retention capacity was evaluated through three different layer thicknesses soils (11.25, 22.5 and 45 cm), and two homogeneous soils (sand and sandy loess) as control in 90 cm long columns. The soil columns were completely saturated and then drained with a positive pressure of 2 cm at the bottoms of the soil columns. The changes of water storage with time were measured by weighing the columns and by measuring the soil water content profiles using TDR probes, respectively, at 1, 5, 24, 48, 72 and 96 hours of drainage. The hydraulic properties of the sand and sandy loess were measured in the laboratory and also optimized using numerical simulations for the two homogeneous soil columns. Comparing the measured hydraulic parameters with the optimized ones, we found that the optimized parameters could decrease the maximum relative error from 19% to 9%. These optimized properties were then used to simulate the drainage process of deep profiles in more typical field conditions. The results showed that: 1) The laboratory observations and simulations all confirm that the amount of water retention decreases as the thickness of the layers increased. When the combined pressure caused by the suction of the underlying coarser layer plus the hydrostatic pressure within the finer sandy loess layer exceeds the air entry value (AVE) of the finer sandy loess, the amount of the water retention capacity does not increase with decreasing thickness of layer in the textural soil. In this study, we found the minimum thickness of the fine sandy loess is about 5 cm; 2) The water content within the finer textural layers decreased only slightly from saturation, with almost all of the water loss occurring from the coarser textural layers. This phenomenon can be found in the water content profile. The water content profiles in all layered soil columns showed distinct breaks at the layer interfaces, and the water content of coarser layers decreased from saturated to only 0.04 cm3/cm3. Though the water content of finer sandy loess layers changed little, it also can be found that the water content of finer soil increased with the decreasing thickness. The results of this research can provide some recommendations for mine reclamation in arid and semiarid regions.