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Hydrodynamic Dispersion in an Unsaturated Dune Sand

机译：不饱和沙丘中的水动力分散

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Solutes spread relative to the mean displacement position during water flow in soils as a result of meandering through the (partially) saturated pore complex. Spreading is characterized by the hydrodynamic dispersion coefficient in the convection-dispersion equation (CDE). This coefficient has been extensively studied for saturated soils. In this study hydrodynamic dispersion coefficients for nonaggregated dune sand were determined as a function of volumetric water contents, , ranging from saturation to 0.08 cm3cm-3 in columns of 5-cm diam. and 25- to 40-cm length. Unit-gradient flow experiments were conducted to measure solute breakthrough curves (BTCs) using four-electrode salinity probes at several column depths. Transport parameters for the CDE and the mobile-immobile model (MIM) were determined by optimizing analytical solutions to observed BTCs. A maximum dispersivity, , of 0.97 cm was found at = 0.13, whereas for saturated flow 0.1 cm irrespective of pore-water velocity ranging from 208 to 5878 cm d-1. For the MIM, the mobile water fraction, m/, gradually deceased from almost unity at saturation to a minimum of 0.85 at = 0.15 followed by a slight increase with further desaturation. The exchange time between the mobile and immobile phases, 1/, was 0.1 to 0.2 d for > 0.15 presumably because of the relatively homogeneous flow with convective solute mixing. For lower , the exchange became much slower since flow predominantly occurs in water films enveloping sand particles. The Peclet number for molecular diffusion, Pe, decreases as the role of transverse diffusion increases at lower because of smaller v and thinner water films while the resistance increases for solute exchange between mobile and immobile phases. These combined effects lead to a maximum dispersivity value at intermediate water contents in the case of the nonaggregated dune sand.
机译：通过（部分） 饱和孔隙复合物的曲折作用，溶质相对于土壤中 水流的平均位移位置扩散。对流扩散的特征在于对流扩散方程 （CDE）中的流体动力 扩散系数。对于饱和 土壤，已经对该系数进行了广泛的研究。在这项研究中，确定 非聚集沙丘的流体动力扩散系数是体积 水分含量的函数，其范围从饱和度到0.08 cm 3 cm直径为5厘米的 列中的 -3 。和25到40厘米长。进行了单位梯度 流动实验，以在几个 柱深度使用四电极盐度探针测量溶质突破 曲线（BTC）。通过优化对观察到的BTC的解析解 来确定CDE和mobile-immobile 模型（MIM）的传输参数。在= 0.13时发现最大分散度为0.97 cm ，而对于饱和流为0.1 cm，则孔隙水速度范围为208至5878 cm d 均不考虑 。 -1 。对于 MIM，移动水分数 m /从饱和状态下的 逐渐减小到最小的0.85（在= 0.15处），然后< sup> 随饱和度的增加而略有增加。流动相和固定相之间的交换时间（s / 1 /）为0.1到 0.2 d，持续时间> 0.15，大概是因为 流量相对均匀对流溶质混合。较低时，交换 要慢得多，因为流量主要发生在包裹沙粒的水 薄膜中。由于v变小和水膜变薄，分子 扩散的Peclet数P e 随横向扩散 的作用增加而降低。 sup> ，而流动相和固定相之间的溶质交换阻力增加。在未凝结的沙丘情况下，这些综合作用导致在中等含水量下的最大 分散性值。

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    《Soil Science Society of America Journal》 |2003年第3期|703-712|共10页
  • 作者

    Nobuo Toride; Mitsuhiro Inoue; Feike J. Leij;

  • 作者单位

    Dep. of Agricultural Sciences, Saga Univ., Saga 840-8502, Japan,George E. Brown Jr. Salinity Laboratory, 450 West Big Springs Road, Riverside, CA 92507;

    Arid Land Research Center, Tottori University, Hamasaka 1390, Tottori 680-0001, Japan,George E. Brown Jr. Salinity Laboratory, 450 West Big Springs Road, Riverside, CA 92507;

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