基于COMSOL软件的绿洲盐渍化土壤中多离子耦合运移模型构建

了解盐分离子在土壤中的迁移规律可以为盐渍土的综合治理以及高效利用提供科学依据,在COMSOL多孔介质和地下水流模块模拟非饱和土壤水流的基础上,自定义偏微分方程组构建盐渍化土壤SO42–、Ca2+、Na+、Cl–、Mg2+耦合运移模型,考虑阳离子交换过程以及硫酸钙的沉淀溶解反应,并通过新疆绿洲膜下滴灌田间试验对模型进行检验,对比不同活度系数估算方法对模拟结果的影响。结果表明,各离子模拟值与实测值吻合较好,平均相对误差介于9.15%~28.57%,决定系数介于0.41~0.88,该模型能够较好地反映土壤中盐分离子的动态变化规律;在膜下滴灌条件下,膜下0~40 cm土层的盐分离子有不同程度的淋洗,Cl–和Na+的淋洗效果好于Ca2+和SO42–;活度系数的估算对模拟结果的准确性有重要影响,尤其是盐分含量较高时,采用通用的函数关系可能会带来较大的模拟误差。

Modeling of multiple ions coupling transport for salinized soil in oasis based on COMSOL

Abstract: Soil salinization constrains sustainable development of agriculture in arid area. Understanding dynamics of soil salt ions is helpful for the comprehensive treatment and high-efficient utilization of salt-affected soils. COMSOL is a flexible numerical simulation software based on finite element theory, with which one can freely define any type of function capable of describing material properties, sources or sinks, and boundary conditions. In addition, one can define a unique set of partial differential equations for describing certain physics phenomena that are not included in the preset modules in COMSOL. Based on these strengths, we reported a modeling study of SO42-, Ca2+, Na+, Cl-, and Mg2+ dynamics in salt-affected soil using COMSOL. Soil water flow was described using the Richards equations in porous media and subsurface flow module. Salt ions transports were simulated by the advection-dispersion equations in the presence of cation exchange, precipitation and dissolution of calcium sulfate, which were built in the user-defined partial differential equations module. The cation exchange was described by the Gapon equation, and the chemical reaction between Ca2+ and SO42- was described using the second-order equation. We further verified the model with an example of mulched drip irrigation with different irrigation amounts. The simulated soil water contents and ion concentrations in soil solution were generally in good agreement with the experiment measurement. The mean absolute error values for soil water contents ranged from 0.023 to 0.033 cm3/cm3, and the root mean square error values for those ranged from 0.030 to 0.040 cm3/cm3. For all the ions in soil solution, the mean relative error values ranged from 9.15% to 28.57%, and the coefficients of determination ranged from 0.41 to 0.88. It indicated that the model was capable of describing the dynamics of soil salt ions under field conditions. In the mulched drip irrigation system, all the concentrations of salt ions in soil solution decreased in the upper layer (around 40 cm) of the mulched soil after the irrigation, and then increased gradually due to water uptake of root and chemical reaction, or both. As Ca2+ and SO42- in soil solution were replenished by the dissolution of calcium sulfate, their concentrations increased more rapidly than those of Cl- and Na+, which indicated that Cl- and Na+ were leached more easily. However, all the ions gradually accumulated on the exposed soil surface, and the accumulation amount of Cl- was the biggest due to its strong mobility in soil. In addition, the simulation results based on different activity coefficient equations, i.e., Davies equation and the exponential equation fitted from the measured values, were compared. The activity coefficient values calculated from the Davies equation were generally larger than those calculated from the fitted exponential equation. As a result, the simulated Ca2+, SO42-, Na+, and Mg2+ concentrations in soil solution based on the Davies equation were generally lower than those based on the fitted exponential equation, especially for Ca2+ and SO42-. The results suggest that the calculation method of activity coefficient has obvious effect on the model accuracy, and general activity coefficient equation might lead to considerable simulation errors for saline soil.