Jiao Huiqing, Sheng Yu, Zhao Chengyi, Li Baoguo. Modeling of multiple ions coupling transport for salinized soil in oasis based on COMSOL[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2018, 34(15): 100-107. DOI: 10.11975/j.issn.1002-6819.2018.15.013
    Citation: Jiao Huiqing, Sheng Yu, Zhao Chengyi, Li Baoguo. Modeling of multiple ions coupling transport for salinized soil in oasis based on COMSOL[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2018, 34(15): 100-107. DOI: 10.11975/j.issn.1002-6819.2018.15.013

    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.
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