UNSATCHEM Module

The SWMS_2D model was expanded by Šimůnek and Suarez (1994) to also simulate the transport of major ions in variably-saturated porous media, including major ion equilibrium and kinetic non-equilibrium chemistry. The resulting UNSATCHEM-2D code was intended for prediction of major ion chemistry and water and solute fluxes in soils during transient flow. Since the solution chemistry in the unsaturated zone is significantly influenced by variations in water content, temperature and CO2 concentrations in the soil gas phase, all of these variables were included in the model. The CO2 transport and production model was based on the SOILCO2 model (Šimůnek et al., 1993) described below. The major variables of the chemical system in UNSATCHEM-2D were Ca, Mg, Na, K, SO4, Cl, NO3, H4SiO4, alkalinity, and CO2. The model accounted for various equilibrium chemical reactions between these components, such as complexation, cation exchange and precipitation-dissolution. For the precipitation-dissolution of calcite and dissolution of dolomite, either equilibrium or multicomponent kinetic expressions could be used, which included both forward and back reactions. Other dissolution-precipitation reactions considered included gypsum, hydromagnesite, nesquehonite, and sepiolite. Since the ionic strength of soil solutions can vary considerably in time and space and often reach high values, both the modified Debye-Hückel and the Pitzer expressions were incorporated into the model to calculate single ion activities. The UNSATCHEM-2D module was incorporated into the HYDRUS software in 2009.

The actual FORTRAN model simulating water, heat, carbon dioxide, and multicomponent solute movement with major ion equilibrium and kinetic chemistry in one-dimensional variably saturated media. Relevant features of the UNSATCHEM module are:

Parts of the code were adopted from the UNSATCHEM program developed by Šimůnek et al. (1996).

Other Relevant References:

Šimůnek, J., T. Vogel and M. Th. van Genuchten, The SWMS_2D code for simulating water flow and solute transport in two-dimensional variably saturated media, Version 1.1., Research Report No. 132, U. S. Salinity Laboratory, USDA, ARS, Riverside, CA, 1992.

Šimůnek, J., and D. L. Suarez, Modeling of carbon dioxide transport and production in soil: 1. Model development, Water Resour. Res., 29(2), 487-497, 1993.

Suarez, D. L., and J. Šimůnek, Modeling of carbon dioxide transport and production in soil: 2. Parameter selection, sensitivity analysis and comparison of model predictions to field data, Water Resour. Res., 29(2), 499-513, 1993.

Šimůnek, J., and D. L. Suarez, Two-dimensional transport model for variably saturated porous media with major ion chemistry, Water Resour. Res., 30(4), 1115-1133, 1994.

Šimůnek, J., D. L. Suarez, and M. Šejna, The UNSATCHEM software package for simulating one-dimensional variably saturated water flow, heat transport, carbon dioxide production and transport, and multicomponent solute transport with major ion equilibrium and kinetic chemistry, Version 2.0, Research Report No. 141, U.S. Salinity Laboratory, USDA, ARS, Riverside, California, 186pp., 1996.

Šimůnek, J., and D. L. Suarez, Sodic soil reclamation using multicomponent transport modeling, ASCE J. Irrig. Drain. Engineering, 123(5), 367-376, 1997.

Suarez, D. L., and J. Šimůnek, UNSATCHEM: Unsaturated water and solute transport model with equilibrium and kinetic chemistry, Soil Sci. Soc. Am. J., 61, 1633-1646, 1997.

Šimůnek, J., D. Jacques, M. Th. van Genuchten, and D. Mallants, Multicomponent geochemical transport modeling using the HYDRUS computer software packages, J. Am. Water Resour. Assoc., 42(6), 1537-1547, 2006.

Gonçalves, M. C., J. Šimůnek, T. B. Ramos, J. C. Martins, M. J. Neves, and F. P. Pires, Multicomponent solute transport in soil lysimeters irrigated with waters of different quality, Water Resour. Res., 42, W08401, doi:10.1029/2006WR004802, 17 pp., 2006.

Buchner, J. S., J. Šimůnek, J. Lee, D. E. Rolston, J. W. Hopmans, A. P. King, and J. Six, Evaluation of CO2 fluxes from an agricultural field using a process-based numerical model, J. Hydrology, doi: 10.1016/j.jhydrol.2008.07.035, 361(1-2), 131– 143, 2008.

Šimůnek, J., M. Šejna, H. Saito, M. Sakai, and M. Th. van Genuchten, The HYDRUS-1D Software Package for Simulating the Movement of Water, Heat, and Multiple Solutes in Variably Saturated Media, Version 4.0, HYDRUS Software Series 3, Department of Environmental Sciences, University of California Riverside, Riverside, California, USA, pp. 315, 2008.

Ramos, T. B., J. Šimůnek, M. C. Gonçalves, J. C. Martins, A. Prazeres, N. L. Castanheira, and L. S. Pereira, Field evaluation of a multicomponent solute transport model in soils irrigated with saline waters, J. of Hydrology, 407(1-4), 129-144, 2011.