Submitted to: Laboratory Publication
Publication Type: Other
Publication Acceptance Date: March 5, 1995
Publication Date: N/A
Interpretive Summary: Due to the complexity of processes involved, prediction of water and ion concentrations in the unsaturated zone require use of computer models and many of the problems for irrigated agriculture require prediction of transient conditions and representation in 2 and 3 dimensions where salt concentrations increase at the soil surface with time. This report provides the documentation for the UNSATCHEM2-D computer model. The model provides for prediction of major ion chemistry and water movement in space and time. The model is able to simulate 2 dimensional problems, such as water and salt distributions for furrow and drip irrigation systems. It is to be used as a tool to evaluate the impact of management changes on salt distribution in and below the rootzone. It should be especially useful for assessing the need for surface leaching and reclamation in drip irrigation systems.
Technical Abstract: This report documents the development of the two-dimensional finite element code UNSATCHEM-2D for modeling major ion equilibrium and kinetic non-equilibrium chemistry in variably saturated porous media. The model is intended for prediction of major ion chemistry and water and solute fluxes for soils under transient conditions. Solution chemistry in the unsaturated zone is significantly influenced by variations in water content, temperature and CO2 concentrations in the soil gas, thus these are also calculated by the model. The flow equation incorporates a sink term to account for water uptake by plant roots. The diffusion in both liquid and gas phases and convection in the liquid phase are considered as CO2 transport mechanisms. CO2 production model is described. The model accounts for equilibrium chemical reactions such as complexation, cation exchange and precipitation-dissolution. For the precipitation-dissolution of calcite and dissolution of dolomite, either equilibrium or multicomponent kinetic expressions are used which include both forward and back reactions. The program is written in ANSI standard FORTRAN 77. Computer memory is a function of the problem definition. This report serves as both a user manual and reference document. Detailed instructions are given for data input preparation. Selected example input and output files are also provided.