Record Search Query: [Keyword='Soil Moisture']
UNSATCHEM-2D Model: Element Code for Modeling Major Ion Equilibrium and Kinetic Non-Equilibrium Chemistry
Entry ID: USDA_ARS_UNSATCHEM-2D
Abstract: UNSATCHEM-2D is a two-dimensional finite element code 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. Since the solution
chemistry in the unsaturated zone is significantly influenced by variations in
water ... content, temperature and CO2 concentrations in the soil gas, all these
variables are also calculated by the model. The flow equation incorporates a
sink term to account for water uptake by plant roots. The heat transport
equation considers the heat transport by conduction and by convection with
flowing water. 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 major variables of the chemical system are Ca, Mg, Na,
K, SO4, Cl, NO3, H4SiO4, alkalinity, and CO2. The model accounts for
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 are used which include both
forward and back reactions. Other dissolution-precipitation reactions
considered include gypsum, hydromagnesite, nesquehonite, and sepiolite. Since
the ionic strength of soil solutions can vary considerably with time and space
and often reach high values, both modified Debye-Huckel and Pitzer expressions
were incorporated into the model to calculate single ion activities.
The program may be used to analyze water and solute movement in unsaturated,
partially saturated, or fully saturated porous media. UNSATCHEM-2D can handle
flow regions delineated by irregular boundaries. The flow region itself may be
composed of nonuniform soils having an arbitrary degree of local anisotropy.
Flow and transport can occur in the vertical plane, the horizontal plane, or in
a three-dimensional region exhibiting radial symmetry about the vertical axis.
The water flow part of the model can deal with prescribed head and flux
boundaries, as well as boundaries controlled by atmospheric conditions.
The governing flow and transport equations are solved numerically using
Galerkin-type linear finite element schemes. 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.
[Summary provided by the USDA.]
Access Constraints The program and manual are available upon request from:
USDA-ARS U.S. Salinity Laboratory
450 W Big Springs Road
Riverside, CA 92507-4617
Creation and Review Dates