Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 23, 1997
Publication Date: N/A
Interpretive Summary: Sustaining irrigated agriculture requires controlling rootzone salinity as well as maintaining a favorable water content and ion composition for plant growth. Evaluating the effects of management changes on drainage water composition and volume is also required. The complexity of the processes involved requires development of computer models to aid decision makers. In this paper we describe the development of the UNSATCHEM model which is designed to prediction water transport and major ion and boron chemistry under dynamic conditions. The model includes several unique features including prediction of the soil gas phase composition, consideration of the adverse effect of sodicity and high pH on soil hydraulic properties, and prediction of the reduction in water consumption by plants under water, salt or oxygen stress. A user friendly Windows based interface is included for parameter input and graphic display of output. The model is well suited for assessing water quality for irrigation including cyclic use of different water qualities and reclamation of soils high in salinity, sodicity or boron In addition to applications related to agriculture, the model provides a more realistic simulation of soil and ground water solution composition as well as for simulations of carbonate redistribution and/or climate change with time. Examples are given to demonstrate the utility of the model for prediction of major ion composition.
Technical Abstract: This model we couple one dimensional unsaturated water and solute transport with a major ion chemistry routine, and plant water uptake. The model is developed for predicting major ion chemistry in the soil zone and in recharge to ground water. The model has several unique features, including expressions relating reductions in hydraulic conductivity to chemical factors, prediction of CO2 partial pressure in the rootzone based on a CO2 production, multiphase transport submodel, kinetic expressions for silicate weathering and calcite precipitation dissolution and dolomite dissolution, B adsorption using the constant capacitance model and the option to use Pitzer ion interaction expressions for high ionic strength. The chemical submodel considers equilibrium ion exchange,as well as various equilibrium and kinetic expressions for precipitation and dissolution of soil minerals, including gypsum, Mg carbonates, and sepiolite. The use of a predictive submodel for CO2 production and transport allows for the calculation of carbon dioxide concentrations with depth and time. This avoids the assumption of constant CO2 distribution or constant pH, which is required by previous models. Use of kinetic expressions for carbonate chemistry allows for a more realistic simulation of soil and ground water solution composition as well as for simulations of carbonate redistribution and/or climatic change with time.