Modeling water quality for soils containing gypsic horizons

Abstract

A computer simulation model was developed to simulate the impact of irrigating soils containing gypsic horizons on soil water quality and percolated water quality. The model simulates saturated - unsaturated solute movement using the mixing cell approach to simulate dispersion and movement of soluble salts. Dissolution and precipitation of slightly soluble salts and the formation of ion pairs is considered as a function of temperature. Van Genuchten closed form equation is used to find soil water retention function. Finite difference method was applied to Richards equation for moisture movement simulation in the profile. The model predicts the concentration of the major cations and anions in each segment. The regression coefficients of the observed vs. simulated concentration of the major ions were higher than 0.961 in all the three replicates with slopes ranging between 0.717 and 0.940. Running the model at 1 and 41 °C showed significant differences in Ca, SO₄, and HCO₃ concentrations in the percolated water. However, the differences in Cl, Na, and Mg concentrations were not significant. The presence of high Ca and SO₄ concentrations in the irrigation water reduced gypsum solubility in soils containing a layer of gypsum compared with the presence of Mg and Cl in the irrigation water.