A hybrid computer program is developed to predict the water and salt outflow from a river basin in which irrigation is the major water user. A chemical model which predicts the quality of water percolated through a soil profile is combined with a general hydrologic model to form the system simulation model. The chemical model considers the reactions that occur in the soil, including the exchange of calcium, magnesium, and sodium cations on the soil complex, and the dissolution and precipitation of gypsum and lime. The six common ions of western waters, namely calcium (Ca(++)), magnesium (Mg(++)), sodium (Na(+)), sulfate (SO4(=)), chloride (Cl(-)), and bicarbonate (HCO3-) are considered in the study. The model is tested on a portion of the Little Bear River Basin in northern Utah. The model successfully simulates measured outflows of water and each of the six ions for a 24-month period. (Author)