Model for Predicting Simultaneous Distribution of Salt and Water in Soils

Knowledge of water and salt movement in soils is necessary for development of a management scheme for controlling the quality of irrigation return flow. A computer model was developed to predict the distribution of water and salts in the root zone under varying initial and boundary conditions. The model consists of water flow and salt flow sub-models. The water flow sub-model considers the numerical approximation of the general water flow equation with modification for water loss by evapotranspiration (and thus root extraction). The salt flow sub-model considers the mass flow of salts, chemical exchange, precipitation or dissolution of CaCO3, and CaSO4, and formation of undissociated Ca and Mg sulphate. The model was tested under laboratory and field conditions by comparing predicted values with experimental measurements. Satisfactory agreement was noted for the water content distribution in almost all the experiments. The model yielded approximately correct values of total salt distribution in the field and one of the column experiments. The agreement between the measured and predicted values for the two other column experiments was poor. The poor agreement seems to result from the irregular dissolution of the applied powdered salts. The distribution of individual ions was not accurately predicted by the model. The disagreement between the predicted and measured values was large at high salt concentration. Complex ion formation, insufficient description of exchange and activity coefficients at high salt concentration are suggested for this lack of agreement. Further development and field testing of the model are needed