Submitted to: International Soil Management Workshop
Publication Type: Proceedings
Publication Acceptance Date: March 2, 1996
Publication Date: N/A
Interpretive Summary: Assessment of the suitability of a water for irrigation is of increased importance as marginally suitable waters and drainage waters are more frequently utilized. Proper assessment of water suitability requires that we predict the resultant distribution of salts in the rootzone, and evaluate potential changes in salinity related to changes in irrigation management, such as irrigation system, leaching fraction, amendment applications, etc. We describe the preliminary development of a user friendly model designed expressly for assessment of irrigation water. By describing unsaturated water flow, major ion chemistry and plant water uptake, it provides the capability to predict transient changes in water content, solute composition and plant water uptake. Reductions in water uptake are then related to yield loss. The user can subsequently evaluate the impact of other management changes and determine under what circumstances the irrigation water might be useable.
Technical Abstract: We present the preliminary development of the computer model tentatively named Soil Salinity. The model is intended as a management tool for predicting the salinity and sodium distribution with depth and time. The model couples variably saturated water flow to CO2 production and transport, solute transport, a generic plant growth submodel and a major ion chemistry submodel. 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 calculated by the model. The model considers that adverse salinity or water stress conditions reduce water consumption and reduce biomass yield proportionally to the reduction in water consumption. The model accounts for equilibrium reactions such as complexation, cation exchange and precipitation-dissolution among the major chemical components. The example presented demonstrates the applicability of the model for evaluating the impact of water quality and irrigation management on soil salinity and ion composition. The model provides predictions of water content, dissolved ions and exchangeable cations with time and depth. It is anticipated that in many instances users will not have a complete input data set, thus default parameters are available for some functions. For functions, for example, hydraulic properties, where default parameters are not appropriate, we make available a small menu based on soil texture.