Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)


Civil and Environmental Engineering


J. Paul Riley


A computerized mathematical model has been developed to simulate the system hydrology and water-salinity (as indicated by seven major ions constituting the salt) of a river basin, in which irrigation is the major beneficial water user. The overall system simulation model consists of three submodels: (1) a general hydrologic submodel programmed on a hybrid computer; (2) a chemical submodel to predict the quality of percolated water through the soil profile; and (3) a biological transformation submodel in respect of microbial nitrogen transformations, which is utilized by the chemical submodel. Both the chemical and biological submodels are programmed on a digital computer and are combined with the general hydrologic submodel to form the system simulation model. The overall model operates on monthly time intervals with variable spatial resolution.

The hydrologic portion of the model simulates the various hydrologic processes which are linked together by the continuity-of-mass principle and predicts the monthly runoff from the area. The chemical quality component of the model considers the complex chemistry of soilwater-plant system, including cation exchange on the soil complex, the dissolution or precipitation of lime (CaCO3), and calcium and magnesium sulphates ion pairs in solution.

The biological transformations submodel uses the kinetic approach in simulating the microbial nitrogen transformations within the root zone of a soil profile. The transformation reactions included are: (1) hydrolysis of Urea-nitrogen; (2) immobilization of ammonium-nitrogen; (3) mineralization of organic-nitrogen; and (4) immobilization of nitrate-nitrogen. The chemical composition of return flow is a function of these chemical and biological processes within the soil profile, in addition to blending of undiverted flows, evapotranspiration, and mixing of subsurface return flow with groundwater. Uptake of nitrate by aquatic biomass in the surface runoff of return flow has also been considered. The seven ions considered in the study are calcium, magnesium, sodium, sulphate, chloride, bicarbonate, and nitrate. The total dissolved solids outflow is a summation of these individual ions.

In order to demonstrate its capabilities, the hydro-quality model is applied to a large irrigated area of the Snake River Plains, near Twin Falls, Idaho. The model successfully simulated measured outflows of water and the concentration of seven ions for a 24 month period. The correlation coefficients range from 0.79 to 0.96 for the quantity and quality components of outflow, except for the sulphate ion, the correlation coefficient of which is 0.66. The model is general in nature and with suitable adjustments it can be applied to other areas and also to various kinds of management situations, including land application of waste water and studies involving nitrate pollution of groundwaters.