Abstract

Drainage water compositions were calculated with a computer simulation model from irrigation water compositions, leaching fractions, aragonite and gypsum solubilities, and measured partial pressure of CO₂. The calculated compositions were compared with measured values obtained from lysimeters filled with Pachappa soil, cropped with alfalfa, and irrigated with eight synthetic waters typical of rivers in the western U. S. Linear regression analysis of predicted vs. measured values for Na⁺ and SO₄^2- concentrations, sodium-adsorption-ratio, electrical conductivity, and salt burden resulted in essentially one-to-one relationships. The gain in salt burden of drainage water at high leaching fractions due to mineral dissolution was adequately described by assuming the soil solution was saturated with respect to aragonite. Some evidence for Mg²⁺ precipitation was found.

The utility of the simulation model is demonstrated for evaluating the salinity, sodicity, and pollution hazards of irrigation waters. The evaluation of irrigation water quality in terms of minimum leaching fractions commensurate with maintaining satisfactory salinity, and sodicity levels is illustrated using recent crop tolerance data for alfalfa and the hydraulic conductivity characteristics of several representative soils. For salt-tolerant plants leaching fractions between 0.05 to 0.1 should be safe for six of the eight waters. Salt burdens of drainage waters are predicted to decrease with decreasing leaching fraction and partial pressure of CO₂. The predicted relative decrease in salt burden ranged from 0.32 to 0.85 for a decrease in leaching fraction from 0.33 to 0.04.

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