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Estimation of the Salt Burden of Irrigation Drainage Waters¹

ABSTRACT

The problem of predicting salt concentrations and mass emissions below irrigated fields is studied with three models, ranging from an inert proportional model frequently used by water quality regulatory agencies, to steady state and transient models which include chemical reactions as well as salt transport. Calculations of salt movement are made for four waters and three leaching fractions, for depths up to 450 cm below the surface.

Results suggest that the proportional model will overestimate salt burden for waters which tend to precipitate Ca²⁺-salts and will underestimate the salt burden of waters which tend to dissolve native CaCO₃ in the soil. The extent of the error is a function of the leaching fraction (LF) and the irrigation water composition. The steady state model, which does not consider ion exchange, predicts a greater tendency toward mineral dissolution than occurs when Ca²⁺ ions are brought into solution through ion exchange, but calculates a more accurate salt balance than the proportional model. For instance, with a saline irrigation water at a LF = 0.1, the steady state and transient model mass emission predictions were 22 and 36% less, respectively, than the estimates of the proportional model. At the other extreme, with a high-quality irrigation water at a LF = 0.4, the mass emission predictions of these two models were 198 and 201% higher, respectively, than the predictions of the proportional model.

The transient stage of salt movement was found to persist until Mg²⁺ had reached steady state, which required about 12 years/m at a moderate leaching fraction of 0.25. It was concluded that the steady-state and proportional models are not appropriate for estimating the salt burden of irrigated fields on underlying ground water.

Key Words: salt movement • environmental impact • 208 planning • magnesium exchange

¹ Contribution of Dep. of Soil and Environ. Sciences, Univ. of California, Riverside, CA 92521.

² Associate Professor of Soil Physics and Professor of Soil Science, respectively.

Received for publication June 2, 1979.

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