The effect of a change in irrigation water quality on the salt load of the deep percolate of a saline sodic soil — a computer simulation study |
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| Authors: | J H Moolman |
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| Institution: | (1) Hydrological Research Unit, Geography Department, Rhodes University, Grahamstown 6140, P.O. Box 94, Republic of South Africa |
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| Abstract: | Summary Recent studies conducted in the Breë River Valley in South Africa, have indicated that the return flow from the flood irrigated soils (using saline borehole water) add substantially to the salt load of the receiving river. In the present study an irrigation return flow model, developed at the USBR, was used to predict what the effect of a change in irrigation water quality would have on the chemical composition of the deep percolate of a saline-sodic soil. Three scenarios were investigated, i.e. irrigation with saline borehole water (TDS = 1,279 mg/1), irrigation with low electrolyte water (TDS=91 mg/1), and irrigation with the low electrolyte water in the presence of surface applications of gypsum. The results indicate that the present flood irrigation practices lead to deep percolation losses of ca. 155 mm/ha/a. In the case of the borehole water the TDS content of the deep percolate ranges between 5,500 and 6,400 mg/l, equivalent to salt loads of 8.59 and 9.98 t/ha/a respectively. By replacing the saline borehole water with the low electrolyte water the TDS content of the deep percolate will, over a five year period, be reduced to 2,100 mg/1, and the salt load to 3.3 t/ha/a. Surface applications of gypsum to prevent clay dispersion and hydraulic conductivity failures will, when compared to the low electrolyte scenario, increase the salt load by 1.2 t/ha/a. However, it will still be 4.84 t/ha/a less than is presently the case. It is concluded that by using the better quality irrigation water in combination with gypsum applications, the salt load in the receiving river attributable to irrigation return flow, can be reduced by approximately 10%. |
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