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This article is presented by:
M. Dhakshinamoorthy
Professor, IMTI, Trichy
MANAGEMENT STRATEGIES FOR POOR QUALITY WATER
Quality of available water is so important in determining the crop and type of irrigation system selected. Irrigation water quality is based on the salinity hazard, sodium content and quality of other materials which may present in amounts which can accumulate to toxic levels in soil. The following parameters are determining the water quality rating.
Total salt concentration
Total salts can be estimated by measuring electrical conductivity. Electrical conductivity (EC) is expressed as dSm-1.The salinity hazard based upon the above classification does not always give a true answer as to whether or not the water may be used. The total salt content of irrigation water will give only a general qualitative assessment of its quality.
Sodium Hazard:
The sodium hazard irrigation water is determined both by the absolute and relative concentrations of cations. The total sodium can be readily measured and that total percentage along with its relationship between sodium and other cations (Ca, Mg) can be determined by calculating the Sodium Adsorption Ratio ( SAR). The following equation is used.
Where Na+, Ca++, Mg++ represent the concentrations in milli equivalents per litre of the respective ions. Good quality water usually has an SAR of 10 or less.
In addition to SAR, the relationship between SAR and Soluble Sodium Percentage (SSP) is estimated to give the estimated Sodium Percentage(ESP), then the ESP is related to SAR for classification purposes. The validity of sodium hazard prediction may be confirmed by examining the relationship between SAR and ESP of the soil.
Residual Sodium Carbonate (RSC)
Toxic constituents frequently found in irrigation water are bicarbonates, boron, iron, fluoride and dissolved gas like Hydrogen Sulphide, Residual Sodium Carbonate exists in the water when the Carbonate plus Bicarbonate contents exceeds the Calcium plus Magnesium concentrations. RSC can be computed from the following equation using equivalents per million.
RSC values above 2.50 would cause the irrigation water to be classified as unsuitable.
