Critical Review of Irrigation Water Quality Parameters for Assessing Sodium and Bicarbonate Hazards and Gypsum Application for Quality Improvement

Traditional parameters such as sodium adsorption ratio (SAR), residual sodium carbonate (RSC), and exchangeable sodium percentage (ESP) for assessing water/soil quality have been critically reviewed. Limitations of these parameters especially related to neglect of potassium and the assumption that calcium and magnesium have similar effect on soil flocculation are highlighted. It emerged that the role of Mg in water is least understood rendering these parameters to underestimate the problems related to sodium and bicarbonate hazards. Latest research for devising new parameters to overcome the limitations by incorporating relative dispersive and flocculating role of 4 cations namely Na, K, Mg and Ca is reviewed. Some of the new parameters such as sodium to calcium adsorption ratio (SCAR), cation ratio of structural stability (CROSS), optimized cation ratio of structural stability (CROSSopt), exchangeable dispersion percentage (EDP), residual sodium bicarbonate (RSBC), and modified adjusted residual sodium (mod. RNa) have been included and their relative merits over the existing parameters discussed. Since most of these equations have been developed for specific soil water and agro-climatic conditions, final forms of these equations in generalized forms have been included for testing and evaluating their constants for the Indian monsoonal environment. Equations have been developed and reported for the first time to determine the gypsum requirement of high SAR alkali water to manage high SAR and high RSC and for managing Mg/Ca ratio of water having high Mg/Ca ratios. Application of these new parameters has great significance as India is at the verge of increasing use of recycled municipal wastewaters, polluted surface water, agricultural drainage water, and even groundwater associated with high concentrations of K and Mg. These parameters may also help to answer some of the unresolved issues such as resodification of alkali lands and productive use of water having high magnesium concentrations. It is concluded that research actions to test these new parameters should be initiated as early as possible for switchover from traditional to these new parameters.