Saline water management for irrigation in India

Abstract

Much (32–84%) of the ground water surveyed in different Indian States is rated either saline or alkali. Because of the continental monsoonal climate, the basic principles of saline water management need some adaptation, e.g. providing for a leaching requirement is not appropriate when the growing season for post-monsoon winter crops starts with a surface-leached soil profile, because it would increase the salt load. High salinities during the initial stages of growth are particularly harmful. Further, if benefits are to be gained from frequent saline irrigation, the amount of water applied per irrigation needs to be reduced. This is not possible with most widely practiced surface irrigation methods, but can be achieved with sprinkler and drip methods. However, in India the large-scale use of such systems is not yet technically or economically feasible. Another management goal is simultaneously to encourage the utilisation of carried over rainwater in the soil profile/shallow watertables. Tolerance limits of crops to the use of saline water in different agro-ecological regions of India are available, and have been observed to vary with soil type, rainfall and anionic/cationic constituents of salinity. Multi-location trials on the appropriate use of saline and non-saline water reveal the benefits of irrigating with non-saline canal water during the initial stages of growth, as well as cycles of saline and non-saline water during the pre-sowing irrigation period. Monsoon-induced salt leaching decreases with increasing clay content, SAR iw, and is enhanced with increasing chloride salinity. Additional doses of phosphorous to alleviate the effects of chloride toxicity, and the use of organic materials to enhance the efficiency of applied nitrogen are recommended under saline-irrigated conditions. Contrary to the general belief that soils irrigated with high-SAR saline water may regain their infiltration capacity when the electrolytic concentration of ingoing water is greater than the flocculation value, irreversible reductions are induced under cyclic saline-rainwater infiltration where sub-soil layers, ingressed with clays from the plough layer, control steady intake rates. Thus, the use of gypsum (SAR iw > 20) is advocated. Gypsum is also needed for soils irrigated with saline water with an Mg:Ca ratio > 3 and rich in silica. Other cultural practices, such as furrow planting, increasing the plant density and post-seeding irrigation in crops like mustard, also prove useful. Water-quality standards which were too conservative have been replaced by site-specific guidelines where factors such as soil texture, rainfall and crop tolerance have been given due consideration.