A. Quality of Irrigation
Water Whatever may be the source of irrigation water viz., river, canal, tank, open well or tube well, some soluble salts are always dissolved in it. The main soluble constituents in water are Ca, Mg, Na and K as cations and chloride, sulphate, bicarbonate and carbonate as anions. However, ions of other elements such as lithium, silicon, bromine, iodine, copper, cobalt, fluorine, boron, titanium, vanadium, barium, arsenic, antimony, beryllium, chromium, manganese, lead, selenium, phosphate and organic matter are also present.
Among the soluble constituents, calcium, sodium, sulphate, bicarbonate and boron are important in determining the quality of irrigation water and its suitability for irrigation purpose. However, other factors such as soil texture, permeability, drainage, types of crop etc., are equally important in determining the suitability of irrigation water. Following are the most common problems that result form using poor quality water.
Salinity – If the total quantity of salts in the irrigation water is high, the salts will accumulate in the crop root zone and affect the crop growth and yield. Excess salt condition reduces uptake of water due to high concentration of soil solution.
Permeability – Some specific salts reduce the rate of infiltration into the soil profile.
Toxicity – When certain constituents of water are taken up by plants, which accumulate in large quantities and result in toxicity and reduces yield.
Miscellaneous – Excessive Nitrogen in irrigation water causes excessive vegetative growth and leads to lodging and delayed crop maturity. White deposits on fruits or leaves may occur due to sprinkler irrigation with high bicarbonate water. Based on the characteristic features of majority of ground waters in use by the farmers in different agro-ecological regions of the country, the various indices which describe the nature of hazards on soils and crops, irrigation waters have been broadly grouped into good, saline and alkali waters.
Majority of natural ground waters have pH between 7.2 and 8.5 and are either in equilibrium or even super saturated in respect of calcite and dolomite. Water with pH less than 7.2 seems to be unsaturated in respect of calcite. Water samples with pH > 8.4 invariably have SAR more than 10. High pH is associated with waters containing residual alkalinity and a high carbonate: bicarbonate ratio. Water having residual alkalinity contains carbonate and bicarbonate ions in varying proportions depending on pH. The ratio of CO3 ions in ground waters generally vary between 1:10 and 1:2, marginally saline waters have low SAR, the usual range being up to 20. Hardly 10-15 per cent of the total ground waters have both high SAR (>20) and high salinity.
Factors affecting suitability of water for irrigation The suitability of particular water for irrigation is governed by the following factors.
• Chemical composition of water (TSS, pH, CO3, HCO3, Cl, SO4, Ca, Mg, Na and B).
• Total concentration of soluble salts or salinity (EC).
• Concentration of sodium ions, in proportion to calcium and magnesium or sodicity (SAR).
• Trace element boron may be toxic to plant growth, if present in limits beyond permissible.
• The effect of salt on crop growth is of osmotic nature. If excessive quantities of soluble salts accumulate in the root zone the crop has extra difficulty in extracting enough water from salty solution, thereby affecting the yields adversely.
• Besides this, total salinity depends on the extent to which exchangeable sodium percentage (ESP) of soil increase as a result of adsorption of sodium from water. This increase depends on sodium percentage
. • Soil characteristics like structure, texture, organic matter, nature of clay minerals, topography etc.
• Plant characteristics like tolerance of plant varies with different stages of growth. The germination and seedling stages are usually more sensitive to salinity.
• Climatic factors can modify plant response to salinity. Tolerance to saline water irrigation is often greater in winter than in the summer. Rainfall is the most significant factor for the leaching of salts from the plant root zone. Temperature also plays a vital role.
• Management practices also play great role. Wherever saline water is used for irrigation, adoption of management practices which allow minimum salt accumulation in the root zone of the soil is necessary.
The primary parameters that have to be considered to ensure effective irrigation management for salt control are the water requirement of crop and quality of irrigation water. Correct irrigation should restore any soil water deficit to control salt levels.
Use of poor quality water
Besides the salinity and alkalinity hazard of water, some industrial effluents and sewage water are also problem waters that can be reused by proper treatment. The complex growth of industries and urbanization (Urban development) leads to massive increase in wastewater in the form of sewage and effluent. Waste water supplies not only nutrients but also some toxic elements such as total solids of chloride, carbonate, bicarbonate, sulphate, sodium, chromium, calcium, magnesium, etc., in high concentration. Besides this, the effluent or wastewater creates BOD (Biological Oxygen Demand). These wastewaters when used for irrigation lead to surface and sub surface source of pollution due to horizontal and vertical seepage.
Points to be considered
• Application of greater amounts of organic matter such as FYM, compost etc., to the soil to improve permeability and structure.
• Increasing the proportion of calcium, through addition of gypsum (CaSO4) to the irrigation water in the channel, by keeping pebbles mixed gypsum bundles in the irrigation tank.
• Mixing of good quality water with poor water in proper proportions so that both the sources of water are effectively used to maximum advantage.
• Periodical application of organic matter and raising as well as incorporation of green manure crops in the soil.
• Irrigation the land with small quantities of water at frequent intervals instead of large quantity at a time.
• Application of fertilizer may be increased slightly more than the normally required and preferably ammonium sulphate for nitrogen, super phosphate and Di Ammonium Phosphate (DAP) for phosphorus application.
• Drainage facilities must be improved.
• Raising of salt tolerant crops such as cotton, ragi, sugar beet, rice, groundnut, sorghum, corn, sunflower, chillies, tobacco, onion, tomato, garden beans, amaranthus and lucerne.
Projected waste-water utilization – It is estimated that 2,87,000 million m3 of waste water can be reusable.
Hence, these waste waters can be properly treated as follows:
• Dilute with good quality water in the ratio of 50:50 or 75:25.
• Alternate irrigation with waste water and good quality water.
• Treat the effluent water through fill and draw tanks, lime tank, equalization tank, settling tank, sludge removal tank, aerobic and anaerobic treatment tanks etc.
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