Irrigation Water: Nature, Suitability and Improvement

After reading this article you will learn about:- 1. Nature of Irrigation Water 2. Harmful Effect of Poor Quality of Irrigation Water 3. Criteria for Suitability of Irrigation Water 4. Improvement in the Quality.

Nature of Irrigation Water:

In arid regions of India, i.e. Uttar Pradesh, Haryana, Punjab, Rajasthan, Gujarat etc. water available for irrigation usually contains soluble salts which include ions of calcium, magnesium, sodium, chloride, sulphate, bicarbonate and sometimes borate and nitrate. Their relative amounts depend on the source of water. Well water from drier areas of Uttar Pradesh, Haryana, Gujarat and Rajasthan are very saline.

Harmful Effect of Poor Quality of Irrigation Water:

If water which contains excessive amounts of soluble salts is applied to the field, these salts increase the osmotic pressure of the soil solution. Consequently, crops cannot absorb enough water, quickly enough to make up the rapid loss of water by transpiration on hot summer days. So they wilt permanently and the yield is decreased. Excessive amounts of chloride when present in irrigation water is very harmful to crops.

Borates even when present in small amounts are also very harmful to most crops. If excessive amount of sodium ions are present in irrigation water, these will be adsorbed by the clay micelle; the clay will de-flocculate, consequently destroying the soil structure and water and air will not able to circulate properly in the soil.

Criteria for Suitability of Irrigation Water:

Irrigation water may contain soluble salts which may include carbonates, bicarbonates, chlorides and sulphate of sodium, magnesium and calcium. When saline irrigation water is applied to the fields, soluble salts, if present in the soil, dissolve to increase the concentration of salts in the soil solution, and sodium ions replace calcium ions from the clay micelle to destroy the soil structure.

So irrigation water should contain enough calcium salts when calcium ions instead of sodium ions will be adsorbed by the clay micelle because calcium ions are held more tightly than sodium by the clay micelle.

If the soil contains free calcium carbonate, this is dissolved in the irrigation water to release enough calcium ions into the water as shown in the following equation:

Calcium ions prevent the sodium ions from being absorbed by the clay micelle. So irrigation water containing higher amounts of soluble salts may be used if the soil contains enough lime. If not, only low salinity water should be used. Irrigation water also contains suspended materials.

On the basis of the above ideas, the suitability of any water for irrigation depends on:

(i) Total soluble salts in the water as determined by its electrical conductivity

(ii) Total solids in the water

(iii) The ratio of sodium to calcium in the water as determined by its (a) sodium absorption ratio (b) soluble sodium percentage and (c) residual sodium carbonate value

(iv) The quantity of sodium salts present in the soil as determined by its exchangeable sodium percentage, and

(v) Presence of borates in the water

Na⁺, Ca⁺⁺ and Mg⁺⁺ are the concentration of the sodium, calcium and magnesium in the water in milliequivalents per litre. All carbonates (CO₃–) and (HCO₃ -)in the water in milliequivalents per litre in excess of calcium (Ca⁺⁺)and magnesium (Mg⁺⁺) in mill equivalent per litre in water, occur in the form of sodium carbonate and bicarbonate and therefore increase the pH of the water.

Residual Sodium Carbonate (R.S.C.) = (CO₃ — + HCO_a) – (Ca⁺⁺ + Mg⁺⁺) Irrigation waters have been grouped into classes of low, medium, high and very high, depending upon their suitability for irrigations as shown below:

Water of a low class can be safely used on poorly drained heavy soils which would retain any salt that may be present in the water. Medium class should be used on permeable light soils which contain enough calcium carbonate and have low water table. Water of high class should not be used.

The pH of most of Indian rivers varies from 7.8 to 8.4. On the basis of the above standards, waters of rivers Kosi, Brahmaputra and Gandak are excellent for irrigation, due to their low salt content (Electrical Conductivity varies from 142 to 253 micro mhos/cm at 25°C) and low sodium content (Sodium Absorption Ratio varies from 0.17 to 0.26).

Waters of a few other important rivers like Beas, Kaveri, Ganga, Sutlej and Narmada are also fairly good for irrigation due to their medium content of soluble salt (Electrical conductivity varies from 269 to 339 micro mhos per am at 25°C) and low sodium content (S. A.R.) varies from 0.46 to 0.99).

Three south Indian rivers Krishna, Hagari and Tungabhadra contain very saline water (Electrical conductivity varies from 1392 to 1730 micro mhos/am at 25°C) which should not be used for irrigation, even when the S.A.R.’s of the first two rivers are below ten.

Improvement in the Quality of Irrigation Water:

A little saline irrigation water can be improved by adding gypsum or farmyard manure to water. A saturated solution of gypsum should be mechanically added at a constant rate to flowing water when sodium bicarbonate is converted to calcium carbonate.

Sodium ions may also be removed from irrigation waters by passing them through high cation exchange capacity calcium and hydrogen charcoal, which absorb sodium ions from them as indicated in the following equations:

The used charcoal may be converted again to hydrogen charcoal by treating it with dilute acids.