Quality of Irrigation water

 Quality of Irrigation water

Quality of Irrigation water:-

• Every type’s water is not suitable for irrigation.
• The water containing some impurity, which is harmful to plants growth, is not suitable for irrigation.
• The quality of irrigation water largely depends upon the soil which is to be irrigated.
• A particular type of water which is suitable for irrigation for particular soil may or may not be also suitable for irrigation for some other soil.
• The various types of impurities which make the water unsuitable for irrigation are-

          1. Sediment concentration in water

          2. Total concentration of soluble salt

          3. Proportion of sodium ion to other cation

          4. Concentration of potentially toxic element present in water

          5. Bicarbonate concentration as related to the concentration of calcium plus magnesium

          6. Bacterial concentration

 

1. Sediment concentration in water:-

• The effect of sediment present in water influence by the type of soil to be irrigated.
• When fine sediment is deposited by irrigation water on sandy soil it improves the fertility of the soil.
• When the sediment carried from the eroded area, it may reduce the fertility of the soil and also decrease the permeability of the soil.
• Water containing sediment create problem in irrigation canal.
• Sediment increase canal siltation and maintenance costs.
• Groundwater or surface water from reservoirs, pound, etc do not have sufficient sediment to cause any serious problem in irrigation.

 

2. Total concentration of soluble salt:-

• Salt of calcium, magnesium, sodium and potassium, present in the irrigation water may prove injurious to plans.
• When these salt present in excessive quantities, they cause a reduction in the osmotic activities of plants.
• The injurious effects of salts on plant growth depends upon the concentration of salts left in the soil.
• These salts may prevent adequate aeration, causing injurious to plant growth.
• The effects of salts on plants growth depend largely upon the total amount of salts present in the solution.
• The salinity concentration of the soil solution after the consumptive water has been extracted from the soil is given by

              Where C_s = Salinity concentration of soil solution

                           Q = Quantity of water supplied

                           C_u = Consumptive use of water

                           R_e = Effective rainfall

                           Cu – R_e = Used irrigation water by plants

                           C = Concentration of salt in irrigation water

                           CQ = Total salt applied to soil with Q amount of irrigation water­

• The salt concentration is generally expressed in ppm or mg/l.
• Salinity concentration in excess of 700 ppm is harmful to some plants.
• Salinity concentration more than 2000 ppm is injurious to all crops.
• The salt concentration is generally measured by determining the electrical conductivity of water.
• Electrical conductivity is expressed in micro mhos per centimetre.
• When its value is up to 250 micro mhos/cm, it is called Low conductivity water (C1).
• When its value is between 250 to 750 micro mhos/cm, it is called Medium conductivity water (C2).
• When its value is between 750 to 2250 micro mhos/cm, it is called High conductivity water (C3).
• When its value is above 2250 micro mhos/cm, it is called very High conductivity water (C4).

Table.3.1.

3. Proportion of sodium ion to other cation:-

• Most of the soils contain calcium and magnesium ions and small quantities of sodium ions.
• The percentage of sodium ion is generally less than 5% of the total exchangeable cations.
• If the percentage of sodium ion increases tom about 10% or more, the aggregation of soil grains breaks down.
• The soil becomes less permeable.
• It starts crusting when dry.
• Its pH increase towards the pH of alkaline soil.
• High sodium soils are plastic, sticky when wet
• The proportion of sodium ions present in the soil is measured by Sodium Absorption Ratio (SAR).
• It represents the sodium hazards of water.
• SAR is calculated as-

• When the value of SAR lies between 0 to 10, it is called Low Sodium Water (S1).
• When the value of SAR lies between 10 to 18, it is called Medium Sodium Water (S2).
•  When the value of SAR lies between 18 to 26, it is called High Sodium Water (S3).
• When the value of SAR is more than 26, it is called Very High Sodium Water (S4).
• The SAR value can be reduced by adding Gypsum (CaSO4) to the irrigation water or to the soil.

 Table.3.2

4. Concentration of potentially toxic element:-

• A large number of elements such as Boron, Selenium, etc may be toxic to plants.
• The concentration of Boron above 0.3 ppm may cause toxic to certain plants.
• The concentration of Boron above 0.5 ppm is dangerous to nuts, citrus fruits and deciduous fruits.
• The most tolerable crops tolerate Boron concentration till 4 ppm
• Boron is generally present in various soaps.
• The waste-water containing soap should be used with great care in irrigation.
• Selenium, even in low concentration is toxic and must be avoided.

5. Bicarbonate concentration as related to the concentration of calcium plus magnesium:-

• High concentration of bi-carbonate ions may result in precipitation of calcium and magnesium bi-carbonates from soil solution, increasing the relative proportion of sodium ions and causing sodium hazards.

6. Bacterial concentration:-

• Bacterial concentration in irrigation water is not a serious problem.
• If the crops are directly irrigated with high bacterial concentrated water, it should not be eaten directly without cooked.