Standard of Irrigation Water
Completely pure water for irrigation cannot be predicted. However, the presence of impurities in the water should have some reasonable boundaries so that crop yield is not impaired.
Standard irrigation water means water that has reasonable limits of impurities. The following are the impurities that may be present in the water.
(a) Sediment Concentration
The sediment of fine silt improves the fertility of the land. While other kinds of sedimentation decrease fertility. Again, too much sedimentation causes problems in canals and reservoirs. Therefore, excessive suspended sediment should not be contained in water.
(b) The concentration of Soluble Salt
The presence of calcium, magnesium, sodium, and potassium salts may be harmful to crops if it exceeds the permissible limit. The concentration of soluble salt can be measured electrically or computed by laboratory testing.
The concentration is usually expressed as follows:
i) When the concentration of soluble salt is expressed in P.P.M. the amount in excess of 700 P.P.M. is harmful to crops.
(ii) The electrical conductivity of saline water is expressed in micromhos/cm. The value up to 250 micromhos/cm is beneficial for all crops. But, if the value exceeds 250 micromhos/cm is very injurious for crops.
(c) The proportion of Sodium Ions
All soils generally contain ions of calcium, magnesium, and sodium. The percentage of Sodium ions present in the soils less than 5 is suitable for crops.
If the percentage increases to more than 10, it is harmful to crops. The proportion of sodium ions is designated by a factor which is known as Sodium Absorption Ratio (S.A.R.).
The water having the value of S.A.R. between ‘0’ and ’10’ is helpful for all crops. The value of more than 10 is unsuitable for irrigation.