Learn about factors to determine the quality of irrigation water.
Irrigation water quality
Water, either from precipitation or ground, never remains pure. As it moves, it is converted into liquid form from vapour. It gets mixed with atmospheric gases and after reaching on the earth it is mixed with various salts, minerals and even heavy metals.
Irrigation water quality refers to the kind and amount of salts present in the water and their effects on crop growth and development.
High salt concentrations influence osmotic pressure of the soil solution and affect the ability of plants to absorb water through their roots.
However, an appropriate evaluaion of the water quality prior to its use in irrigation will help in arresting any harmful effect on plant productivity and ground water recharge.
The suitability of water for irrigation is determined in several ways including the degree of acidity or alkalinity (pH), EC (Electrical Conductivity), Residual Sodium Carbonate (RSC), Sodium Adsorption Ratio (SAR), Permeability Index (PI) and Total Hardness (TH) along with the effects of specific ions.
The assessment of water quality criteria for irrigation is based on the consideration of the related aspects like the possible effects on the physico-chemical properties of the soil and the impact on crop yield.
Classification of Irrigation water quality
Evaluation of Salt Content in Irrigation Water (USSAR)
Classification of irrigation water based on total salt content
Residual Sodium Carbonate (RSC)
Carbonate associates quickly with Ca and Mg and form CaCO3 and MgCO3.
The Na replaces Ca and Mg and synthesizes Na2CO3 which again causes sodium hazard called as Residual Sodium Carbonate.
The Residual Sodium Carbonate (RSC) Index of irrigation water or soil water is used to indicate the alkalinity hazard for soil. In the dispersed soil structure, the plant roots are unable to spread deeper into the soil due to lack of moisture.
Sodium Hazard of Irrigation Water
Sodium Adsorption Ratio (SAR) is commonly used as an index for evaluating the sodium hazard associated with an irrigation water supply.
The SAR is defined as the ratio of the sodium (Na) to the square root of calcium + magnesium (Ca + Mg).
Where all cation measurements are expressed in milliequivalents per liter (meq/L).
Irrigation waters having high SAR levels can lead to the build‐up of high soil Na levels over time, which in turn can adversely affect soil infiltration and poor aeration.
Sodicity hazard also classified as S1, S2, S3 and S4. (RRB SO)
Boron Hazard of Irrigation water
Relationship between EC, SAR and RSC
Specific Ion Toxicity Hazard
Sodium
Among the soluble constituents of irrigation water, sodium is considered most hazardous.
Excess of sodium ions characterizes the water as saline or alkaline depending upon its occurrence in association with chloride/ sulphate or carbonate/ bicarbonate ions.
For some time in the past, the quality of irrigation water used to be evaluated with respect to sodium based on Soluble Sodium Percentage (SSP) calculated as below.
It has been useful in characterizing water, since a high value indicates soft water and low value hard water. When water with excess of sodium (SSP = 66) is used for irrigation, part of it is adsorbed by the soil. Both, soils and plants are adversely affected by high sodium irrigation water.
Sodium soils are relatively impermeable to air and water. They are hard when dry, difficult to till and plastic and sticky when wet. These adverse physical conditions prevent germination and are generally unfavourable for plant growth.
Even though, sodium is not as essential as other nutrients, it is taken up freely by many plants and it may be specifically toxic to plants.
Magnesium
It is believed that one of the important qualitative criteria in judging the irrigation water is its Mg content in relation to total divalent cations, since high Mg content in relation to total divalent cations, since high Mg adsorption by soils affects their physical properties.
A harmful effect on soils appears when Ca: Mg ratio decline below 50.
Chlorides
The occurrence of chloride ions in irrigation water increases with increase in EC and sodium ions. Therefore, these ions are most dominant in very high salinity water.
Unlike sodium ions, the chloride ions neither effect on the physical properties of the soil, nor are adsorbed by the soil. Therefore, it has generally not been included in modern classification system However, it is used as a factor in some regional water classification.
Sulphate
Sulphate salts are less harmful when compared to chlorides. This is because when both the ions occur in this concentration, only half of the sulphate ions contribute to salinity due to the fact that approximately half of the sulphates gets precipitated as CaSO4 while the other half remains in soluble form as Na-MgSO4 in the soil. That is the reason, the potential salinity of irrigation is calculated as Cl- + ½ SO4–.
Fluorine
Fluorides are only sparingly soluble and are in only small amounts. The concentration of fluoride ranges from traces to more than 10 mg L-1 in natural water, and surface water do not exceed 0.3 mg L-1 unless they are polluted.
Irrigation with fluoride saline water (upto 25 mg L-1) has not been found to affect yield of wheat.
Therefore, it is doubtful if fluoride requires any monitoring in India.
At present, the average concentration of fluoride has not been observed to be very high (10 mg L-1).
Nitrate
Very frequently ground water contains high amount of nitrate. When such type of irrigation water is applied on soils continuously, various properties of soils are affected.
Lithium
Lithium is a trace element may be found in most of saline ground water and irrigated soils. It has been found that 0.05-0.1 ppm of lithium in water produce toxic effects on growth of citrus. It has also been reported that saline soils of varying degrees found in India contain lithium up to 2.5 ppm. Fortunately, the germination of majority of crops is not affected with this level of lithium content.
State wise brackish water
Inland water resource in the form of Brackish water.
