Agriculture for Engineers 4 (3+1)

Lesson 12. Quality of Irrigation Water

12.2 Quality of irrigation water

The five generally recognized criteria for judging the quality of irrigation water are: 

1. Salinity hazard: Continuous use of water having high salt content will convert a normal soil into a saline soil. On the basis of electrical conductivity (EC) measurements, the waters were divided into four classes as follows:

Conductivity (dS/m)	Class	Symbol	Inference
0.00 – 0.25	Low salinity	C1	(i) Can be used for most soil for most crops (ii) Little likelihood of salinity
0.25 – 0.75	Medium salinity	C2	(i) Can be used with moderate leaching (ii)               Moderate salt tolerant crops should be grown
0.75 – 2.25	High salinity	C3	(i) Cannot be used where drainage is restricted
2.25 – 5.00	Very high salinity	C4	(i) Not suitable for irrigation

                   

2. Alkali hazard: The continuous use of water having high concentration of Na will convert a normal soil into an alkali soil. The sodium adsorption ratio (SAR) developed by USSSL expresses the relative activity of Na ions in cation exchange reactions with the soil. The exchangeable Na percentage (ESP), which the soil will attain when the soil and water are in equilibrium, can be predicted approximately from the value of SAR of water. Accordingly, the waters are divided into four classes with respect to the Na hazards as follows:

SAR value	Class	Symbol	Inference
0 – 10	Low Na water	S1	(i) Can be used for all soils with little danger of harmful Na level development. (ii) The Na sensitive crops are affected.
10 – 18	Medium Na water	S2	(i) Sodium hazard likely in fine textured soil. (ii) Can be used on soils having high permeability.
18– 26	High Na water	S3	(i) May produce harmful level of exchangeable Na in most soils except gypsiferous soils. (ii) Requires special management practice like good drainage, high leaching and addition of organic matter and gypsum.
> 26	Very high Na water	S4	Unsatisfactory for irrigation except at low and perhaps medium salinity of irrigation water, special management as above should be made.

         

 

The USSSL has prepared the diagram for use of water having different values of EC as well as SAR.

3. Bicarbonate hazard: The bicarbonate ions are primarily important because their tendency to precipitate Ca and to some extent Mg, in the soil solution as their normal carbonates e.g.

Ca   +   2HCO₃   →   CaCO₃   +   CO₂   +   H₂O

The CO₃^-2 ions are seldom present in water but HCO₃^- ions may be present in appreciable proportion of the total anions present in irrigation waters. Based on the theory of precipitation of Ca and Mg, Eaton (1950) suggested the concept of “Residual Sodium Carbonate” commonly known as RSC. The RSC can be found out by following equation:

RSC   =   (CO₃^2- + HCO₃^-)   -   (Ca²⁺ + Mg²⁺)

Where; concentrations of all ions are expressed in meq/l.

The standard for RSC as given by USSSL as follows:

RSC (meq/lit)	Quality of irrigation water
Less than 1.25	Probably safe for most purpose
1.25 – 2.50	Marginal can be used on light textured soil with adequate leaching and application of gypsum
More than 2.50	Not suitable for irrigation purposes

 

4. Boron hazard: Boron is very toxic to plants at low concentration in the soil solution. Because boron tends to accumulate in the soil from even low concentration in the irrigation waters, it is necessary to consider this constituent in assessing the quality of irrigation waters. The USDA has suggested the type of crops to be grown with respect to boron content in irrigation water. The limits are as under:

Boron content of irrigation water (ppm)	Boron tolerance of crops	Crops to be grown
0.3 – 1.0	Sensitive	Citrus, Apricot, Peach, Apple, Pear, Plum, walnut
1.0 – 2.0	Semi-tolerant	Sweet potato, Oats, Sorghum, Maize, Wheat, Barley, Radish, Peas, Tomato, Cotton, Potato, Sunflower
2.0 – 4.0	Tolerant	Carrot, Cabbage, Onion, Beans, Sugar beet, Alfalfa, Date

      

5. Other hazards:

(i) Chlorides: The grading of irrigation waters based on chloride content as proposed by Schofield is as under:

Chloride (meq/lit)	Class
0 – 4	Excellent
4 – 7	Good
7 – 12	Permissible
12- 20	Doubtful
More than 20	Unsafe

 

(ii) Other elements: The safe limit for other elements present in irrigation water is as follows:

Element	For waters used continuously on all soil (ppm)	For used up to 20 years on fine textured soil at pH 6.0 to 8.5 (ppm)
Al	5.00	20.00
Arsenic	0.10	2.00
Cu	0.20	5.00
Fluorine	1.00	15.00
Lead	5.00	10.00
Lithium	2.50	2.60
Mn	0.20	10.00
Mo	0.01	0.05
Se	0.02	0.02
Zn	2.00	10.00
Fe	5.00	20.00

12.3 Suitability of irrigation water

The suitability of irrigation water (SI) will be determined by following factors.

(a) Quality of irrigation water: Amount, nature and proportion of various cations and anions present in the water.

(b) Nature of the soil to be irrigated: Texture, structure, drainage, permeability, depth of water table, chemical composition of the soil, pH, and CaCO₃ content will determine the effect of irrigation water on the soil.

(c) Nature of the crop plants to be grown: water, which may not be suitable for very sensitive crop, may be excellent for tolerant crops.

(d) Climatic conditions: High temperature and less humidity will require more number of irrigation.