Lesson 12. Quality of Irrigation Water

12.1 INTRODUCTION        

All natural waters used for irrigation contains inorganic salts in solutions which are derived originally from the rocks or solid phase material through which water percolates. The most common dissolved constituents are chlorides, sulphates and bicarbonates of Ca, Mg and Na. The concentration and proportion of these salts determine the suitability of water for irrigation. Other constituent such as B, Li, F or other ions, which have a toxic effect on plants, may occur in lesser amounts in irrigation water. If water used for irrigation contains excessive quantities of the constituents noted above, it might affect the growth of plants in three ways viz.,

(a) As a result of adverse changes in the physical characteristics of the soil,

(b) The increased osmotic pressure of the soil solution may decrease the physiological availability of moisture to plants,

(c) Accumulation of certain ions in the soil solution may have a specific toxic effect upon the physiological processes of the plant.

 Therefore, the question arises “What should be the ideal quality of water to be used for irrigation?” Different workers for judging the quality of waters have proposed various standards.

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

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   =   (CO32- + 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.

Last modified: Tuesday, 13 August 2013, 4:39 AM