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Salinity Hazard

Salt content in irrigation water

The excess of salts content is one of the major concerns with water used for irrigation. A high salt concentration present in the water and soil will negatively affect the crop yields, degrade the land and pollute groundwater.

The suitability of water reuse for irrigation with high salt content depends on the following factors:

- Salt tolerance of the type of crop

- Characteristics of the soil under irrigation

- Climate conditions. The quality of the irrigation water plays an essential role in arid areas affected by high evaporation rates and cause high concentrations of salt accumulating in the soil.

- Soil and water management practices

In general water reuse for irrigation purposes must have a low to medium salinity level (i.e. electrical conductivity of 0.6 to 1.7dS/m). (See table below).

Special account should be taken to coastal areas where the infiltration of sea water poses a high risk of salinity in the water that is then pump from wells to be used in irrigation. For example in Spain the overexploitation of groundwater resources for agriculture lowered the water table and as a consequence caused the seawater intrusion in the coastline.

Hazard TDS (ppm or mg/L) dS/m or mmhos/cm
None <500 <0.75
Slight 500-1000 0.75-1.5
Moderate 1000-2000 1.5-3.00
Severe >2000 >3.0

Salinity with moderate content of salts can be used if moderate leaching occurs.

Water with high saline (ECi>1.5) and sodium (SAR>6) should not be used for water irrigation. Nevertheless, in some places with water shortage, water with high salinity concentration is used as a supplement for other sources and therefore a good management and control is essential and the salt tolerance of the plants must be considered.

If water with a very high salinity is used (extreme water shortage circumstances) the soil must be permeable, drainage must be adequate, water must be applied in excess to provide considerable leaching and salt-tolerance crops should be selected.

Real hazard!! A percentage of 21% of total irrigated land is estimated to be damaged by salt (see table below).

Salinization of soils on Irrigated Lands

Country Irrigated Land Damaged by Salt

(million Ha)

Total irrigated Land Damaged by Salt

(percent)

India

China

Pakistan

USA

Uzbekistan

Iran

Turkmenistan

Egypt

Subtotal

World Estimate

7.0

6.7

4.2

4.2

2.4

1.7

1.0

0.9

28.1

47.7

17

15

26

23

60

30

80

33

21

21

Source: Adapted from F.Ghassemi, A.J.Jakeman, and H.A. Nix, salinisation of Land and Water Resources (Sydney: University of New South Wales Press, 1995)

If a farmer annually applies 10,000 tons of irrigation water to a Ha of crops, which is typical, between 2 and 5 tons of salt will be added to that land every year. Unless these salts are flushed out, enormous quantities can build up over the course of years or decades.

Measurement units of salinity

Salt concentration is taken by the total dissolved solids (TDS) expressed in mg of salt per liter of water (mg/L) or grams of salt per cubic meter of water (g/m3)
(i.e. mg/L= gr/m3 = ppm).

Salt concentration can also be measure by the electrical conductivity of irrigation water (ECi).

Electrical conductivity is normally expressed in millimhos per centimeter (mmhos/cm) or deciSiemens per metre (dS/m) or microSiemens per centimetre (1.e. 1dS/m = 1000μS/cm).

The relationship between salt concentration (C) and electrical
conductivity (EC) is approximately C = 640 EC.

Another way of estimating salt concentration is by measuring the electrical conductivity of water extracted from a saturated soil sample (ECe).

The approximate relationship between the electrical conductivity of irrigation water (ECi) and soil salinity is ECe = 1.5 ECi, if about 15 percent of the applied water is draining from the crop root zone.

Salt tolerance of different crops

The yield of different crops in relation with the salinity content of the water used for irrigation, depends on the type of crop, soil and environmental conditions.

The most distinct signs of injury from salinity is reduced crop growth and loss of yield. Crops can tolerate salinity up to certain levels without a measurable loss in yield (salinity threshold). When the salinity level is bigger than the threshold, the crop yield reduces linearly as salinity increases.

Common name Average root zone salinity threshold (ECse) ECi threshold for crops

sand loam clay

Field Crop

- Cotton

- Wheat

-Sunflower

- Rice

- Corn grain sweet

- Sugar cane

7.7 (+)

6

5.5

3

1.7

1.7 (-)

12.1

9.4

7.5

4.8

3.2


4.3

6.9

5.3

4.3

2.7

1.8


2.5

4.0

3.1

2.5

1.6

1.1


1.4

Fruits

- Olive

- Peach

- Grapefruit

- Orange

- Grape

- Apple



4 (+)

3.2

1.8

1.7

1.5

1 (-)

5.1

4.7

3.0

2.9

3.3

2.0

2.9

2.7

1.7

1.7

1.9

1.2

1.7

1.6

1.0

1.0

1.1

0.7

Vegetables

- Zucchini

- Broccoli

- Pea

- Tomato

- Potato

- Onion

4.7 (+)

2.8

2.5

2.3

1.7

1.2 (-)

7.3

4.9

3.2

3.5

3.2

2.3

4.2

2.8

1.8

2.0

1.8

1.3

2.4

1.6

1.1

1.2

1.1

0.8

Management Practices for Irrigating with Saline or Sodic Water

The following consideration should apply:

- Adequate internal drainage. This measure is intended to avoid free movement of water in the root area.

The appropriate leaching requirement depending on tolerance levels for specific crops should apply to avoid the accumulation of salt. For example if natural drainage is not adequate, a drainage system must be installed.

- Higher water availability in soil. At high salt concentrations plants will not absorb all the normally available water.

- Proper management and control of SAR and salinity controls. Ex. add soluble calcium such as gypsum (calcium sulphate) to decrease the SAR to a safe value.
Monitor of salt and sodium with saline-alkali soil tests every 1 to 2 years.

Related pages:

Bicarbonate hazard of irrigation water

Irrigation water lab analysis

Irrigation water quality

Nutrients in irrigation water

SAR hazard of irrigation water

Toxic ions hazard of irrigation water


For more information check the following pages: groundwater contamination, source of groundwater pollution, contaminants (seawater intrusions, nitrates, arsenic, iron), reducing groundwater contamination.

Click here for definitions concerning groundwater, or to learn more about its properties, its origin and quantities, its sources in Europe.

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