Efficient Sand Integration

Water Supply

Irrigation - Water Supply

Water quality

Good quality irrigation water is required to produce high yielding vegetable crops of good quality. Irrigation water can have important effects on crops and soils and so it is important to understand the quality of your irrigation water.

In developing water quality guidelines, emphasis is placed on sustainability in agricultural practice which aims to ensure that:

  • The supply of inputs is sustainable;
  • The natural resources are not degraded;
  • The environment is not irreversibly harmed;
  • The welfare and options of future generations are not jeopardised by the production and consumption activities of the present generation; and
  • Yields and produce quality are maintained and improved.

In terms of water quality, the focus for sustainable farming systems is on adopting management practices that maintain productivity and minimise the off-farm movement or leaching of potential aquatic contaminants (Department of Education, Science and Training, State of the Environment Advisory Council, 1996)..

How is irrigation water quality measured?

The most important factors are measures of

The water quality alone does not give the complete story. Water quality needs to be considered along with the soil type, environment and delivery system and production system in which the water is to be used.

Guidelines for irrigation water quality

Salinity

Salinity is the measure of the total salts or ion concentration and is usually reported as total dissolved solids (TDS) in milligrams per litre (mg/L). It is quicker and cheaper to measure electrical conductivity (EC) of water samples which is well correlated to total dissolved solids. The common units for EC are deciSiemens/metre (dS/m) or milliSiemens/metre (mS/m). To convert from deciSiemens/metre to milliSiemens/m multiply by 100.

Salinity measures the total salts concentration in water and includes ions such as calcium, magnesium, potassium, nitrate as well as sodium and chloride.

In Western Australia, EC in milliSiemens per metre (mS/m) is multiplied by 5.5 to obtain an estimate of TDS in mg/L.

Total Dissolved Solids (TDS) (mg/L) = EC (mS/m) x 5.5

For example water with an EC of 100 mS/m has approximately 550 mg/L or ppm of total dissolved solids. Note that 1 mg/L is equivalent to 1 part per million (ppm) and 1 gram per cubic metre (g/m 3).

The following is a general guide to water quality for overhead irrigation

Table 1: EC TDS use

EC

TDS

 

0-80 mS/m

0-440 mg/L

Suitable for all vegetables

80-230 mS/m

440-1,200 mg/L

Suitable for most vegetable species except the most sensetive eg. beans, radish, squash

230-550 mS/m

1,200-3,000 mg/L

Only suitable fro the more salt tolerant vegetables eg. spinach, asparagus, beetroot

In WA on sandy soil, poor carrot germination has been noted in summer with irrigation water of 1,400 mg/L. Some visual symptoms of salt damage were visible in maturing carrots in hot weather with 1,200 mg/L.

Sodium and chloride

Both sodium and chloride can be toxic to plants. Calcium and potassium deficiencies may be induced when irrigation water is high in sodium.

Irrigation water high in sodium and/or chloride may cause leaf burn, especially on salt sensitive plants, starting at the leaf tip and progressing back along the leaf margins or edges. Leaf symptoms are usually more severe in hot weather.

Sodium and chloride can be toxic to plants with a low salt tolerance when taken up by their roots or absorbed through leaves.

Other ions

What are ions?

When salts dissolve in water they separate into the components called ions. The ions have a positive or a negative charge, those with the positive charge are called cations and those with the negative charge are anions. For example, common salt, sodium chloride, NaCl dissolves and separates into Na + (cation) and Cl -(anion).

pH

pH is the scale used to measure acidity (pH lower than 7) and alkalinity (pH higher than 7). Distilled water has a pH of 7 which is neutral. pH is actually a measure of the hydrogen ion (H +) concentration and is measured on a negative logarithmic scale such that low pH means high hydrogen ion concentration. The logarithmic scale means that the hydrogen ion concentration at, for example, pH 5 is ten times higher than at pH 6. Most water sources are in the range pH 5 to 8. Acidic water has a pH less than 7 and if moderately to highly acid (less than pH , has the ability to corrode metal pumps, pipes and fittings.

Iron (Fe)

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Dissolved iron in irrigation water can be a problem. On exposure to air, dissolved iron which is present in the Fe 3+ state is readily oxidized to form a rusty coloured precipitate that can block nozzles. Where iron bacteria are found, water that contains as little as 1.0 mg/L of iron can result in blocking of screens and pump impellers. The iron bacteria extract the iron out of the water and convert it into a rust coloured sludge.

Alkalinity

Excess alkalinity as measured by sodium carbonate and sodium bicarbonate can be a problem when irrigating heavy textured soils if calcium and magnesium concentrations are low. The result can be adverse effects on soil structure. Alkalinity is not a problem on the Swan Coastal Plain sands that have no structure.

Preferred and critical levels for irrigation water quality are shown in the following table


Table 2: Water Analysis Guidelines

Analyte

Example value

Preferred

Critical

Unit

CO3

18

 

 

mg/L

Ca

83.4

 

 

mg/L

Cl

100

<175

700

mg/L

ECond

87.8

<150

200

mS/m

HCO3

153

 

 

mg/L

Hardness

260

<60

350

mg/L

K

12

 

 

mg/L

Mg

11.8

 

 

mg/L

N_NH3

0.31

 

 

mg/L

N_NO3

0.03

 

 

mg/L

N_total

0.53

<5

25

mg/L

Na

53.3

<115

460

mg/L

P_SR

0.02

 

 

mg/L

P_total

0.06

 

 

mg/L

SO4_S

108

 

 

mg/L

TDS_calc

480

<500

1200

mg/L

pH

8.5

>6

9

 

 

 

 

 

 

 

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