Irrigating with saline groundwater in the GMID


Shallow groundwater is a valuable water resource used by many irrigators in the Goulburn Murray Irrigation District (GMID), particularly in times of low water allocation and high water prices. However, careful planning and management is essential for maintaining aquifer quality and avoiding negative impacts of irrigating with saline water on crop or pasture yield and soil structure.

Maximum salinity levels

Get your groundwater tested regularly, before and during the irrigation season.

The recommended maximum salinity of irrigation water used on perennial pastures (white clover/ryegrass mix) growing on loam soils is 800EC (800 µS/m).

Keep the salinity level of the applied water constant for the season. Varying the irrigation water salinity can upset your soil structure – particularly avoid swapping from undiluted groundwater to ‘fresh’ channel water in alternating irrigations.

Salinity Measurement

CAUTION: Always determine the units of measurement before making a decision about how to use your groundwater.

Remember that EC units are “micro-Siemens/cm at 25⁰C (µS/cm) and that 1000EC = 1000 µS/cm = 640ppm = 1 dS/m

Salinity Electrical Conductivity (EC units) Parts Per Million (PPM)
Extreme 20000 12800
  15000 9600
High 10000 6400
  8000 5120
Medium 5000 3200
  2000 1280
  1000 640
Low 500 320
  200 128
  100 64
  50 32
  0 0

Timing

Use groundwater earlier in the season when temperatures are milder. The ability of pasture to handle saline irrigation water is made worse by high temperatures. Later in the irrigation season there is likely to be more local pumping occurring. This can cause intermittent supply due to lower groundwater levels.

Avoid irrigating with groundwater at germination time. Plants are most vulnerable to saline water at germination and until they have their first ‘true’ leaves. Once established, sub clover-based annual pastures are slightly more tolerant of saline water than white clover based perennial pasture.

Leach Salts

Prevent the buildup of salts in the root zone when using ground water. The periodic application of gypsum will help leach salts and maintain soil structure. Calcium supplied in gypsum will displace sodium allowing it to be leached.

More frequent irrigation and applying extra water will also help with leaching. Take care to avoid water-logging on soils with poor drainage. Precision irrigation systems (eg centre pivots, lateral move sprinklers etc) tend to have less deep drainage, and therefore a greater risk of salt accumulation.

Soil Fertility

Maintain soil fertility on pasture irrigated with saline water. It will help keep your clover productive. If clover population is severely impacted by salinity, additional nitrogen fertiliser may be required to maintain productivity.

Test your soil regularly. It is the salinity of the soil which affects how well plants grow. The salinity of the irrigation water affects the salinity of the soil, but they don’t relate directly.

Soil salinity depends on a range of factors such as soil chemistry, and leaching ability (downward drainage). Some paddocks may handle more saline water than others. Risks are usually higher on heavier, loam/clay soils than light/sandy soils because less leaching of salts occurs.

Protect the Long Term Water Quality

Don’t over pump your aquifer. Continually running your groundwater pump at rates above its potential to recharge will increase the risk of drawing in more groundwater from adjacent aquifers which could be of higher salinity. Permanent degradation in aquifer salinity could result.

Groundwater volumes and quality will vary, especially if winter and spring rains haven’t been sufficient to replenish the aquifer. Extra pumping activity during a dry season could also mean that the volume you pumped last year may not be available this summer. Have a contingency pasture management plan to allow for reduced aquifer yields.

Calculate your Mixing Ratio

To work out the right mix of groundwater to achieve your target salinity level, use the formula below:

Mixing ratio =        Groundwater salinity (EC) – Target Salinity

Target Salinity – salinity of other water

Example:

Groundwater salinity is 2000 EC

Channel water salinity is 100 EC

Target salinity level is 800 EC

(2000 – 800) 1200
____________ = _____ = 1.7
(800 – 100) 700

In this example the irrigator will need to mix 1.7ML of channel water to every ML of groundwater to achieve a salinity of 800 EC in the ‘blended flow’.

Salinity

Salinity refers to the concentration of soluble salts in soil or water. All natural waters contain some dissolved salts such as sodium, magnesium and calcium. Sodium chloride (table salt) is the most common of all the salts. The level of salt in water affects its suitability for irrigation, stock and domestic use.

Salinity Tolerances

Table 1 Salt tolerance of forage species
Species Salinity (EC) Comments
Sensitive

Clovers (white, red, cluster, subterranean)

Up to 800 Suitable for use with all crops. Above 800EC sensitive plants will suffer some yield loss.
Moderately sensitive

Balansa clover, maize, Persian clover, strawberry clover, faba beans, lucerne

800 – 1500 Sensitive plants have increasingly reduced growth. Moderately sensitive plants should suffer little or no yield decline.
Moderately tolerant

Berseem clover, sorghum, tall fescue, phalaris, perennial ryegrass, cocksfoot, wheat, paspalum

1500 – 3000 Moderately sensitive plants will suffer increasing yield loss. Moderately tolerant plants should suffer little yield loss with good management at the lower end of this range. At the upper end, some yield loss occurs.
Tolerant

Tall wheatgrass, puccinella, bermuda grass, barley (grain), millet, canola.

3000 – 5000 Moderately tolerant plants will suffer increasing yield decline. Only tolerant plants should be grown with very good irrigation /soil management. Towards the high end of this range some yield decline will occur for some of the tolerant plants

 

Table 2 – Salt tolerance of fruit varieties
Up to 500EC 500 – 900EC 900 EC and above
Passionfruit
Strawberry
Apple
Peach
Grape
Pear
Lemon
Plum
Apricot
Quince
Raspberry
Orange
Olive
Fig
Cantaloupe

 

Table 3 – Salt tolerance of vegetable varieties
Up to 800EC 800 – 2300EC 2300-5500EC
Lettuce
Carrot
Sweet corn
Potatoes
Celery
Onion
Cabbage
Cauliflower
Broccoli
Tomato
Spinach
Asparagus

 

Table 4 – Tolerance of livestock to saline water
Stock type Production decline begins EC EC Maximum
Poultry 3,100 6,250
Pigs 3,100 6,250
Horses 6,250 10,900
Dairy cattle (lactating) 4,700 9,300
Beef cattle 6,250 15,600
lactating Ewes Weaners 6,000 10,000
Mature sheep (dry feed) 9,300 21,800

NOTE: Growth rates, animal condition and possible health will start to decline once salt levels exceed these limits. Livestock need to be introduced slowly to water at these upper levels of salt to minimise the impacts listed above.

CAUTION: other ions and elements in water can become toxic as salinity levels increase. The pH (acidity/alkalinity) also affects how toxic these elements are, and could limit stock use.

Disclaimer

This publication may be of assistance to you but the State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.

Groundwater Fact Sheet March 2019

Soil Salinity Surveys on offer for North Central irrigators

Irrigating in a dry season

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