Home About Us Our Services Publications Testing Submissions Contact Us Links

Irrigation in drought

Patterns of rainfall in Australia are changing: in particular, the southern rainfall belt is slowly moving southwards, over the Southern Ocean. The rain is still falling; just not as much is hitting the land.

One consequence of less rain and less frequent rainfall is low river flows. Rivers are not just drainage channels, taking overland runoff to the sea. They also interact dynamically with groundwater. In times of high flow, rivers recharge groundwater, both vertically (into the deep aquifers) and laterally (sideways into the soil). During periods of low flow, groundwater leaks out of the soil into the rivers.

Much soil in Australia has salt reserves, either just below the root zone or, as a common consequence of poor land management practices, actually at the surface, in salt pans. This salt has accumulated over millions of years from tiny quantities in rainfall (raindrops form preferentially on microscopic salt particles in the air, which are themselves dried salt spray). Under native vegetation, deep tree roots draw up water, lowering the water table, so inflow from rain keeps the surface soil layers relatively low in salt. But during the past century or so of extensive clearing, the removal of trees has allowed rainfall to raise the water tables, particularly at low landscape positions. This has brought the saline deep water to near (in some cases above) the soil surface.

When river flows get low, the saline groundwater no longer has the pressure of the river water opposing it, and so it leaks into the river. This is why the trickles in recent years in the Murray–Darling River system have become highly salty (among other things, threatening Adelaide’s water supply).

If you rely on river water for irrigation, you might already have noticed that the water is getting saltier. So what can you do about it?

Short-term solutions

In the short term you have two options:

• Use scheduled deep irrigation. The aim of deep irrigation is to use water with a given salinity (tolerable by crops) to flush accumulated salt out of the surface soil into the deeper layers. Regular scheduling of deep irrigation will keep the soil at a supportable level of salinity, rather than allowing the soil to dry and thus concentrate the salt at the plants’ roots. For more detailed information, see Managing irrigation water quality.
• Dilute the saline water with fresher water. If you have an alternative supply of less saline water, such as dam water, town water or treated effluent, you can use this to dilute the saline water to a level that the plants can tolerate. This allows you to continue to use the saline water in quantities that allow regular flushing so that the salt does not build up in the soil.

Long-term solutions

In the long term you have two more options:

• Improve your capture of rainfall or source alternative supplies, such as treated effluent, for dilution as above.
• Switch to more salt-tolerant species:

Salt tolerance of some crops

From K.I. Peverill, L.A. Sparrow & D.J. Reuter, 1999. Soil Analysis – An Interpretation Manual.

Testing

You can test your water quality yourself with a simple electrical conductivity meter, or we can test it for you. The EC meter has the advantage of immediate results. Lab testing offers the advantages of accuracy and an extensive breakdown of water quality components. SESL’s Irrigation Water Assessment package tests for all of the following:

• pH
• Salinity hazard: electrical conductivity, total dissolved salts
• Hardness
• Sodium hazard: sodium adsorption ratio (SAR), adjusted SAR, residual sodium concentration (RSC), calculated gypsum requirement
• Cation/anion balance
• Lime deposition potential
• Saturation index
• Irrigation blockage potential
• Plant nutrition contribution: potassium, calcium, magnesium, ammonium, nitrate, phosphate, sulphate, iron, manganese, zinc, copper, boron
• Plant toxicity potential (foliar and root): bicarbonate, sodium, chloride, aluminium, boron

Further information

http://www.sesl.com.au/fertileminds/200804/Water quality.php

http://www.sesl.com.au/fertileminds/200805/Salinity.php

Download this article as a PDF