Dryland salinity is the build up of salts in the soil surface and groundwater in non-irrigated areas. Although salts are a natural part of the Australian landscape, dryland salinity refers to excess salt that affects soil, native vegetation, biodiversity, crops and water quality.
How does it occur?
Dryland salinity is usually the result of three broad processes - groundwater recharge, groundwater movement and groundwater discharge.
Groundwater recharge occurs naturally, but is often accelerated by excessive clearing of native vegetation, particularly on hill slopes. When there are not enough deep-rooted plants to use the available rainwater the excess seeps past the root zone and enters the groundwater system.
Recharge is greatest where:
- soils are shallow, overlying fractured rocks
- soils or rocks are highly permeable (allowing the free passage of water)
- vegetation is shallow-rooted or absent and the rainfall is greater than the rate of evaporation and plant water use for a period of the year.
Some land management practices such as excessive tillage and long fallow periods may also increase groundwater recharge in susceptible areas.
Groundwater moves through the landscape via aquifers in porous and permeable rocks and soil masses. These may be "confined" (closed to the surface by a waterproof rock unit), or "unconfined" (open to the surface). The rate of movement through these aquifers depends on how porous the material is and on the resistance to flow. Water movement may occur over a regional scale (eg. several hundred kilometres, as in the Great Artesian Basin) or be highly localised (eg. a few hundred metres in a near-surface aquifer).
As increasing quantities of water enter these aquifers, groundwater pressure rises. A discharge, seepage, or flow of water will occur when the watertable meets the surface. Frequently, discharge points first appear where there is a change in slope of the ground, at a change in rock type, or along a rock fracture.
Where saline water rises to within two metres of the surface, water can be taken up by plants or can evaporate through the soil. Evaporation results in the dissolved salts being left behind and concentrated as deposits at the soil surface.
The water cycle and dryland salinity
1 Perennial, deep-rooted native vegetation uses most of the available water and prevents leakage to the groundwater system.
|1 Replacing native vegetation with annual crops and pastures that have shorter growth cycles and shallow root systems reduces water use and increases leakage to the groundwater system.|
|2 Evapotranspiration rates are high.||2 Evapotranspiration rates are lower.|
3 Surface run-off and erosion rates are low.
|3 Surface run-off and erosion rates increase.|
4 The water table remains well below the surface and a range of productive agricultural land uses are carried out.
4 The water table rises, bringing salts stored deep in the soil to the surface where they affect plant growth (production) and water quality, thereby limiting agricultural production.
Another cause of dryland salinity is erosion that exposes saline subsoils. The removal and loss of native vegetation, as well as some agricultural practices can increase erosion rates and expose naturally saline sub-soils. Erosion as a result of salinity can also contribute sediment to rivers and streams causing further decline in water quality.
In this situation, rising groundwater may have little role in the development of salinity problems. Management responses are therefore different from those required for areas affected by rising watertables.
Sodic soils (soils that have a high concentration of sodium ions in comparison to other ions like calcium and magnesium) can also cause salinity. When wet, sodic soils disperse causing the soil aggregates to separate and block the soil pores. On drying, sodic soils are often hard and dense, forming a crust on the soil surface. The poor soil structure reduces water infiltration and there is little or no leaching of salts below the root zone. Sodic subsoils can create a perched watertable causing waterlogging of the root zone.
What are the impacts of dryland salinity?
Salt near the soil surface makes it harder for plants to take water from the soil and eventually they die from dehydration. With agricultural crops, a yield loss of up to 30% can occur before the evidence of salinity becomes visible.
Salinity rarely occurs in isolation from other natural resource problems. This accumulation of salt usually results in the death of native vegetation, causing a loss of biodiversity. Introduced pastures and crops fail to thrive or die leaving patches of soil with no ground cover. The bare areas create a soil erosion hazard. Salt crystals can often be observed on these bare areas. The salts are dissolved by rainwater and washed into nearby watercourses, causing river salinity.
What is being done?
Dryland salinity management is most effective when implemented over a whole catchment or sub-catchment as integrated works programs. The best combination of salinity management practices will vary for each catchment.
Catchment Management Authorities are responsible for developing a Catchment Action Plan (CAP) (incorporating catchment blueprints) for their catchment. The CAP defines the salinity targets, as well as other natural resource objectives, for a region and describe the best management actions the community must undertake in order to help meet the targets. The CAPs are consistent with the;
- NSW Salinity Strategy
- National Action Plan for Salinity and Water Quality, and
- Murray-Darling Basin Salinity Management Strategy.
What can be done at the local level?
Contact your local Catchment Management Authority to discuss how local action can integrate with and help implement priorities outlined in their Catchment Action Plan.
Strategies to address salinity problems should be mainly directed at reducing groundwater recharge. This can be done by introducing changes to land use and land management practices at sites where there is high recharge, for example, the upper reaches of a catchment and the tops of hills.
One of the best ways to reduce groundwater recharge is to maintain adequate vegetation cover throughout a catchment, particularly on sites of potentially high recharge. The greater the amount of water that the vegetation intercepts or uses, the more effective it will be in addressing the problem of excessive leakage to the subsurface.
Maintaining soil health is also very important as healthy soil can hold more water, allowing less water to leak through to recharge the groundwater.
Specific management practices that may be implemented by land managers to prevent dryland salinity include:
- Protect native vegetation, particularly in areas of high recharge. Areas of high recharge typically occur in the upper parts of a catchment on hills and ridge-tops with thin soils. These areas typically have low land capability and relatively low agricultural value if cleared.
- Rehabilitate degraded native vegetation in areas of high recharge. Catchment Management Authorities can provide information for landholders about meeting biodiversity targets and reducing salinity recharge.
- Reafforestation and tree planting in areas of high recharge. It is important that species appropriate to the site conditions are selected, although species with known high water use would be desirable.
- Agroforestry may offer landholders the opportunity to offset the cost of tree planting to address salinity problems by supplementing their income with income from forest products.
- Deep-rooted perennial pasture species such as lucerne and phalaris are better for year round water management than shallow-rooted annual species, such as clover-based pastures. Often, a mixture of species, which have different growth habits and climatic requirements, can maximise water use.
- Improve grazing management to stop overgrazing, which reduces pasture cover.
- Control pests such as rabbits is also necessary to maintain ground cover and healthy soil.
- A minimum ground cover of about 70% is considered appropriate in most areas. Lack of ground cover allows too much water to leak through the soil profile and can lead to erosion, which can expose saline subsoils.
The overall aim is to minimise deep drainage to halt salinity and to produce healthy vigorous crops.
- Manage the soil - maintain satisfactory levels of fertility, pH and structure to maintain soil health and encourage the growth of high yielding crops.
- Minimise the use of long fallows - a system of crop and pasture rotation or double-cropping may be better.
- Introduce conservation farming techniques to maximise water use and control erosion
- Adopt crop rotations. Introducing crops such as canola, lupins and peas into a wheat rotation will help improve soil structure and fertility.
Managing discharge sites
The goal of most reclamation programs is to lower the watertable to a level that is below the root zone of crops and pastures with the expectation that full productivity will be restored. Measures that are commonly recommended to treat salt-affected sites include:
- Fencing off the area to restrict stock allowing for maximum vegetation growth.
- Revegetation with salt-tolerant grasses, herbs, shrubs and tree species, preferably those that have some productive value. Building up mounds so plants can grow above the salt.
- Using light surface tillage and ripping to assist plant growth.
- Using gypsum and/or fertilisers to maximise plant growth.
- Applying mulch to improve soil conditions, reduce evaporation, aid plant growth and protect soil against erosion.
- Establishing water drainage control works such as contour banks to direct surface water away from a site and prevent erosion.
- Establishing banks and subsurface drains to intercept shallow subsurface water flow and direct it away from the site to prevent waterlogging.
- Using groundwater pumping to lower watertables. Although positive effects can be localised around a bore this may not be economical for large dryland farms. Consideration needs to be given to the disposal of the saline water.
References and resources
- A series of four booklets have been produced by the NSW Salt Action Teams and include:
Book 1 Dryland salinity - the basics (Book1DrylandSalinity.pdf, 877KB)
Book 2 Identifying saline aeras (Book2DrylandSalinity.pdf, 215KB)
Book 3 Investigation and assessment techniques (Book3DrylandSalinity.pdf, 542KB)
Book 4 Productive use of saline land and water (Book4DrylandSalinity.pdf, 258KB)
- The NSW Salt Action Teams have developed a Glove Box Guide to Salinity to help land mangers identify and manage salinity in the southern inland catchments of the Murray and Murrumbidgee. This is available on the CRC for Plant Based Management of Dryland Salinity website.
- Salt Magazine contains stories about land mangers managing dryland salinity on their properties through a variety of different means. This is available on the CRC for Plant Based Management of Dryland Salinity website.
- The newsletter Focus on Salt contains information on current salinity research and development. This is available on the CRC for Plant Based Management of Dryland Salinity website.
What would you like to do next?
Page last updated: 26 September 2013