Catchment management for salinity control PDF (34kb)

Dryland salinity is a groundwater-driven process that occurs on a catchment scale. A whole catchment approach is therefore required for the successful management of dryland salinity.

Catchments and salinity

A surface water catchment is the area of land drained by a river or creek and its tributaries. As groundwater catchments don't always coincide with surface water catchments, additional hydrogeological information is necessary to define groundwater catchments.

The cause of dryland salinity originates in the catchment area upslope of the saline area. It is important to define this area in terms of both surface water and groundwater flows.

Investigating and planning for salinity management

Biophysical information is necessary for planning, and important datasets include topography and drainage, remnant vegetation, land systems, waterlogging status, groundwater flow systems (GFS), salinity, salinity risk potential and recharge area maps.

Investigations to gather such biophysical information may be necessary, along with interpretation of monitoring results to determine groundwater flow directions within the catchment. Use of a GIS (Geographic Information System) platform will allow overlay of different datasets; this being a very useful tool in planning.

The various natural and man-made assets within the catchment need to be identified, along with the degree of threat posed by existing salinity and potential salinity spread. The relative value of each asset (e.g. agricultural land, water resources, infrastructure, biodiversity) will help determine the priority given to management action.

Priority areas and priority actions will also be determined by the type of GFS operating within the catchment. Local flow systems respond relatively quick to on-ground works targeted within identified high recharge zones. On the other hand, regional systems require broadscale intervention and being sluggish in nature, they respond much more slowly.

Secondary impacts of salinity management actions also need to be considered during the planning process. These can include potential reduction in surface water harvest, environmental weed risk and safe disposal of drainage water. Environmental, social and economic considerations will differ according to the scale of the plan (catchment, sub-catchment or property level), and this in turn will influence the formulation of priority actions and priority areas for management change.

Management options

Depending on the priorities set within a catchment, a mix of management options can be applied. Sometimes these may apply only in specific areas to target those natural and man-made assets that are valued most highly.

Management options for salinity can be generally summarised under three categories:

• Living with salt - using saline resources productively.
• Engineering - pumping and drainage options to increase discharge
• Recharge reduction - increasing total water use over the catchment.

Living with salt

There will always be saline land that cannot be reclaimed to its former use. This land needs to be protected and managed productively (salt-tolerant fodder shrubs/grasses, saline wetlands, woodlots). Maintaining vegetative cover on saltland reduces the risk of salt washoff and erosion while also reducing recharge on-site.

Engineering

Engineering options exist for the control of both surface water and groundwater flow. Surface drains are less than one metre deep and include contour banks, interceptor banks, diversion drains, W drains and spoon drains. They are designed to remove excess surface and shallow subsurface water from the landscape, thereby reducing recharge and waterlogging potential.

Groundwater drains and groundwater pumps are designed to lower shallow watertables; the level of drawdown being determined largely by pumping/drainage rate and soil permeability. Groundwater can be viewed as a potential resource for industries such as inland saline aquaculture, salt harvesting, water desalination, and energy production from solar ponds.

Recharge reduction

Recharge reduction tackles the source of the salinity problem by manipulating the total catchment water balance - increasing water use over the catchment. The success of recharge reduction will depend on the level of land management/land use change that is implemented and the responsiveness of the groundwater system to such change. Recharge reduction is a longer-term approach to salinity problems.

Increasing water use can be achieved in many ways, but at the scale required to control salinity it invariably involves a significant change from annual to perennial vegetation. The perennial component will often include trees and shrubs and herbaceous plants such as lucerne grown as permanent pasture or in a phase farming system.

Maximising annual crop and pasture productivity through improved rotations and better agronomic management remains an important component of recharge reduction. A recharge reduction strategy incorporates treatment of associated land degradation problems such as acid soils, erosion and soil fertility decline. The claying of non-wetting sands, improvement in soil organic matter content, and removal of barriers to root growth will all increase total water use and decrease recharge. Fencing off and re-invigorating remnant vegetation provides an avenue for both recharge reduction and maintenance of catchment biodiversity.

In any catchment, there will be identified high priority recharge areas that require preferential treatment - they are the areas where recharge reduction strategies will work most efficiently and effectively. However, in targeting stream, land and urban salinity problems, major change over a large proportion of the catchment is required, and is often an integration of recharge reduction, engineering and living with salt options.

Last update: June 2007

Authors: Chris Henschke and Trevor Dooley, Salinity Program Consultants.

Agdex: 512

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