Fact Sheet 1. Local flow systems in deeply weathered rocks
Regions
Local flow systems in deeply weathered rocks are found in south-west Western Australia; Eyre Peninsula, South Australia; and Dundas Tablelands, Victoria.
Critical attributes that determine groundwater behaviour in response to land management
- Scale: Local
- Landform: Gently undulating
- Aquifer: Saprolite
- Aquifer transmissivity: Low
- Groundwater salinity: Moderate - high
- Land use: Grazing and cropping
- Catchment size: Small - moderate
- Annual rainfall: Moderate
- Salinity manifestation: Break of slope and valley floor
- Salinity rating: S2, S3
- Temporal distribution of recharge: Seasonal
- Spatial distribution of recharge: Catchment
Discussion
These groundwater systems typically occur within ancient landscapes. They are formed through deep weathering in early Tertiary times. They occur most commonly where deep chemical alteration of the upper regolith of granitic terrain has resulted in extensive zones of pallid clay and silt; a very effective medium for storing salts introduced as aerosols through rainfall and concentrated in the saprolite through evapotranspiration. In these simple systems, groundwater recharge generally occurs on the slopes of catchments. Vertical movement dominates groundwater hydrogeology, and is compounded by lateral convergence on the lower landscape, causing rising watertables and ultimately saline groundwater discharge at breaks of slope or valley floors. In these systems the timeframe between clearing land of native vegetation and the onset of groundwater discharge is short, on average 20 years. Once full and without intervention, the time for the groundwater systems to empty out excess water is likely to be much longer, due to the low permeability of the rock material. The critical attributes of this groundwater flow system for salinity management are the low permeability of the aquifers and the relatively high groundwater salinity levels.
Potential options and their suitability for salinity management
Recharge management
Pasture agronomy Recharge may be managed (in part) by pastures but the sluggish nature of groundwater flow together with contemporary elevated groundwater conditions mitigate the opportunity to manage salinity within a timeframe acceptable to most stakeholders.
Cropland agronomy As above. Biological management of recharge in these systems is unlikely to deliver salinity benefits within an acceptable timeframe. Adopting cropping systems and rotations that incorporate lucerne and woody vegetation may reduce recharge, but watertable reductions are unlikely to be realised within the short term.
Woody perennial vegetation As above. The behaviour of these systems is controlled by their inherent low permeability, and accordingly their slow responsiveness to land management. This option may be effective in the long term.
Plantation forestry Provides effective recharge management, and may be effective in managing salinity in the medium to long term.
Engineering watertable management
Surface drainage Drainage may reduce run-off flowing onto or ponding on saline land. This may in turn limit salt wash-off to streams, minimise erosion of saline soils and increase agricultural productivity.
Sub-surface drainage This is a technically feasible option, but limited permeability together with a high cost factor makes it unacceptable in most regions unless high value assets require protection.
Groundwater pumping Limited application because of the poorly permeable nature of the weathered rock material and high cost factor.
Managing saline resources
Halophytic vegetation Generally applicable where diffuse saline groundwater discharge within semi-arid slowly permeable landscapes affects extensive soil salinity.
Salt-tolerant grasses/clovers Widespread application, increased productivity and protect saline land from soil erosion.
Saline horticulture & silviculture Where it is economically feasible, saline horticulture and silviculture may have application, particularly in marginal to salt-affected land.
Salt harvesting Lack of pumpable aquifers and higher rainfall/evaporation ratios mitigate against salt harvesting.
Saline aquaculture Lack of pumpable aquifers limits the availability of saline groundwater resources, nevertheless saline farm dams provide some opportunities.
Combining options
The use of two or more of the above options (appropriate to the prevailing climate, soil type and landscape position) typically may have a beneficial salinity management effect.
Table of Contents for the Australian Dryland Salinity Assessment 2000
Before you download
Most publications are downloadable as PDF files. Adobe Acrobat Reader is required to view PDF files.
If you are unable to access a publication, please contact us to organise a suitable alternative format.
Key
Links to an another web site
Opens a pop-up window