Fact Sheet 5. Local flow systems associated with colluvial fans
Regions
Local flow systems associated with colluvial fans are found in the coarser sediments in shallower terrain of the footslopes of the Dividing Ranges, Victoria, New South Wales and Queensland; and the Lofty Ranges of South Australia.
Critical attributes that determine groundwater behaviour in response to land management
- Scale: Local
- Landform: Colluvial fan (footslope terrain)
- Aquifer: Unconsolidated colluvial sediments
- Aquifer transmissivity: Low - moderate
- Groundwater salinity: Low
- Land use: Grazing
- Catchment size: Small
- Annual rainfall: Moderate - high
- Salinity manifestation: Break of slope seeps
- Salinity rating: S2, S3
- Temporal distribution of recharge: Seasonal with episodic overprinting
- Spatial distribution of recharge: Upper slopes and crests
- Type areas: Warrenbayne, Victoria
Discussion
These groundwater systems typically occur where coarse colluvial slope wash material overlies a massive, unjointed bedrock. This leads to a contrast in hydraulic conductivity between the permeable slope wash and the relatively impermeable underlying rock. Most groundwater flows down slope via the colluvial material, which is usually 1_10 m thick on the highest slopes, and up to 20 m thick in the valley floors. Groundwater salinity concentrations are low. Discharge is usually at the break of slope where the colluvial material becomes finer, and where hydraulic gradients flatten. This provides potential for groundwater utilisation upslope of discharge zones, and means that the timeframes for management of salinity are relatively brief, with positive benefits from effective recharge management possible within a decade or so of their application. The relatively high rainfall of many of these systems and relatively high soil fertility mean that water efficient biological options are feasible for controlling salinity.
Potential options and their suitability for salinity management
Recharge management
Pasture agronomy Existing pasture options are unlikely to be able to reduce recharge sufficiently to prevent these systems filling up, although they may provide some scope to slow groundwater level rises.
Cropland agronomy (poor cropping land due to slopes) Existing cropping options are unlikely to be able to reduce recharge sufficiently to prevent these systems filling up or to provide a substantial decrease in rates of groundwater level rise.
Woody perennial vegetation Woody perennial vegetation offers considerable potential for reducing recharge to levels necessary for controlling salinity.
Plantation forestry Plantation forestry that extracts groundwater directly from the aquifer at break of slope positions provides potential for reducing recharge to levels necessary to controlling salinity and in some instances providing economic returns.
Engineering watertable management
Surface drainage Surface drainage on the slopes of colluvial fans is unlikely to provide a reasonable salinity benefit, although under some circumstances it may be possible to divert water that would otherwise have the opportunity of contributing to groundwater recharge.
Sub-surface drainage Sub-surface drainage may provide opportunities where deeper drainage lateral to the slope has the opportunity to intercept
fresher groundwater before it moves down-slope. It is equally true, however, that low-yielding groundwater pumping may afford the same result in a more cost-effective manner.
Groundwater pumping The transmissive nature of these aquifers means that low-yield groundwater pumping is technically feasible, and the relatively low salinity of much of the groundwater means that in many cases the pumped water can have productive uses.
Managing saline resources
Halophytic vegetation Halophytic vegetation is more suited to semi-arid, high salinity lands.
Salt-tolerant grasses/clovers Establishment of salt-tolerant grasses is an appropriate response to saline seeps in these low salinity systems.
Saline horticulture & silviculture This option is appropriate given that in some regions there are opportunities for small scale developments based on limited groundwater pumping. It may be restricted due to the low salinities encountered.
Salt harvesting The relatively low salinities and low yield of these systems mean that salt harvesting is not a viable option.
Saline aquaculture The low yield of these systems provides little opportunity for saline aquaculture.
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
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