Fact Sheet 11. Regional and intermediate flow systems within fractured basaltic rocks
Regions
Regional and intermediate flow systems within fractured basaltic rocks are found in the basalt plains of western Victoria.
Critical attributes that determine groundwater behaviour in response to land management
- Scale: Regional
- Landform: Plains
- Aquifer: Fractured volcanic rocks comprising numerous individual lava flows
- Aquifer transmissivity: Moderate - high
- Groundwater salinity: Low - high
- Land use: Grazing
- Catchment size: Large
- Annual rainfall: Moderate - high
- Salinity manifestation: Discharge zones associated with low lying land and lakes
- Salinity rating: S1, S2
- Temporal distribution of recharge: Seasonal with episodic overprinting
- Spatial distribution of recharge: Diffuse, highest within higher permeability basalts (`stoney rises')
- Type areas: Lough Calvert area, Victoria
Discussion
When plant growth is at a minimum. Due to the long history of agricultural land use in these regions, groundwater levels have already risen across the catchment. These attributes mean that while the scale of land use change that must be adopted is considerable, there are some viable options for reducing dryland salinity. These must focus on either reducing recharge through land use alternatives such as plantations, re-establishment of native vegetation; or pumping of less saline groundwater for horticultural or other purposes. Surface drainage is also an option, although as the region drains internally, the issue of disposal of saline water is substantial. Historically, drains have fed existing lakes within surface depressions. The timeframes involved in reducing existing salinity using many of these options are considerable, given the already elevated groundwater levels.
Typically this groundwater flow system occurs in extensive basalt landscapes that are relatively flat. Groundwater is free to migrate through the permeable fractured rock aquifer across sub-catchment boundaries. Saline discharge was a component of this landscape prior to widespread land use change, and this has increased since European settlement. The groundwater system is characterised by a relatively permeable aquifer, low hydraulic gradients, and diffuse recharge that is dominated by distinct, but large areas where recharge occurs particularly rapidly. Groundwater salinity is controlled by interactions with other sources, either from discharge of saline water from other aquifers or from saline lakes, or from recharge of fresh water from higher permeability basalts. Recharge is greatest during the cooler months,
Potential options and their suitability for salinity management
Recharge management
Pasture agronomy While it may be possible to manage groundwater recharge locally with the aid of high water-use pasture systems, it will be difficult to realise a regional salinity benefit from these practices. Groundwater is free to migrate through the permeable fractured rock aquifer across sub-catchment boundaries. This means that very large areas would have to be treated to achieve effective recharge reduction in proportion to the scale of the groundwater processes. Watertables are already well established at elevated positions in the landscape, and the flow system is, thus, strongly buffered against changes imposed by biological intervention.
Cropland agronomy Cropping systems in this region may enhance groundwater recharge and few biological options that will lessen the impact appear to exist.
Woody perennial vegetation Woody vegetation may be effective in local recharge management and important in localised high recharge areas such as `stoney rises' and scoria cones. Where such vegetation is used locally for these purposes, it cannot be expected to deliver benefits in terms of the management of regional salinity issues. Even widespread adoption of woody vegetation across the plains is unlikely to deliver salinity benefits within an acceptable timeframe because of the existence of elevated groundwaters.
Plantation forestry Plantation forestry may provide a medium-term salinity benefit where it is practised over very large areas. In the short to medium terms, the reduction in runoff experienced as a consequence of higher water use may result in increased stream salinity (reflecting the reduction in dilution flows).
Engineering watertable management
Surface drainage Surface drainage may provide a level of recharge management, since under some circumstances it may remove surface water that would otherwise become groundwater recharge. It is most unlikely however, that localised drainage will deliver salinity benefits through mitigation of watertables associated with the regional aquifer. Surface drainage may help manage localised soil salinity by lessening runoff from affected areas and reducing potential for soil erosion.
Sub-surface drainage While it may be technically feasible to apply sub-surface drainage to the management of watertables, this activity would under most circumstances only be economically viable where high value assets must be protected.
Groundwater pumping Groundwater pumping is technically viable where there are highly transmissive aquifers. In some areas low salinity groundwater makes this option suitable for limited irrigation-based activities. These practices, however, require careful management to avoid inflows of saline groundwater from more saline aquifers. Groundwater pumping is technically feasible where it is necessary to protect high value assets.
Managing saline resources
Halophytic vegetation Halophytic vegetation is more suited to the semi-arid lands where salinity is higher and the landscape less prone to waterlogging.
Salt-tolerant grasses/clovers Salt-tolerant species are likely to be an important land management option within the cooler and wetter lands common to the basalt plains.
Saline horticulture & silviculture Salt-tolerant crops are likely to be an important land management option in saline areas, particularly where groundwater salinity is low to moderate.
Salt harvesting Salt harvesting from some saline lakes has been practised in the past, and may provide some local opportunities for saline industries.
Saline aquaculture Existing saline lakes may lend themselves to the establishment of saline aquaculture; and recreational fishing is already practised. The throughflow nature of this highly transmissive system enables some flushing of such lakes.
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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