Fact Sheet 3. Local flow systems in fractured rocks
Regions
Local flow systems in fractured rocks are found in the central highlands of Victoria and the Great Dividing Range of New South Wales.
Critical attributes that determine groundwater behaviour in response to land management
- Scale: Local
- Landform: Hills, rolling hills
- Aquifer: Fractured rocks
- Aquifer transmissivity: Moderate
- Groundwater salinity: Low - moderate
- Land use: Grazing
- Catchment size: Small
- Annual rainfall: Moderate - high
- Salinity manifestation: Discharge zones associated with break of slope and valley floors; some structural control
- Salinity rating: S1, S2
- Temporal distribution of recharge: Seasonal
- Spatial distribution of recharge: Diffuse, highest within areas of skeletal soils - usually in upper parts of catchments
- Type areas: Boorowa River catchment, New South Wales
Discussion
Groundwater recharge in all parts of the landscape causes saline groundwater discharge both to land and streams. This mainly occurs where the hydraulic gradient reduces with slope and bedrock variation. Groundwater flow systems are characterised by a large number of small local flow systems correlating very closely with topographic catchments. Recharge, occurring in phase with seasonal rainfall patterns and in areas high in the landscape, produces a distinctive filling and draining response. This response pattern is more attenuated lower in the landscape. Rocks are usually only superficially weathered, with groundwater flow generally converging on the downslope regions. Ephemeral and perennial stream networks receive groundwater discharge as baseflow. Salt storage in the catchments is usually low, but off-site impacts are caused by the volumes of saline stream water exported from the system. Response times to changed groundwater conditions can be rapid (one or two decades), with equilibrium conditions taking significantly longer to establish. The main management issue is the need to consider off-site impacts, whether these relate to the impact of increased vegetation on reducing run-off, and thus increasing stream salinity levels; or the impacts of extracted/drained groundwater on stream salinity levels.
Potential options and their suitability for salinity management
Recharge management
Pasture agronomy Likely to have low effectiveness if applied over the entire area, but moderate effectiveness if deep-rooted pastures are widely applied over lower slopes in conjunction with more radical land use change (woody vegetation/plantation forestry) over higher slopes. A major impediment to perennial pasture establishment is pervasive low soil acidity.
Cropland agronomy (usually poor cropping country) Likely to have low effectiveness. Only benefit will be in returning higher gross margins. Will also promote higher levels of erosion.
Woody perennial vegetation Likely to be effective in reducing recharge only if implemented over large areas, and particularly important in high recharge areas.
Plantation forestry Likely to be effective in reducing recharge over much of the area, and particularly important in high recharge areas.
Engineering watertable management
Surface drainage The main issue limiting the use of surface drainage is that of salt export, as well as the economics of implementation.
Sub-surface drainage Subsurface drainage is not a feasible option due to the extent of drains required. Issues of salt export and economics of implementation limit the use of subsurface drainage.
Groundwater pumping Groundwater pumping is technically viable using low capacity technology. Although low salinity of groundwater in this region makes this option suitable for irrigation-based activities, yield restrictions will introduce complications. The main issue is that while groundwater salinities are low, the total volume required to be pumped to balance recharge is large and salt disposal issues arise. Any groundwater pumping option will need to be integrated into a total water management plan.
Managing saline resources
Halophytic vegetation Salinities are low, and the area of land with high salinity is minimal. Cold climate halophytes will be required.
Salt-tolerant grasses Salt-tolerant species are likely to be an important land management option, where saline land is a reality but where groundwater levels permit the establishment of grasses.
Saline horticulture & silviculture The area of land with high salinities is restricted.
Salt harvesting Minimal salinity at levels required.
Saline aquaculture Minimal salinity at levels required.
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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