Fact Sheet 9. Regional flow systems within unconfined sediments
Regions
Regional flow systems within unconfined sediments are found on the mallee plains of Victoria.
Critical attributes that determine groundwater behaviour in response to land management
- Scale: Regional
- Landform: Aeolian plains
- Aquifer: Unconsolidated sands
- Aquifer transmissivity: Moderate
- Groundwater salinity: High
- Land use: Cropping and grazing
- Catchment size: Large
- Annual rainfall: Low
- Salinity manifestation: Expansive salinas
- Salinity rating: S3
- Temporal distribution of recharge: Episodic
- Spatial distribution of recharge: Diffuse across the region
- Type areas: Mallee plains, Victoria & South Australia
Discussion
This groundwater system typically occurs within unconsolidated sediments, usually sands and silts, related to large-scale Aeolian landscapes. The system covers extremely large and flat areas, is regionally unconfined and rarely reaches great thickness. In the Mallee Region of Victoria, saline discharge and the presence of saline areas were characteristic components of the landscape prior to widespread land use change. This has also increased since European settlement. The groundwater system is characterised by high salinity, a relatively permeable and extensive aquifer, very low hydraulic gradients, and episodic recharge that occurs diffusely across the plains. Groundwater salinity is controlled by evaporative concentration in the subsurface and discharge of water from hypersaline lakes. The region is also characterised by low soil fertility. Due to the long history of agricultural land use in this region, groundwater levels have already risen across the catchments, although these are yet to translate into major regional lateral flow out of the aquifer due to the continued flat hydraulic gradients and lower transmissivities. These characteristics mean that the scale of land use change that must be adopted to reduce recharge to the system is considerable. It must:
- be able to intercept irregular and extreme recharge pulses; and
- deal with already increased groundwater volumes moving through the system.
Salinity is always likely to be a component of the landscape. Viable options for dealing with dryland salinity must therefore focus on either using the saline water as a resource, or managing saline land for its conservation and recreational values.
Potential options and their suitability for salinity management
Recharge management
Pasture agronomy Likely to have low effectiveness even if applied over the entire area, due to the inability of shallow-rooted systems to intercept episodic recharge, and the lengthy time required for groundwater to recede to where salinity benefits might accrue.
Cropland agronomy As above.
Woody perennial vegetation While woody perennial vegetation provides the opportunity to manage groundwater recharge, and perhaps even episodic recharge, it is unrealistic to expect that it might be adopted on a scale sufficient to influence salinity within a super-regional groundwater flow system in semi-arid terrain. Even if regional recharge management could be achieved, the groundwater system would remain strongly buffered against change, and the length of time required for groundwater to recede would be excessive.
Plantation forestry It is technically feasible to manage groundwater recharge with plantation forestry within this groundwater flow system. Given that the region has a semi-arid climate, it is unlikely that this option will ever be adopted on a scale sufficient to influence dryland salinity. Even if large scale forestry were practised, the strongly buffered regional groundwater system would not recede within a timeframe acceptable to most stakeholders.
Engineering watertable management
Surface drainage The main issue limiting the use of surface drainage is that when recharge to these systems does occur, it is highly diffuse and infiltrates through the relatively permeable soils before a surface drainage system is likely to be able to divert it.
Sub-surface drainage Sub-surface drainage may be a viable option where watertables must be artificially lowered to protect high value assets. Under most circumstances, groundwater pumping will provide a more cost-effective solution.
Groundwater pumping Groundwater pumping is technically viable from these relatively transmissive aquifers, and the option has application in situations where high value assets need to be protected, or where there is potential for industries that rely on a supply of saline groundwater (see below). Alternatively, diversion of pumped groundwater to natural depressions in the land surface which act as evaporation basins may provide some opportunities for protecting high value assets.
Managing saline resources
Halophytic vegetation Halophytic vegetation is well-suited to high salinity areas and the semi-arid climate.
Salt-tolerant grasses/clovers Salt-tolerant grasses are less suited to the high salinity, high aridity areas.
Saline horticulture & silviculture Most salt-tolerant horticultural/tree crops will be poorly productive under conditions of extreme salinity and aridity.
Salt harvesting The high salinities and relatively high transmissivities of these systems mean that salt harvesting from either pumped or discharged groundwater is likely to be a technically viable option.
Saline aquaculture The ability to pump saline groundwater from regional aquifers presents opportunities to cultivate a range of otherwise marine products in inland areas.
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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