Groundwater flow system fact sheets
Background
Australian groundwater flow systems define hydrogeological provinces with similar geological and geomorphic characteristics and landscape processes that give rise to the incidence of dryland salinity.
Hydrogeological processes determine the responsiveness of the groundwater systems to change and therefore govern the extent, scale and mix of interventions required to manage dryland salinity. The mix of options that might be reasonably considered to manage salinity will vary considerably (e.g. local groundwater flow systems in deeply weathered terrain will need different solutions to large regional groundwater flow systems comprising alluvial aquifers operating at the scale of river basins).
Context for action
The responsiveness of groundwater systems to change will dictate what can be effectively achieved through groundwater recharge and discharge management within reasonable timeframes.
Attributes of groundwater systems that determine their capacity to transmit groundwater exert considerable influence over the capacity to manage salinity either through biological options for managing recharge, engineering options for managing watertables, or options for developing saline industries. Highly transmissive local flow systems might be expected to respond to most salinity management options far more readily than very sluggish local flow, or perhaps regional flow systems operating over hundreds of kilometres.
Biological options that promise to reduce recharge by 50% might be seen in a very positive light, particularly if they achieve increased productivity of farming systems through greater water use efficiency. However, the underlying groundwater system could take a long time to change because of:
- the sluggish nature of the aquifer;
- its regional extent; or
- strong convergence upon a lower discharge site.
Therefore salinity benefits might not be evident for some 15 to 60 years, or indeed much longer (depending on the groundwater flow system).
Engineering options will be required in very sluggish groundwater flow systems because of difficulties in extracting groundwater from slowly permeable materials. The ability to reliably harvest saline groundwater from slowly permeable landscapes also influences the range and efficacy of many commercial options that might be considered in the quest for saline industries such as salt harvesting or saline agriculture or aquaculture.
Acknowledgment: Much of the background material presented below on recharge management options has been taken from the National Dryland Salinity Program publication Assessing the causes, impacts, costs and management of dryland salinity, L. Martin & J. Metcalfe, LWRRDC Occasional Paper 20/98 Revision Number One.
Biological options for managing groundwater recharge
Biological systems for managing groundwater recharge most commonly involve the widespread reintroduction of perennial vegetation in the landscape, either in the form of deep-rooted high-water-using perennial pastures, cropping practices which afford improved water use efficiency in croplands, or the adoption of of woody vegetation ranging from fodder shrubs to plantation forestry.
Each of these biological systems varies in its capacity to control groundwater recharge. This variance has been taken into account in rating the efficacy of each salinity management option. Broad judgments are made, largely on the basis of woody vegetation being more effective in recharge management than perennial pastures, which are in turn considered more effective than cropland management. In the overall assessment, however, the means of controlling recharge is considered much less important than consideration of the timeframes in which recharge reduction options translate into salinity benefits through groundwater responses.
Maintenance of remnant native vegetation
It is easier and more economic to maintain existing native vegetation than to replace it once it has been disturbed or cleared.
However, in order to maintain the vegetation in good condition it is necessary to adopt a management regime that will ensure its long-term viability. The main management practices required to maintain and, where necessary, rehabilitate native vegetation involve:
- fencing remnant vegetation to control grazing stock;
- eradicating rabbits and controlling weeds;
- maintaining fire breaks;
- minimising pesticide drift into native vegetation;
- confining recreational activities to specified areas; and
- re-establishing plant species that are absent from the ecosystem.
Advantages
- Saving remnant vegetation helps in recharge control.
- Preserved areas help to set recharge control targets.
- Areas of remnant native vegetation that remain are the most important systems in terms of biodiversity outside tropical rainforests.
- High heritage and conservation values are maintained.
Limitations
- Due to the limited amounts of remnant native vegetation left to protect there may only be modest benefits to catchment-scale recharge control.
- There is evidence that degraded remnant vegetation provides incomplete recharge control.
- Much of the existing remnant vegetation is not self-sustaining since it is too scattered to regenerate effectively.
- It may not be possible to do enough to save downstream systems due to rising watertables and existing damage may be irreversible.
- The time it takes for recharge control to reduce saline discharge may be very long (hundreds to thousands of years in some places).
- Native vegetation can harbour pest plants and animals, and therefore requires ongoing management.
Annual crops and pastures
There are a number of ways that water use can be increased in conventional crops and pastures. Although many are relatively simple measures, greater effort may be required in planning and management (particularly with pastures) if they are to be effective for reducing recharge. The following measures increase water use by annual crops.
Improving agronomy
- Lifting yields of cereal and legume crops though improved management, better crop rotations and improved pasture management (during the pasture phase) may improve the water use efficiency of crops.
Removing impediments to root growth/reduced tillage
- In some cases tillage to enable better root growth will aid in reducing recharge, while in others, reducing tillage will help decrease recharge rates. Determining whether or not it is appropriate to till serves to highlight the complexity of salinity management and how options are very specific to local conditions.
Eliminating fallow periods
- Fallow periods have been used traditionally to allow water stores to increase, and therefore improve productivity. However, recharge is increased and can lead to more rapid degradation than if fields are continuously under vegetation cover.
Opportunity or response cropping
- This occurs when favourable conditions such as suitable soil moisture levels are used to determine when to plant.
Phase cropping
- Phase cropping involves the rotation of annual cropping systems with perennial pasture systems (e.g. following 5-7 years of a continuous cropping phase with 5-7 years of lucerne. The lucerne helps to remove water that has escaped past the root zone of shallow rooted crops in previous years).
Alley cropping
- Annual crops are planted in alleys among belts of perennial plants which provide shelter, increased water use and, depending on the species used, possibly grazing, timber and/or habitat.
Advantages
- In cropped areas there is the potential for a high rate of adoption of this solution, particularly when there is the possibility of higher economic return.
- Costs can be kept to a minimum, and may be offset by productivity increases.
- This solution doesn't require substantial land use change.
- Productivity is likely to increase.
Limitations
- Although there is room to improve farming practices so that water use is maximised, it is difficult to significantly reduce recharge under annual crops and pastures (and almost impossible in wet years); increased plant water use and productivity may come from reduced soil evaporation and hence may not significantly reduce recharge.
- Skilful management and widespread commitment on every farm in a catchment is required to consistently achieve high water use.
- Increasing the water use by annual plants is unlikely to prevent salinisation on its own.
- Annuals do not use water that falls in intense or prolonged rainfall events, or that falls when plants are not growing.
- Reducing recharge under annuals is a problem in well-drained (e.g. deep sands), low-fertility and waterlogged soils and on heavily grazed pastures.
- When annual rainfall exceeds 600 mm the scope for recharge control by modified agronomic practices in cropped areas is limited.
- Continuous cropping practices may be unsustainable where there is soil structure decline, acidification, and herbicide resistance.
Traditional perennial species
The deeper roots of trees, shrubs and some perennial pasture species give greater water-use potential than annual, shallow-rooted plants. Perennial pasture systems have repeatedly been shown to be superior to annual systems for controlling deep drainage. Lucerne has attracted particular interest for its ability to substantially reduce recharge.
Advantages of using perennial pasture system
- Land use is complementary (e.g. in crop/pasture rotations shallow-rooted species are replaced with deeper-rooted more water efficient species).
- Adoption rates are likely to be higher than for more substantial land use change strategies.
- Gives a cash flow if well managed.
- Water use will increase and make some difference to recharge.
Limitations
- When changing from annual to perennial pasture species, better management is required (e.g. restricted grazing).
- May only deliver marginal benefits in terms of recharge.
Changing to new perennial species
There is potential for reducing recharge by changing to new deep-rooted perennial woody plant crops such as jojoba, oil mallee species and broom brush. These may provide some of the economic returns required to finance the scale of revegetation that is required to make a difference to salinity problems. Such woody plants may also be important in agroforestry and alley farming options (e.g. the State Salinity Strategyfor Western Australia indicates the need for 3 million hectares of revegetation over the next 30 years. Governments cannot finance this scale and rate of planting, but could provide commercial incentives to increase farmer motivation to adopt revegetation as part of agricultural practice and farm business).
Alternatively, sustainable agriculture utilising commercially motivated revegetation can be seen as complementary to biodiversity conservation. Biodiversity conservation may
require woody plant crops as they have the potential to provide commercially viable industries while at the same time addressing the salinity problem.
Advantages
- These crops may prove effective for controlling recharge and re-establishing water balance in low rainfall areas.
- Perennial species may provide economically viable alternatives to traditional crops (e.g. oil mallee in the wheat belt of Western Australia).
- Evidence is emerging that mixtures of trees/woody plant crops and annual plant crops may be more productive than monocultures of either.
Disadvantages
- Uncertainty about market prospects for the crops if there is insufficient planning and economic analysis.
- Time lag involved in establishing crops can place additional financial strains on already struggling enterprises.
- Cultural change required for their cultivation may be a disincentive.
- Deep-rooted perennials which thrive in a new environment will always have the potential to invade remnant native vegetation. Native non-local plants can be as great a weed threat as exotics.
Trees
Under favourable conditions, trees can extract large quantities of water from the soil by transpiration, and can directly intercept and evaporate rainfall. They use more water to a greater depth in the root zone than shallower-rooted species, reducing deep drainage.
Trees actively use water for a greater part of the year than most crops and pastures and are the only generally effective way of lowering watertables in the absence of massive engineering. Trees are particularly useful for reducing recharge in higher rainfall areas (greater than 500 mm/year) and can minimise recharge even given large episodic rainfall events. Other types of vegetative land cover are unable to provide effective recharge reduction when rainfall events are episodic.
Situations most suited to using trees to manage salinity include those where:
- there are shallow soils (less than 5 m) over weathered and fractured rock;
- the soils are deep sandy or permeable (less than 5 m);
- the country is steep and broken;
- timber processing industries are situated nearby;
- shade or windbreaks are needed;
- the region was naturally treed prior to settlement;
- wildlife protection is desired; and
- groundwater is close and fresh (less than 5 m).
Advantages
Few would argue with the contention that planting trees can reverse land degradation if revegetation is extensive and strategically placed. In addition to effective recharge control through higher water use and consequent lowering of watertables, planting trees confers other important advantages:
- enhancement of regional biodiversity by providing habitat for other plant and animal species;
- aesthetic improvements to the appearance of the land;
- returns for both landowners and the community;
- erosion and wind control;
- windbreaks for stock;
- biological weed and pest controls;
- increased production (see alley farming); and
- potentially sustainable income sources.
Impediments
Establishment of trees is expensive. Using them to manage salinity or provide income is a long-term strategy. After the trees are planted, it will be some years before watertable levels start to be affected and many more years before the trees reach maturity and provide other desired benefits.
Another major obstacle is that for effective reduction in recharge there may need to be 30_50% reforestation in a catchment (and even more if trees are to be harvested). This is a particular issue for those landowners in recharge areas who do not themselves experience salinity problems. These landowners are unlikely to see the value in losing large areas of their land at high cost when they are not personally impacted by the salinity itself.
Impediments associated with planting trees to manage recharge include:
- costs: initial planting costs are high and unless there is a perception of a solid economic return it can be difficult to convince farmers to plant trees;
- cash flow: if growing trees for harvest, it may be 25 years before there is an economic return;
- difficulties with integrating trees into farming systems;
- lack of species and systems suitable for lower rainfall areas;
- need for infrastructure to support agroforestry enterprises, such as sawmills and markets;
- risk of fire;
- loss of productive land with non-commercial tree planting (at least in the short term);
- the need to harvest trees for a return on investment, which means plantings will need to be staged to ensure recharge control;
- the time lag before trees begin to have an effect on recharge.
Engineering options for managing watertables
Engineering strategies to reduce recharge include drainage to intercept and redirect surface and groundwater, and groundwater pumping of fresh water. These strategies can be costly to implement and are hence usually only used in urban areas. Deep subsurface drainage and pumping have been shown to be cost-effective only in situations where:
- the land is very valuable (towns, nature reserves, infrastructure);
- the soils and aquifers are permeable and in hydraulic connection (water flow is connected); and
- there is a safe option for effluent disposal.
Engineering strategies are often combined with vegetation strategies to increase the effectiveness of salinity management plans.
In areas prone to flooding, inundation (water ponding at the soil surface), waterlogging (excess water in the root zone of plants) and where agronomic solutions are inadequate, drainage is often a solution. Areas with excess surface water contribute salt, sediment, nutrients, and pesticides to streams, rivers, wetlands and estuaries. Collection and storage for later re-use of these surface waters can increase plant growth and water use and reduce recharge.
Advantages
Surface drains
- Low cost measures which are cheaper to construct and maintain than it is to take land out of production.
- Cause minimal disruption to land use.
Subsurface drains
- Effective for removal of groundwater from waterlogged areas.
- Can lower watertables sufficiently to enable productive use of affected land.
Groundwater drains
- Interception trenches are a relatively cheap water interception and storage strategy.
- Can lower watertables.
- Enable unusable land to be reclaimed for production.
Groundwater pumps
- If pumping works, salinisation problems can be minimised and production of conventional agricultural systems boosted at the same time.
- Capital assets in towns can be protected by removing the problems associated with salinity and waterlogging
Limitations
Surface drains
- Value for controlling waterlogging may be limited in some areas.
- Effluent may cause problems off-site (salts, nutrients).
- Cannot be built in sodic soils.
Subsurface drains
- Much more costly than surface drainage measures.
- Storage or disposal of collected water can be costly.
Groundwater drains
- Deep drains are costly to build.
- Storage and disposal of collected water can be costly unless it has a beneficial use.
Groundwater pumps
- Capital cost is high; can be expensive to set up and maintain.
- Applicable areas are limited to high yielding aquifers.
- Only treats the symptoms.
- Limited area of influence in most systems.
- If using water for irrigation it must be carefully managed.
- Water storage systems that leak can recharge the system.
- Industry infrastructure is required for the irrigated crops.
- Disposal of water to the environment.
Saline industries
It is apparent that throughout much of Australia salinity will continue to expand over coming decades in spite of our best efforts to contain it. In many instances there will be little choice other than to adapt to more saline conditions by further developing the range of saline industries.
How difficult it is to develop these new industries depends upon the nature of biophysical and landscape processes operative within each groundwater flow system. Options that require large amounts of saline groundwater (e.g. salt harvesting) will be established with greater ease where the landscape is made up of highly transmissive regional aquifers. Industries based upon the grazing of salt-tolerant grasses may find greater application in the more humid regions within upland regions made up of fractured rock aquifers.
The fact sheets
The following fact sheets describe each hydrogeological province in terms of the biophysical and landscape context in which salinity occurs, the attributes that determine groundwater responsiveness, and the processes that operate in the landscape to affect salinity. The likely efficacy of salinity management options has been rated. The ratings are based on a set of mainly quantitative criteria and expert judgement (Table 27).
The evaluation process
Listing the attributes of each salinity/groundwater flow system provides a common basis for defining hydrogeological performance in terms readily appreciated by the Australian salinity and groundwater specialists, and a common basis for considering the responsiveness of each system. Specifying the biophysical and landscape determinants of each system has allowed the experience and knowledge gained over many decades of salinity research to be considered, in addition to more recent hydrogeological modelling, particularly that achieved within the case studies of the Audit. In this sense the fact sheets represent the outcome of an `expert' decision making process.
Fact sheets: version 1
The information in each fact sheet illustrates our knowledge and understanding of the general salinity and groundwater processes that prevail in Australian groundwater flow systems. The discussion of management options presented under each of the groundwater flow systems is intentionally generic. Readers should use these comments as a starting point for the consideration of options at a catchment level and refine with more detailed local information. It is anticipated that over time this material will be revised as new information becomes available.
Table 27. Definitions of the relative ratings that apply to the attributes of groundwater flow system as listed within fact sheets.
|
Attributes |
Rating |
Meaning/value |
|
Scale (of groundwater processes) |
Local |
Groundwater flows over distances less than 5 km within the confines of sub-catchments |
|
Intermediate |
Groundwater flow over distances of 5 to 30 km and may occur across sub-catchment boundaries |
|
|
Regional |
Groundwater flow occurs over distances exceeding 50 km at the scale of river basins |
|
|
Aquifer transmissivity |
Low |
Less that 2 m2/day |
|
(ability to transmit groundwater |
Moderate |
2 m2/day to 100 m2/day |
|
through the aquifer) |
High |
Greater than 100 m2/day |
|
Groundwater salinity |
Low |
Less than 2000 mg/l |
|
Moderate |
Ranging from 2000 mg/l to 10 000 mg/l |
|
|
High |
Greater than 10 000 mg/l |
|
|
Catchment size |
Small |
Less than 10 km² |
|
Moderate |
Ranging from 10 km² to 500 km² |
|
|
Large |
Greater than 500 km² |
|
|
Annual rainfall |
Low |
Less than 400 mm |
|
Moderate |
Ranging from 400 mm to 800 mm |
|
|
High |
Greater than 800 mm |
|
|
Salinity rating |
S1 |
Loss of production |
|
S2 |
Saline land covered with salt-tolerant volunteer species |
|
|
S3 |
Barren saline soils, typically eroded with exposed sub-soils |
|
|
Responsiveness to land |
Low |
Salinity benefits accrue over timeframes that management exceed 50 years |
|
Moderate |
Salinity benefits accrue over timeframes ranging from 30 to 50 years |
|
|
High |
Salinity benefits accrue over timeframes less than 30 years |
|
Local flow systems in deeply weathered rocks |
|
|
Intermediate flow systems within sedimentary sequences infilling large valleys |
|
|
Local flow systems in fractured rocks |
|
|
Local flow systems in deeply weathered fractured rocks |
|
|
Local flow systems associated with colluvial fans |
|
|
Intermediate flow systems in fractured rock aquifers |
|
|
Local flow systems in fine grained unconsolidated sediments |
|
|
Regional flow systems in alluvial aquifers |
|
|
Regional flow systems within unconfined sediments |
|
|
Local flow systems associated with sand dunes |
|
|
Regional and intermediate flow systems within fractured basaltic rocks |
|
|
Intermediate and local flow systems in fractured basaltic rocks and layered sedimentary rocks |
Table of Contents for the Australian Dryland Salinity Assessment 2000
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