Salinity - Management - South Australia
South Australia
Introduction
Water balance
As the groundwater system fills and eventually reaches a new equilibrium, the amount of water entering the landscape as recharge and the amount of water leaving as discharge is balanced. However there is a time lag between when changes in land use or improvement in water balance occurs and evidence of a response. It will take decades to reverse the water rise in most groundwater systems (see figure below).
Re-establishing the water balance requires farming systems with similar water use to that of deep-rooted native vegetation. Designing and implementing such farming systems is a major challenge.
Recharge processes are generally faster than discharge processes. If it takes 30 to 50 years for our fastest groundwater system to fill with water, then it is reasonable to expect that it might take at least 30 to 50 years for it to empty back to where it was. If the system takes 100 years or more to fill, we can again expect at least a similar amount of time to establish the original equilibrium. This is an important issue for management as the degree of recharge reduction and the time taken have important consequences on land use options during any adjustment period, and the degree of change sought. Beneficial effects of land use options may well occur before the system has returned to an equilibrium.
Salt balance
As more water moves through an aquifer, more salt is mobilised. Very long periods of time are needed for catchment salt stores to be reduced to the point where the amount entering the system equals the amount leaving the system, that is, to achieve a salt balance. The net amount of salt that exits a catchment via stream flow indicates the time it will take for the catchment to flush its store of salt, when compared with the total mass of salt stored in that catchment. In some of the more responsive groundwater flow systems, the net output of salt may take about 150 years to flush from the system. In larger catchments (e.g. the Murray groundwater basin), it may take as much as 15 000 years. This means that although management may lower the watertable and allow productive use of land, there may be ongoing salt inflow to streams via groundwater.
This makes managing stream salinity very difficult. It is very important to prevent the interception of groundwater with salt stores in regions where we still have this opportunity.
The reality
The substantial lag times for catchments to come back into water balance and change salt mobilisation mean that it is inevitable that dryland salinity will be a feature of many Australian landscapes for some time. This is true even with widespread adoption of innovative land uses that manage to turn off the recharge tap and re-establish water balance. Ultimately the decisions on the measures to be taken will be influenced by the value of the threatened assets, the capacity to manipulate the environmental processes, the economic feasibility and social acceptance of the proposed actions.
What is the scale of the groundwater systems and how can they be managed?
Groundwater trends
All observation well data within the five agricultural regions were examined and analysed for any trends and relationships with rainfall. Because of the low utilisation of groundwater due to high salinities and low yields in most of these areas, data was only available from observation wells drilled in the five representative catchments in 1990/91. The exception is the Murray Basin where the early recognition of dryland salinity and the widespread use of groundwater for stock supplies enabled a regional observation network to be established in the Coastal Plain area in 1987. Further south in the Upper South East, regional groundwater monitoring has been carried out since 1975, but not specifically for dryland salinity.
The monitoring record in most areas is therefore only moderate in length, and is complicated by the atypical rainfall patterns, namely a very wet 1992/93 season and mostly below average rainfall up until the year 2000. However all of the observation wells could be categorised according to the following trends.
- Continually rising
- Episodic rise (in response to very wet years)
- Correlation with winter rainfall
- No discernible trend
The State-wide distribution of the observation networks and the location of each observation well categorised according to its water level trend is displayed in a GIS coverage available in the dynamic mapping part of the Australian Natural Resources Atlas. Each observation well is also hot-linked to its water level data presented as a hydrograph, together with rainfall data.
Continually rising trend
The continually rising trend is seen only in the regional flow systems of the Murray Basin. This trend is observed where there is no discharge from the system apart from lateral groundwater flow, ie where there is no major pumping for irrigation, or more importantly, where the watertable is too deep for evaporative discharge to occur.
In the Coastal Plain area where the depth to the watertable is greater than 5 m, consistent rises of 7- 10 cm/year are being observed beneath sandy soils despite significant variations in annual rainfall. Similar rises are also observed in the Upper South East where the depth to the watertable is greater than 20 m. Heavier soils in the Bordertown area restrict recharge from rainfall, with rises of the order of only 2 - 3 cm/year. Rises in the drier Northern Mallee area are also low, about 2 cm/year where watertable is greater than 10 m below the ground surface.
Episodic rise
Episodic rises in response to very wet years were found to occur in local and intermediate flow systems in lower rainfall areas (below 500 mm) and where lower permeability aquifers prevent the rapid dissipation of the high recharge volumes by lateral flow. These trends were observed in upper Eyre Peninsula, Yorke Peninsula, the Mid North and in part of the Murray Basin (Coastal Plain) where the normally permeable regional limestone aquifer changes to a calcareous marl (clay).
Because of the mostly below average rainfall since the last major recharge event in 1992-93, groundwater levels have been falling since then, often to levels lower than those preceding this event.
Winter rainfall
In some wetter areas, a very good correlation was found to occur between observed groundwater level trends and the cumulative deviation from the mean winter rainfall (May-August). This winter rainfall correlation was better than using the mean monthly rainfall deviation over the twelve months of the year. This suggests that in many areas, the winter rainfall alone is controlling watertables, irrespective of the rainfall in the other eight months of the year (except in very wet years such as 1992-93). This is not really surprising given the dominance of evaporation over rainfall for most of the year under the Mediterranean climate experienced in southern SA.
Close examination of hydrographs shows that even average, or slightly below average rainfall in winter is still sufficient to cause rising watertables, and several years of below average rainfall are required before watertables begin to fall. In some cases, it is difficult to distinguish between a continually rising trend and a winter rainfall correlation because winter rainfalls in some areas of the Upper South East and the Northern Mallee have been mostly above average since 1981, leading to consistent watertable rises.
Because of the this close relationship with winter rainfall, groundwater levels have fallen during the last 2 - 3 years over large areas of southern SA that experienced below average rainfall for the past three winters (up until the year 2000).
No discernible trend
Observation bores showing no discernible trend are mostly located in discharge areas where the watertable is less than 2 m below the ground surface. Although there are seasonal variations due to winter recharge and summer evaporative discharge, these processes tend to balance each other and consequently, any long term rising or falling trend is quite subdued or non-existent.
Future trends
Obviously, because of the strong relationship between groundwater levels and rainfall as demonstrated above, future groundwater trends will depend on the future rainfall patterns which are notoriously difficult to predict. The greenhouse effect is expected to lead to lower winter and higher summer rainfalls, however the degree to which other cyclical patterns (eg the eleven year solar cycle) will impact on these trends is unknown. It must be remembered that these natural variations in rainfall are only of the order of 10 - 20%, whereas recharge has increased by about 1000% since clearing.
More detailed information is available in the South Australia Dryland Assessment 2000 Report
The groundwater flow systems for South Australia are present in the table below. Within the >300mm rainfall zone, 47,000 hectares in local, 21,000 hectares in intermediate and 500,000 hectares in regional flow systems are coincident with regions within which there are areas with a high risk of dryland salinity.
| Groundwater Flow System Type | Area at risk in 2050(ha) | Percentage of total risk area (%) |
|---|---|---|
| Local and intermediate flow systems in deeply weathered rocks | 45,550 | 8 |
| Intermediate flow systems in sedimentary sequences in large valleys | 1,275 | 0 |
| Local flow systems in fractured or weathered rocks or colluvial fans | 725 | 0 |
| Intermediate flow systems in fractured rocks | 19,463 | 3 |
| Local flow systems in fine grained unconsolidated sediments | 863 | 0 |
| Regional flow systems in permeable alluvial sediments | 484,869 | 85 |
| Regional flow systems in marine sediments overlain with some local flow systems in sand dunes | 14,938 | 3 |
* Area within >300mm rainfall zone
Government responses to dryland salinity as at the year 2000
Government response
A whole-of-government approach to managing the growing salinity problem in South Australia has been adopted with the formation of the State Salinity Committee, consisting of seven agency heads. This body has overseen the formulation of the overarching policy statement Directions for Managing Salinity in South Australia and the more specific South Australian River Murray Salinity Strategy and the State Dryland Salinity Strategy (Government of South Australia 2000a, 2000b, 2000c).
The State Dryland Salinity Strategy aims to reverse the trend of rising salinity and to minimise, and where possible prevent, damage to water resources, the environment and to infrastructure. Management options include:
- reducing recharge (usually with the aid of deep-rooted perennial vegetation);
- utilising discharge (usually with salt-tolerant plants or in industries that can use saline water); and
- disposal of surplus water (usually by drainage).
Key points to emerge from the strategy
- Whole-of-catchment management is required.
- Significant new investment will be needed to support individuals and communities.
- Costs of salinity are borne by the whole community and therefore managing salinity is the responsibility of everyone.
The strategy recommends:
- support for on-ground works;
- developing partnerships with affected communities;
- improving knowledge; and
- commitment to action.
Significant action is already being undertaken to combat the impacts of dryland salinity through the Upper South East Dryland Salinity and Flood Management Plan, (Natural Resources Council of South Australia 1993) with associated drainage, revegetation, farm redevelopment and environmental initiatives.
The Coorong and Districts Local Action Plan (Coorong District Local Action Plan Committee 2000) and associated on-ground works has become a national model: a local community-led implementation of significant on-ground works to increase rainfall utilisation and reduce salinity threats. Similar projects are emerging in other parts of South Australia.
A more difficult challenge is in dealing with the increased recharge from rainfall onto dryland farming areas in the Mallee region which will cause significant saline discharges into the Murray River.
Further information
- South Australia Dryland Salinity Assessment 2000 report
- Australian Dryland Salinity Assessment 2000 report
- National Technical Overview Report of the State-based dryland salinity assessments
- Australian Groundwater Flow Systems Report
- Primary Industries and Resources South Australia
- National Dryland Salinity Program
- National Action Plan for Salinity and Water Quality
Link to Map maker to make a map using this information.
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