Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Jonas Ball, Sinclair Knight Merz Pty Limited, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06750 7
Australia is the driest inhabited continent in the world, with the lowest average rainfall (except for Antarctica) and the lowest run-off from its catchments (McMahon et al. 1982). Around 80% of Australia has a median rainfall of less than 600 mm per year and 50% has a median of less than 300 mm per year (see the Atmosphere Theme Report). The high average rainfall areas include the east-facing slopes of north-east Queensland (greater than 4000 mm per year), west-facing slopes of Tasmania and the Australian Alps (4000 mm per year), and mountainous areas in north-east Victoria and some eastern slopes in the Great Dividing Range (2500 mm per year). Most of Australia is also affected by a high year-to-year variability in rainfall (due to the El Niņo-Southern Oscillation), high evaporation rates and large temperature ranges. In many areas rainfall is also highly seasonal. The large variation in climate and rainfall has resulted in a similar variation in natural environments - ranging from the arid inland areas to the tropical environments of far northern regions to the cooler temperate climates in south-eastern Australia. Reflected in variability of climate and rainfall across Australia are the regional characteristics of inland waters and their associated aquatic flora, fauna and habitats (i.e. ecosystems).
Inland waters were defined by Fairweather and Napier (1998) as:
surface and underground water resources not associated with the sea and that are predominantly fresh, but can be more saline than seawater for natural or cultural reasons.
This definition includes:
- Rivers and streams - Rivers and streams are non-marine waters that flow in one direction at least part of the time. There are a wide variety of river and stream types in Australia due largely, but not exclusively, to the climate. They can be permanent or ephemeral, wide or narrow, deep or shallow, straight or sinuous, have open or confined floodplains or, as in many cases, be a combination of many styles in different parts of the system. During dry periods, rivers can be reduced to a series of disconnected pools, while during wet periods, the main river channel connects to the floodplains, billabongs and riverine wetlands. In many rivers and streams, groundwater discharge contributes substantially to base flow. Depending on groundwater quality, these inflows can affect dry weather water quality as evidenced in areas of saline groundwater discharge.
- Wetlands - The term 'wetlands' is a generic classification for a variety of systems including swamps, marshes, billabongs, floodplain depressions, ponds and mangroves and they are associated with both coastal and inland waters. They can be ephemeral or flowing with fresh, brackish or salt waters, and natural or constructed. Wetlands are of considerable ecological and hydrological significance in the Australian landscape. They assist in flood mitigation, provide habitat and nursery areas for a wide variety of animals, and act as drought refuges and in the general maintenance of biodiversity and numerous ecological processes.
- Natural and artificial lakes and storages - Lakes are distinguished from wetlands by their size, area of open water and their generally greater depths. As with wetlands, they can be natural or artificial. Natural lakes are formed by geological and/or hydrological processes while artificial storages are mainly built instream in a location where the surrounding geology lends itself to a dam wall and with sufficient upstream catchment to capture run-off. Lakes, whether natural or artificial, act in flood mitigation, provide habitat for plants and animals, act as drought refuges and maintain biodiversity and ecological processes.
- Groundwater and groundwater-dependent ecosystems - Groundwater encompasses all water in a persistently saturated state in soil, rock or gravel. The layers of soil, rock or gravel that are able to hold and transmit water are known as aquifers. Water infiltrates through surface soils down through coarse alluvial material to the aquifer. Aquifers occur throughout Australia but are spatially variable. There is growing concern, particularly in the Murray-Darling Basin, over the increasing rates of groundwater extraction particularly where these are beyond sustainable yield. Groundwater is important in maintaining groundwater-dependent ecosystems such as mound springs and perched lakes. Groundwater is also important in maintaining floodplain vegetation such as floodplain trees that have root systems extending down into the aquifer to take advantage of the water during dry periods.
Much of Australia's agricultural production is located in the semi-arid regions inland of the coastal plains. Generally, rainfall in these regions is low and highly seasonal, resulting in river systems that have long periods of low or no flow (drought) and short periods of very high flows (flooding). This 'boom or bust' pattern of water availability has shaped many unique Australian inland aquatic ecosystems (such as floodplain wetlands and billabongs) and also the development of water resource infrastructure. In contrast, the coastal plain, where most of the major urban centres are located, generally receives higher average rainfalls that are less seasonal than those in inland areas. The development of coastal water resources in populated areas is also common, primarily to supply drinking water to urban centres.
Unlike inland aquatic ecosystems, human uses require security and predictability in water supply. Water storages have been constructed to achieve security of supply and many inland rivers have some form of flow regulation to meet human requirements for water. In conjunction with activities in the catchment, the modification of 'natural' flow regimes in river systems is considered to be one of the most significant impacts on Australian inland aquatic ecosystems.
The use of groundwater for both irrigation and drinking water supply has doubled since the early 1980s and many of these resources are also under considerable stress.
Activities in catchments such as land clearing, agriculture, urbanisation and the discharge of pollutants can also have significant effects on inland waters and their ecosystems. The major threats to Australian inland water ecosystems from activities in the catchment are increasing land and water salinisation; destruction of habitat; nutrient enrichment and algal blooms; and the introduction and spread of exotic flora and fauna.
Since 1996, the State of the Environment reporting process has been refined by the development of a set of indicators that define 1) the major pressures, 2) the condition of inland waters and 3) the management responses (Fairweather & Napier 1998). Wherever possible these indicators have been used as a basis for reporting.
The assessment of the condition of inland waters is based around three key management issues:
- water resources - the use of surface water and groundwater for human uses such as agriculture, drinking water and industry
- water quality - including eutrophication and algal blooms, salinity, contaminants, pollutant sources and groundwater pollution
- aquatic ecosystems - assesses the impact of over-extraction, flow regulation, poor water quality and habitat destruction on Australia's unique communities of aquatic flora and fauna.
Major issues in each of the key management areas were assessed using the condition-pressure-response model. An introduction to the major pressures on aquatic ecosystems and water resources, the indicators used to measure their condition and the framework for management responses is presented in following sections.