Inland waters
Theme commentary
Professor Graham Harris, ESE Systems
prepared for the 2006 Australian State of the Environment Committee, 2006
Catchment scale influences
Hydrological condition
Surface water availability and use patterns
Australia is the driest inhabited continent in the world, and more than 80 per cent of the population lives in a narrow coastal strip (see ABS 2004). The bulk of the landmass is arid or semi-arid, with intermittent water flow in many streams. The coastal strip, however, often has a more constant water supply, which is being placed under increasing stress by population increases. While Australia does not appear to be short of water on any of the global maps of per capita water availability—and certainly not as short of water as countries like Kuwait—there is, nonetheless, strong evidence of the stress being placed on Australia’s continental water resources. Australia is the third largest per capita user of water in the world (Radcliffe 2004). The delicate balance between water supply and demand is evident in the extensive regulation of river flows through dams and weirs, extraction and pumping of water for urban and rural use, low water levels in water storages (Figure 1), and water restrictions in many of the major cities as a result of long, dry periods and reduced runoff in recent years.
Figure 1: Percent capacities of selected storages in eastern Australia
Source: adapted from Hanna (2003, Table 1, p. 398)
One of the reasons for this somewhat paradoxical situation is that most of the water is where the people are not, and vice versa. The majority of the runoff from the continent occurs in tropical and subtropical rivers flowing to the north—particularly in northern Queensland, the Gulf of Carpentaria and northern Western Australia (see the Australian Natural Resources Atlas). Over most of the continent (the exceptions being parts of Tasmania and subtropical and tropical high rainfall regions) annual evaporation exceeds rainfall for most, if not all of the year, so that rivers and streams are naturally highly seasonal and would not naturally flow (or be reduced to very low flows) for long periods of the year. In southern Australia, extraction of surface and groundwater is extensive and it supplies agricultural production and the needs of the urban areas. In the Murray-Darling Basin, for example, extractions from the Murray River almost equalled the annual average flow of the river, until a Cap was imposed on further extractions.
One of the facts of life of water supply in Australia is not just the geographical disjunction between supply and demand, it is also the natural climate and runoff variability. This variability and long periods of low flows have made it essential to build large dams. Rivers in southern Australia are extensively dammed and regulated to provide year-round security of supply for urban and domestic use, and irrigation water for use during the summer. Such engineering works are also necessitated by the fact that the natural flow variability in Australian rivers is the highest in the world—this ‘wide brown land’ is characterised by ‘droughts and flooding rains’ (Figure 2). In comparison to the need, Australia stores up to seven years’ worth of water in the major dams . Coupled with the relatively flat topography of this old and weathered continent, the result is that Australia’s major river systems usually possess one or more large, shallow storages. These storages take a long time to empty and a long time to fill after periods of low flow. So Australian water resources have, from the earliest days, been engineered to supply a scarce resource and to provide security of supply in dry periods.
Source: adapted from Department of Primary Industries, Victoria (2004)
Urban areas in Australia cluster around the coast. Coastal development is very rapid as many Australian people become ‘sea changers’ and seek coastal life styles. As a result, the modification of coastal catchments and river systems is extensive, with flow regulation for water supply and flood prevention.
Almost 70 per cent of the water extracted in Australia is used for agriculture . Consumption has stabilised in some catchments, such as the Murray-Darling Basin. Because of climate variability, irrigated agriculture is extensive and growing in area. The majority of irrigation water is used for pasture; lesser amounts are used for some of the higher value crops and for horticulture. Indeed, there is an inverse relationship between the amount of water used and the value added per megalitre. This situation is changing as water markets and other reforms (discussed later) change water use patterns. Water use efficiencies in the pasture and horticulture sectors in regions such as the Murray-Darling Basin are increasing due to improved irrigation practices and the growth of crops that yield greater returns per megalitre of water used (for example, see rice growing in the MIA). There are, therefore, many positive developments.
Urban and industrial use is a relatively small proportion of the total water used (Table 1). Surprisingly, perhaps, urban areas are actually net exporters of water if all sources and sinks are accounted for. Urban areas have large areas of impervious surfaces in their catchments—roads, pavements and roofs—so the hydrology of urban areas is highly modified. On an annual basis, Mitchell et al. (2003) calculated that storm water runoff from urban areas in Canberra exceeds the water imported from storages for domestic and industrial use. The problem here, of course, is variability: storm water runs off rapidly in large volumes and would need to be stored and treated before reuse. Most Australian cities reuse relatively small amounts of water (Table 2), practicing a policy of ‘once through’ from storage to disposal of wastewater (usually to the sea). This situation is changing as recycling and reuse practices are being adopted in response to the recent period of low rainfall and water restrictions. Many Australia cities (for example, Adelaide, Melbourne, Wollongong, Sydney, and Brisbane) either now have water recycling plants in operation, or major developments are planned (Radcliffe 2004).
| Sector | Water consumption (ML) | |
|---|---|---|
| 1996–97 | 2000–01 | |
| Irrigated agriculture | 15 502 973 | 16 660 381 |
| Forestry and fishing | 18 815 | 26 924 |
| Mining | 570 217 | 400 622 |
| Manufacturing | 727 737 | 866 061 |
| Electricity and gas supply | 1 307 834 | 1 687 778 |
| Water supply | 1 706 645 | 1 793 953 |
| Household | 1 828 999 | 2 181 447 |
| Environmental flows | 459 393 | |
| Other | 522 513 | 832 100 |
* Water consumption = mains water use + self extracted water use + reuse water use – water supplied to other uses – instream water use
Source: adapted from ABS (2000, 2004b)
As urban populations grow (particularly in coastal regions and in urban centres in and around Sydney and south-eastern Queensland), supply management is becoming an issue. Forecasts of future water demand frequently exceed the sustainable water yield from the present catchment areas, so new sources of supply will be required to meet predicted demand patterns. If the gap between supply and demand is to be closed, then either new dams will need to be built, or new groundwater systems accessed, or significant increases in reuse and recycling will need to be achieved. In the short-term, demand management is also an important part of the policy mix. Many states and regions, especially south-western Western Australia and south-eastern Queensland, are actively involved in water resource planning of this type. In peri-urban regions there is a growing issue of competition for water between urban and agricultural use. An increased number of inter-basin transfers of water is expected as pressure on water resources grows. Historically, these have been for hydro-electricity generation and for irrigation supplies (for example, the Snowy scheme) or to provide water for urban areas (for example, the Shoalhaven in New South Wales and catchments to the south of Perth in Western Australia).
| State or Territory | 1996–99 | 2001–02 | |||||
|---|---|---|---|---|---|---|---|
| Effluent GL /year | Recycled GL/year | % | Effluent GL/year | Recycled GL/year | % | ||
| Queensland | 328 | 38 | 11.6 | 339 | 38 | 11.2 | |
| New South Wales | 548 | 40.1 | 7.3 | 694 | 61.5 | 8.9 | |
| ACT | 31 | 0.25 | 0.8 | 30 | 1.7 | 5.6 | |
| Victoria | 367 | 16.9 | 4.6 | 448 | 30.1 | 6.7 | |
| Tasmania | 43 | 1 | 2.3 | 65 | 6.2 | 9.5 | |
| South Australia | 91 | 9 | 9.9 | 101 | 15.2 | 15.1 | |
| Western Australia | 109 | 5.5 | 6.1 | 126 | 12.7 | 10 | |
| Northern Territory | 21 | 1 | 4.8 | 21 | 1.1 | 5.2 | |
| Australia (total) | 1538 | 112.9 | 7.3 | 1824 | 166.5 | 9.1 | |
Source: Radcliffe (2003, Table 2, p. 7)
Groundwater availability and use patterns
While surface waters are unconditionally renewable—runoff will continue as long as severe climate change is avoided—groundwater, on the other hand, is only conditionally renewable. This is because some of the groundwater used for irrigation and urban water supply is quite old. Most of the groundwater resources of Australia are recharged annually, but some groundwater supplies are tens to hundreds of thousands of years (if not millions of years) old and response times to changes in recharge are also very long (in the order of centuries to thousands of years). This means that rapid use (at a scale of decades) makes the resource practically non-renewable. Also, there is evidence of overuse of groundwater in both urban and rural areas. Groundwater levels and pressures are declining, and this in an environment where restrictions on the use of surface water are causing an increased use of groundwater resources. Unlike surface water, where there are caps on new licences, for groundwater there are limited caps over a relatively small area of Australia. For example, groundwater levels in the Gnangarra mound aquifer under the city of Perth are declining, and there is evidence of overuse in many major aquifers (Radcliffe 2004, NLWRA 2001c).
| State or Territory | Total Use (GL) 1983–84 |
Total Use (GL) 1996–97 |
Percent Change (‘83–84 to ‘96–97) |
|---|---|---|---|
| New South Wales | 318 | 1 008 | 217 % |
| Victoria | 206 | 622 | 202 % |
| Queensland | 1121 | 1622 | 45 % |
| Western Australia | 373 | 1138 | 205 % |
| South Australia | 542 | 419 | -22 % |
| Tasmania | 9 | 20 | 122 % |
| Northern Territory | 65 | 128 | 97 % |
| ACT | n/a | 5 | - |
| Total | 2 634 | 4 962 | 88 % |
Source: NLWRA (2001b, Table 22)
A recent decision to use a new groundwater source to supplement Sydney’s water supply raises questions of sustainability. Over large areas of the Murray-Darling Basin, groundwater levels have declined during the last decade due to the combined effect of the drought and excessive groundwater pumping (MDBC 2004). There are, however, positive signs in some regions. In the Great Artesian Basin, for example, many bores have been capped and drainage canals have been replaced with pipes to reverse declining groundwater pressures and water levels; this has led to the restoration of some spring wetlands . In some areas, this progress has been offset when old casings have failed and started to leak under the increased water pressure, which in turn has allowed higher pressure saline waters to intrude into less saline aquifers.
Figure 3: Status of bore capping under the Great Artesian Basin Sustainability Initiative
Source: adapted from Hassall and Associates Pty Ltd (2003, Table 3.1, p. 10)
Point source pollution of groundwater is generally controlled by Environmental Protection Agencies in the states and territories. Great improvements in groundwater pollution prevention have occurred in the least few decades, but diffuse source pollution (for example, pesticides, fertilisers, and septic tanks) remains poorly controlled in some jurisdictions.
Paradoxically, groundwater levels in shallow aquifers in many irrigation areas have risen to the levels of the mid-1990s because of recharge under irrigated crops. This is undesirable from a number of viewpoints. Over-application of water coupled with low water use efficiency leads to wastage of irrigation water. Significant groundwater mounds have existed under many irrigation regions where river water is used to irrigate crops. During the last decade, some of these mounds have stabilised or even declined due to improved water use management, and as a result of drought. Also, recharge under dryland crops and cleared pastoral lands exceeds the normal rates of recharge under native vegetation, leading to rising groundwater levels in shallow aquifers and dryland salinity .
Across the continent, there are many good examples of market forces bringing about increased water use efficiencies, better irrigation practices, and control of recharge. Similarly, in dryland and pastoral areas, there is an increasing use of perennial crops such as lucerne or agroforestry plantations to control recharge and bring about other sustainability benefits such as increased biodiversity. So the patterns of groundwater levels are mixed—in some places rising under irrigated crops where river water is over used, but either rising or falling in other areas depending on the source of the irrigation water, the demand for groundwater extraction, and the efficiency of use. In the Murray-Darling Basin, the introduction of the surface water Cap in 1995 has caused many irrigators to switch to using groundwater (MDBC 2003).
Available data suggest that, overall, there are now large areas of Australia where groundwater is being used above a sustainable rate ; groundwater levels are declining as a consequence of overuse and lower rainfall. There is also evidence of a slow improvement in water use efficiency in agriculture as environmental and other concerns influence management practices. On rice crops in the Murrumbidgee Irrigation Area, for example, water application rates have fallen from just over two megalitres of water for each tonne of rice produced in 1985, to just over one megalitre per tonne in 2001. Much of this improvement in water use efficiency can be attributed to market forces and the rising cost of irrigation water.
Ecological aspects of flow regimes
As noted above, Australian inland rivers were historically, and are naturally, very variable in flow. In rivers like the Darling River, flood peak flows were as much as 1000 times the flow in dry periods. Almost all of these rivers have wide floodplains, where the connection between the channel and the floodplain during high flow events was a critical component of the ecology of those ecosystems. Even coastal Australian rivers (which have shorter, steeper catchments and narrower floodplains) are subject to very large fluctuations in flow, with high flow events and flooding following storms. Extensive damming and regulation of river flows has not only altered (and evened out) the river hydrology, it has also reduced or eliminated flooding. There are very few rivers in southern, eastern and Western Australia that are not regulated in some way, particularly in their lowland reaches. Rivers in tropical, northern Australia are less heavily regulated.
Hydrographs of many regulated Australian rivers now show a flow pattern resembling permanent drought, in which the frequency and magnitude of high flow (flood) events have been reduced and the seasonal flow pattern has also been significantly modified. Before settlement, most rivers ran high and in flood after storms or in the tropical ‘wet’, and then almost (or completely) dried up.
Native Australian species are adapted to the natural frequency and magnitude of flood and flow events, frequently taking their cues for migration and reproduction from seasonal flow changes. Overbank flows provided opportunities for feeding and spawning for native fish in these rivers, and these flows allowed species in billabongs and side channels to reconnect with the main flow. Now that all but major floods have been eliminated in regulated rivers, overbank flows are rarer and most rivers run bankfull in summer to provide irrigation water. This has radically altered the seasonal and inter-annual flow patterns. Thus, flow regulation has altered the ecology and biodiversity of these rivers in many substantial and subtle ways. Flow regulation has similarly altered the ecology of wetlands and lakes within regulated river basins. This is also true for major rivers and reservoirs serving urban areas where connectivity between the main channel and the floodplain has also been severed.
The reduction in flood frequencies and the extensive regulation of flows through the building of weirs and dams has changed the physics of most Australian rivers. In these regulated rivers, where floods once regularly scoured out depositional areas, there are now extensive depositional areas and much larger areas of fine sediments .
Before European settlement, there were frequent (if irregular) occasions when the floodplains and billabongs were flushed with freshwater and sediments were deposited; this now rarely happens. The alteration of flow regimes has also changed the physical habitat of most of Australia’s rivers and streams because the scouring action of floods is now much reduced. The changes in land use and the increased erosion of material from the catchments have exacerbated this alteration of the physical regime.
Many people, it seems, forget that it is not just surface waters that have an ecology. There is increasing evidence of a significant stygofauna in underground aquifers, for example in Western Australia (see Humphreys and Watts 2004). This means that alterations to groundwater hydrology are also having effects on the ecology and biodiversity of many poorly-understood species. Over-pumping and salinisation of aquifers will alter ecological communities and eliminate many species. Across Australia, there are also many groundwater dependent ecosystems such as wetlands (including mound springs) that are in decline because of pumping of groundwater. This is discussed further in section ‘Groundwater dependent ecosystems’.
Because of the widespread alteration of the hydrological regime across the southern part of the continent and the evident river degradation that has occurred, there is now an increasing emphasis on the value of the natural capital represented in the aquatic ecosystems of Australia’s riverine and estuarine systems and the need to restore it. The restoration of environmental flows to rivers, wetlands, floodplains and estuaries is a matter of some priority . A number of programmes are now in place to try to achieve this; although, compared with the scale of the problem, progress is slow. It is worth noting, perhaps, that estuaries are as much in need of environmental freshwater flows as rivers and wetlands are. Estuaries are very sensitive to variability and, hence, regulation of freshwater inflows (Webster and Harris 2004). Because of the rapid pace of coastal development and the widespread regulation of coastal rivers and streams, there is an urgent need to restore the hydrological balance of some of our more degraded estuarine systems. Although natural river flows no longer enter many estuaries, estuaries do receive large volumes of urban stormwater runoff of varying quality.
Connectivity—dams and weirs
The regulation of flows, the construction of dams and weirs, the initiation of interbasin transfers and the alterations in the physical habitat of Australia’s rivers and streams is changing the connectivity of these systems and the ecological and evolutionary framework of the continent’s aquatic ecosystems. The very large number of control structures (dams and weirs to impound water, and levee banks to control overbank flows) on Australian rivers means that the lifecycle of many aquatic species is disrupted, particularly that of the native fish . Connectivity is vital for the survival of riverine species; a vital feature of the lifecycle of many species is floodplain connectivity, active upstream and downstream migration or passive downstream drift. There is growing evidence of widespread endemicity in the aquatic flora and fauna of Australian river basins. Because of the great age of the Australian continent, there has been sufficient time, it seems, for evolution of species to occur in relict invertebrate populations that have been isolated in different catchments.
The present hydrological condition of many Australian surface and groundwater systems, particularly in the southern part of the continent, is highly modified from the natural state. The modifications have been designed to increase the security and productivity of human activities and to ensure constancy of supply. All this has been done in an environment of naturally strong seasonal and inter-annual variability. It is important to remember that, before European settlement, the ecology of the continent was adapted and finely attuned to the natural patterns of variability. The human need for constancy and security since then has resulted in significant changes to the hydrology and ecology of the continent being made (see Table 4 for an example of the impact of weirs). Changes to flood frequencies, connectivity and the seasonality of flows are just as important as changes to the overall annually averaged flow regimes.
| Common Name | Number of fish | |
|---|---|---|
| Above Weir | Below Weir | |
| Port Jackson Glassfish | 0 | 237 |
| Long-finned Eel | 5 | 11 |
| Silver Bellies/Biddies | 0 | 83 |
| Hypseleotris Gudgeon | 7 | 5 |
| Flat-tailed Mullet | 0 | 28 |
| Australian Bass | 13 | 49 |
| Bully/Sea Mullet | 6 | 10 |
| Freshwater Mullet | 2 | 1 |
| Flat-head Gudgeon | 71 | 148 |
| Dwarf flathead Gudgeon | 3 | 1 |
| Blue eye | 0 | 49 |
Source: adapted from World Wildlife Foundation Australia (2003, Table 1)
Connectivity—groundwater and surface water
The interconnection between groundwater and surface water is a poorly understood process in Australia. Groundwater and surface water are usually interconnected and interchangeable resources. In many cases it is actually the same water: groundwater becomes surface water, surface water becomes groundwater. Groundwater pumping within a catchment has the effect of reducing baseflow in a gaining stream and, in some cases, it can turn a gaining stream into a losing stream, causing induced recharge. But because this linkage has not been grasped by either government or the general public, Australia’s surface water and groundwater have been managed completely separately. As a result, the same resource has often been double accounted and even double allocated—once as surface water, and a second time as groundwater—but it is actually the same parcel of water.
This has not been recognised because of the long time lag between when groundwater is pumped and when streamflow is actually reduced as a result. The effects of the huge increase in groundwater use during the 1960s, 1980s and late 1990s across Australia have, in some catchments, not yet been observed in reduced streamflows. Even though many of Australia’s surface water catchments are now fully capped, this is still not the case for groundwater use. For example, in some parts of Australia it is still possible to drill a bore on the banks of a fully capped river and call it groundwater and get it approved.
The possible extent of double accounting of groundwater and surface water in the Murray-Darling Basin is described in MDBC (2003) and a description of the possible extent of double accounting in Australia is provided in Evans (2005). The implication of not recognising surface water – groundwater interaction and the continued development of groundwater in many catchments is the reduction in river baseflow. This may occur over months, or very slowly over decades, or even longer. As the baseflow index may typically be between 10 per cent and 50 per cent in Australia (see, for example, Neal et al. 2004), the reduction in baseflow can have the effect of significantly reducing streamflow, especially during the critical, low-flow dry season. This in turn has the effect of reducing the security of supply to surface water users and it can have significant impacts on the environment generally and, more specifically, on stream ecology, even leading to the complete drying out of streams in some cases.
The water cycle and land and vegetation condition
Vegetation
Native vegetation has an important role to play in the overall water budget of the continent through its control of the water balance and hydrology of large areas of the landscape. There has been considerable change in the vegetation of the Australian continent since European settlement in 1788. The native vegetation of Australia evolved in harmony with the variable climate and it is a very efficient user of water. Land clearing causes major changes to the hydrology of catchments; altering interception processes, evapotranspiration rates, and water flows. Land clearing has been extensive and this continues in some places. The basic change that has occurred in the intensive landuse zone is the replacement of structurally and functionally diverse native ‘bush’ that was dominated by perennials of various kinds, with a pastoral and arable cropping regime that is dominated by annual grasses and other crops. Not only is the undisturbed Australian ‘bush’ species-rich, it is also functionally diverse; this ensures that the vegetation is highly efficient in its water use (Pate 1999). A wide range of perennials and annuals with quite different growth strategies and phenologies ensure that almost all of the water arriving in the root zone is utilised. This process of maximising water use efficiency by native vegetation appears to take hundreds of thousands if not millions of years of evolution and ecological development. Subsurface water recharge under native bush is very low compared with the agricultural regime that has replaced it over the large areas of the continent where arable farming dominates.
The National Land and Water Resources Audit (NLWRA) assessed the state of the continental vegetation in 2001. There is a problem in assessing continental-scale vegetation change since settlement because of differences in the systems of reporting used by states and territories. While these are being addressed there are, nonetheless, difficulties in taxonomy and mapping systems. Notwithstanding these difficulties, the overall pattern is clear. Large areas of the wheat and sheep zone in eastern and south-western Australia have severely degraded vegetation. In parts of these regions, less than five per cent of the original vegetation remains, and introduced exotic weeds of various kinds are a problem (see Morgan 2000). Catchment condition, as measured by a number of indicators, is poor (see NLWRA 2002). Habitat fragmentation leads to species loss and reduced biodiversity and, more importantly from the point of view of water and hydrology, it leads to reduced functionality and water use efficiency. Fragmented and less diverse landscapes tend to be ‘leaky’ in terms of water, sediment and nutrient flows. Estimates of the change in the continental water balance since settlement have shown that, in many regions, a significant amount of water that was originally evaporated by the vegetation is now diverted to recharge (see Gordon et al. 2003). This is the fundamental reason for the outbreaks of dryland salinity that have occurred since settlement.
Estimates of the reduction in tree numbers since settlement show that as many as 12–15 billion trees have been lost from the Murray-Darling Basin since European settlement (Walker et al. 1993). The situation is not all one-way, however, because early photographs show fewer trees in some places than now exist. Also, widespread replanting is underway in agroforestry and other revegetation schemes . Some revegetation is targeted towards recharge and salinity control, other revegetation activities aim to restore biodiversity by increasing the extent of native vegetation and the linkages between fragments. At the scale of individual farms and small catchments, some of these efforts have been very successful. Connectivity is as important for the terrestrial landscape as it is in the aquatic realm. Nevertheless all the information to hand—vegetation cover and condition, agricultural extent, and urban development—indicates that the hydrological balance of the Australian continent has been significantly altered by land use change and clearing in the last 200 years (Gordon et al. 2003).
Widespread reforestation, either through plantation forestry or through smaller-scale agroforestry activities, changes the hydrology of catchments. Rapid growth of deep-rooted perennials, such as trees, uses water—more water, in fact, than mature bush—so that during the early stages of the reforestation cycle, water use increases and runoff decreases. Reduced runoff means less water for irrigation or urban supply. This is an important and topical interaction between vegetation and hydrology (Zhang et al. 1999, Best et al. 2003). As pressure on the resource increases, trade-offs will need to be made between revegetation strategies and the various productive and domestic uses of water in catchments.
Erosion
Large-scale estimates of erosion susceptibility and extent were also made by the NLWRA in 2001 (see also Prosser et al. 2001). Erosion of soil and sediment from hillslopes, erosion through gullying and contributions from riverbanks contribute to greatly increased sediment loads to rivers and estuaries (Table 5). Catchment condition is poor and sediment loads are high across large areas of the wheat and sheep zone of eastern and south-western Australia. Sediment loads to rivers are now estimated to be about ten times higher than they were before European settlement. Immediately after clearing of vegetation, sediment loads were more than one hundred times higher than beforehand. Together with the altered physical regime, this has changed the nature of bed sediments and habitats over long stretches of Australia’s inland and coastal rivers (Wasson et al. 1996). These changes have had a strong influence on biodiversity and river function.
| Sediment source | Quantity (106 t/year) |
|---|---|
| Gross sheetwash and rill erosion | 666* |
| Delivery to stream from sheetwash and rill erosion | 50 |
| Gully erosion | 44 |
| Streambank erosion | 33 |
| Total sediment supplied to rivers | 127 |
| Total suspended sediment stored | 66 |
| Total bed sediment stored | 36 |
| Sediment exported from rivers | 25 |
| Total of stores and losses | 127 |
* Does not include sheetwash and rill erosion estimates for the Gulf, Western Plateau or Northern Territory as river budget assessments were not undertaken in these areas.
Source: NLWRA (2001a, Table 5.4 in ‘Water-borne soil erosion’)
In many places across the continent—particularly the upland regions of the east and southeast—gully erosion is the major source of the sediment contributed to rivers and streams (see NLWRA 2001a). The initiation of the gullying process appears to have been rapid once the original land clearing occurred—sometimes more than a century ago. Intense rainfall is characteristic of many warm temperate, subtropical and tropical regions of Australia, so much of the gullying probably began in a single intense storm soon after clearing of the landscape on susceptible slopes. This is a common story in Australian hydrology and geomorphology—all the important things happen during infrequent, intense events; once again reinforcing the story of the importance of climate variability and the frequency and magnitude of individual rainfall and flood events.
Prosser, Olley and others (Wasson et al. 1996, Martin and McCulloch 1999, Prosser et al. 2001) have been able to use geochemical and other techniques to identify the sources of material in river sediments, to trace these to particular events in particular parts of catchments, and to examine trends over time. It appears that across large areas of the landscape, most of the intense periods of erosion are now over and that some kind of new equilibrium is being approached. Much of the eroded material has not yet worked its way through the bigger river systems, with large amounts of sediment stored in river channels in areas of low gradients of rivers (such as in the lower Snowy River), where it causes substantial ecological problems.
Erosion of sediments is much increased after fire. The large-scale fires that burned across the Victorian and NSW high country in 2001 and 2003 caused greatly increased erosion in these areas and led to poor water quality and the deposition of large amounts of soil and other materials in stream beds and lakes downstream of the fires (SAP 2003). Because of the high altitude and low temperatures in those areas, recovery of the vegetation and catchments is expected to take many decades.
All the evidence, therefore, points to close connections between land use, vegetation and its condition, and the hydrology and water quality of inland waters.
Water quality—nutrients and sediments
Increased loads of nutrients and sediments to inland waters increase the fertility of the water (causing eutrophication) and may lead to algal blooms and alteration of aquatic habitats. Just as erosion has increased since settlement so too have the loads of nutrients and other materials . River condition in much of the wheat and sheep zone has been extensively modified and is consequently poor.
The nutrients of most interest are nitrogen and phosphorus because of the important role they play in plant growth and the biological enrichment of receiving waters. In general terms, phosphorus is the key limiting element in freshwater systems and is mostly tightly bound to particulate material; nitrogen is generally the more soluble of the two. Nitrogen is usually the limiting element on coastal and oceanic waters. Nitrogen may be limiting in subtropical and tropical freshwater systems, where microbial process lead to nitrogen being stripped from surface waters. Geochemical tracer work in agricultural catchments seems to indicate that the biggest source of increased phosphorus loads to receiving waters is from gully erosion and streambanks rather than from fertilisers applied to paddocks (Martin and McCulloch 1999, Prosser et al. 2001). Because of generally low population densities across inland Australia there are few major towns discharging sewage effluents into inland rivers (Canberra and Albury-Wodonga being exceptions), so the major contributor to increased phosphorus loads to rivers is land use change and increased erosion . There are, however, nutrient enrichment problems downstream of irrigation areas and intensive dairy regions around the country, where runoff of animal wastes and agricultural chemicals is a regional problem. Estuarine impacts also result from this runoff. This is particularly true in higher rainfall regions.
Australia’s cities mainly discharge major wastewater streams to the oceans, although some coastal communities and suburbs discharge to coastal rivers. Direct discharges, storm runoff, sewer overflows and septic tank infiltration cause local problems with phosphorus enrichment in urban rivers. City water companies have programmes in place to control and improve the quality of these urban flows, through the construction of urban wetlands and other systems to detain and remove nutrients.
Nitrogen sources in landscapes come from fertiliser use and application, animal wastes and sewage discharges (Caraco and Cole 1999). Generally speaking, subtropical and tropical landscapes, such as those that characterise the Australian continent, tend to be poor in nitrogen because of high rates of denitrification. (This is one reason for the prevalence of nitrogen fixing cyanobacteria in water storages—they have a competitive advantage.) So as a result of agricultural development and urbanisation, nitrogen loads to rivers and coastal waters have increased markedly since settlement. Furthermore, the forms of nitrogen have changed from predominantly organic (and less biologically available) forms to inorganic and more biologically available nitrogen forms. This is a particular problem in estuaries, where nitrogen stimulates algal and plant growth, which increases the effect of the changed land use (Webster and Harris 2004).
A number of urban wastewater treatment plants now incorporate both phosphorus and nitrogen removal systems, thereby improving the quality of the water discharged. The initial focus was on phosphorus removal because of the key role played by phosphorus in freshwater systems, but biological nitrogen removal is now also widely used, particularly in waste-streams that reach coastal waters.
Influence of climate variability and change
The debate over climate change is still not settled. While some deny the reality of anthropogenic climate change, others are now convinced that the severe lack of rainfall across large areas of the continent in since 2001 is evidence of a warming and drying trend. Whatever the position, there is strong evidence that, particularly across southern Australia, we are now seeing altered climate patterns and changes in seasonal flow patterns that are quite consistent with the predictions of the climate change models . These models predict reduced rainfall and runoff across southern Australia. Both models and observations agree; there has been a marked warming and drying trend across southern parts of Australia in the last three or four decades, which is apparently due to a southward migration of the subtropical high pressure ridge and a concomitant weakening of the zonal westerlies. In those regions of the continent that are dependent on winter rainfall, there has been a significant reduction in rainfall since the mid 1970s. This extends from Western Australia, through Victoria to eastern Tasmania.
A number of lines of evidence now show that the recent events are part of a longer term warming and drying trend during the last two hundred years (Thresher et al. 2004). The associated warming in southern Australia means that, in many regions, the balance of rainfall and evaporation has changed in favour of much drier conditions. This has been exacerbated by the recent one in one-hundred-year dry period in many regions. Observations have shown that, when compared to the 1982 low rainfall period (which had similarly low rainfall totals), the 2002–05 (and continuing) dry period was much more severe because it was, on average, as much as one degree warmer. The evaporation rate was consequently higher in 2002–04. The low flow conditions in the Murray-Darling Basin have been particularly severe, with storage levels at unprecedentedly low levels and widespread and severe stress being registered. Large areas of mature River Red Gum (Eucalyptus camaldulensis) along the River Murray corridor (about 250 000 hectares) have been killed by the combination of salt ingress to the floodplains, low flows, and the lack of flushing overbank floods. Recent surveys show that the situation is getting rapidly worse; it has been exacerbated by low river levels during this very dry period.
All major southern Australian cities have been on water restrictions in recent years and some are now permanent. The situation in Goulburn is particularly severe, with the city’s reservoirs going below 10 per cent of full storage levels (in late 2005). All major city water companies are studying future supply options, including desalination, new dams, reuse and recycling schemes and demand reduction options.
The situation in Western Australia is particularly severe, with a sharp and sudden drop in runoff in Perth catchments of 50 per cent during the mid-1970s (see Figure 4, and IOCI 2002). If the altered hydrological regime in Australia is a sign of climate change, then Australians shall have to live with a new set of realities. Certainly, water-supply planning benchmarks that used long-term runoff averages have been suddenly changed and planners are dealing with new levels of uncertainty. The reduced rainfall and greater evapotranspiration has also had the effect of significantly reducing recharge, thereby causing groundwater levels to decline.
