Wetland Science
State of the Darling, Stage 1, Hydrological Overview
Authors: Webb, McKeown and Associates
Murray Darling Basin Commission
INTRODUCTION
The report provides an overview of the natural hydrology of the Darling Basin, the development that has occurred, and the effect this is having on river flows and groundwater resources. Although this report is highly technical, some of this information can assist wetland managers. The full report can be downloaded from http://www.mdbc.gov.au/__data/page/15/Darling_Report.pdf A proposed second report will link the hydrological information with important socio-economic data and environmental features of the Basin.
THE DARLING BASIN
The Darling Basin is twice the size of the Murray, but it drains a much more arid area. Climate records show that the system experiences long periods of below average rainfall, affecting river flow and groundwater recharge. In addition, a large part of the flow in the Darling’s tributaries rivers ends up in the wetland areas, rather than in the Darling River itself. As a result, flows in the Darling and its tributary rivers are much smaller and more variable than flows in the Murray and its tributaries.
DAMS AND DIVERSIONS
Dams control about 30% of the Basin flows, considerably less than are controlled by Dams in the Murray. The Dams regulate flows in the eastern tributaries, a short portion of the Balonne River and the last 200 kilometres of the Darling, below Menindee lakes.
More recently, and unique to the Darling Basin, has been the major private investment in large storages on irrigation farms. The total volume of these on-farm storages now rivals that of the headwater dams, and they capture much of the water that enters the Basin’s rivers downstream of the Dams. Current estimates also indicate that there are very large volumes of hillside dams that capture large volumes of runoff before it reaches the Basin’s rivers, although there is considerable doubt about the accuracy of these hillside dam estimates.
The total surface area of the shallow on-farm storages is large and they contribute substantially to the already high evaporation rates in the Basin. There are also large losses from Menindee Lakes. Evaporation rates from all water storages is now estimated to be about 2,000,000 Megalitres per annum, equal to about 25% of the average flow in the Basin’s rivers.
IMPACTS ON FLOWS
The effects of water infrastructure and extraction of water on rivers flows in the Basin include a loss of flow volume, loss of flood events, and increases in periods of low flow. There is now less than half the volume of water flowing from the Darling to the Murray than there would have been under natural conditions. Associated with the reduced volume of flow are large reductions in the size and frequency of floods in all rivers on which there has been major irrigation development. For example, floods that would have been expected to occur on average once every 2 years at the end of the Culgoa will now only occur once every 5 years.
IMPACTS ON FISH AND WETLANDS
Although the report is focused on hydrology and not the environmental or socio-economic issues of the Basin, the report did mention that associated with the hydrologic changes are major reductions in average volumes of flows, and the frequency of major inflows into the large end of system wetlands of the Gwydir, Macquarie and the Condamine / Narran. Also, the numbers of days when wetlands along the Barwon-Darling are joined to the river, and are receiving water, have also dropped – in most cases by a third or more.
There are also restrictions to fish migration. Migration of fish along the Barwon-Darling is now only possible when flows are high enough to drown out the many weirs that now exist. The effect of dams and water use has reduced the frequency of drown outs, and greatly exacerbated the effect of weirs on migration opportunities.
GROUNDWATER
Although groundwater extends throughout the Basin, good quality, useable supplies are limited in their extent. The major aquifers that can be used for irrigation extend up to 300km’s from the lower slopes of the Great Diving Range. They occur in the major tributaries of the Macquarie, Namoi, Gwydir, Border Rivers and Condamine and Balonne Systems. The quality of the water is high relatively close to the rivers, but deteriorates down the long flow path of these systems. There are also several other areas in the Basin where small volumes of good quality water is used.
In most systems in the Basin where there is significant pumping, localised groundwater level declines are being detected, even where the overall extraction remains within the extraction limit. Where there is a localized level decline, there is potential for the migration of poorer quality surrounding groundwater into the fresh aquifers. The declines may also contribute to losses from, or a reduction in discharge to, connected rivers or other surface water bodies including wetlands. Currently, information about the connectivity of surface water to groundwater is patchy, which is hindering their successful management.
THE FUTURE
Future risks for the Basin include climate change, continued increase in surface water use and the current and future groundwater use.
- Current assessments indicate that reductions in average flows of 20% or more may occur by 2030
- Water policy in Queensland and NSW appears to provide scope for further increases in surface water use. The actual extent of this increase is unknown but there may be opportunity for growth in the capture of overland flows and runoff from rainfall.
- Use of groundwater from aquifers may impact on surface water flows, and possibly outflows to other surface ecosystems, such as wetlands. In most instances, the full extent of these impacts is not realised for 10 to 20 years. It is likely that the impact of recent groundwater development has yet to be felt.
- Groundwater use is likely to increase as current entitlements and use remain well below the extraction limits set by governments for many areas of the Basin.
- There are still pockets of historic “over-allocation” of groundwater resources. Even if extractions are limited to within the current limits, with mechanisms put in place in manage local water level and quality impacts, climate variability and change will continue to pose a risk for groundwater supply for the future.
The full report can be downloaded from:
http://www.mdbc.gov.au/__data/page/15/Darling_Report.pdf
Further Information
A proposed second report will link the hydrological information with important socio-economic data and environmental features of the Basin.
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