Inventory and risk assessment of water dependent ecosystems in the Daly basin, Northern Territory, Australia

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Supervising Scientist Report 162
National River Health Program, Environmental Flows Initiative, Technical Report 5

Begg GW, van Dam RA, Lowry JB, Finlayson CM and Walden DJ
Supervising Scientist, 2001
ISSN 1325-1554
ISBN 0 642 24368 9

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About the report

Executive summary

The Daly basin (19 382 km2 in size) lies in the northernmost part of the Northern Territory, Australia, and occupies approximately 36% of the 53 000 km2 catchment of the Daly River. The Daly has the largest flow of all rivers in the Northern Territory. Due to the vast underground aquifers supplying the river, reliable flows of good quality water and areas of high potential soils, the Daly basin serves as an example of a region where water resource and agricultural development is being given serious consideration. Given this situation, and in line with Australian national water reform processes, the potential environmental effects of any such development are also being considered.

As part of a larger assessment of environmental flow requirements for the Daly basin being conducted by the Department of Lands, Planning and Environment (DLPE), eriss was commissioned in March 2000 to undertake an inventory and risk assessment of water dependent ecosystems in the Daly basin. The specific aims were to:

  • map and store in a Geographical Information System (GIS) the area, location and extent of a range of water dependent ecosystems in the Daly basin;
  • establish threats to these ecosystems from forecast and existing water use and land management practices and overlay this information on the GIS-based map;
  • identify which ecosystems are most at risk and where possible provide an assessment of the extent of this risk; and
  • provide a mapping-base describing habitats critical for other key indicator species being investigated in the Daly basin.

The inventory and risk assessment of water dependent ecosystems (hereafter referred to as wetlands) in the Daly basin was conducted in two stages. First, an inventory of the wetlands was undertaken. This included the development of a GIS platform and the collection of data from field surveys and remotely sensed imagery, and available reports and maps. This information was then used in the risk assessment which was undertaken using a framework formally agreed by the Ramsar Convention on Wetlands.

For the purpose of mapping the distribution of wetlands in the Daly basin, heavy reliance was placed on the availability and accuracy of existing information. This included 1:50 000 topographic maps compiled by the Australian Defence Force and the land unit maps of the Daly basin compiled by the DLPE. The two datasets were found to complement each other and, when used in combination, to provide a fairly reliable source of information about the location of wetlands in the area. The wetland features mapped were then reclassified using landform and hydroperiod as a basis and, to assist in the classification process, a ground-truthing exercise and a basin-wide low-level aerial survey were conducted. The assessments made during the course of these surveys also provided information about the nature of the plant communities associated with each wetland type and the forms of land use in their immediate surrounds.

The report contains an overview of the biophysical features of the Daly basin. From a lithologic point of view the area is characterised by three types of karstic rocks known as Tindall limestone, Oolloo limestone and the Jinduckin Formation. All three rock types contain extensive unconfined aquifers that serve as the primary groundwater resources in the area and, as a result, they are responsible for providing the base flow for the majority of the rivers and creeks in the Daly basin. Most wetlands are located in groundwater discharge areas in which the fluctuation of water level and soil moisture are important controlling factors. There is also a proliferation of sinkholes and streamsinks in areas underlain by Tindall limestone and outflows from the aquifers concerned commonly occur in the form of springs that make a significant contribution to the perennial flow of the rivers in the area. The distribution of wetlands and the overall extent of the major wetland types (ie floodplains and damplands) in each of the eleven subcatchments that comprise the Daly basin are described and full advantage taken of the 30-40 year record of historic discharge data on the seasonal distribution and variability of flows in the Daly River and its major tributaries.

To allow the demonstration of wetland risk assessment as a future planning tool, a Land Use Concept Plan was developed in consultation with departmental officers. From this, an overview of the existing and forecast land use activities that may pose a threat to wetlands in the Daly basin is described. Particular emphasis is given to possible agricultural developments on three pastoral properties (Jindare, Claravale and Douglas Stations) and a neighbouring area of crown land (Stray Creek). Whilst the area concerned is said to contain approximately 10 000 km² of soils suitable for rain-fed farming, only 30-40 % of this has the potential to support arable farming and improved pastures on cleared land because of limiting factors such as rainfall, water availability, irrigation potential, soil erosion risk, water quality, availability of power and accessibility. Together with groundwater resources, the potential for the development of dam sites in and around the Daly basin has received much attention in the past. Developments of this nature were given particular attention because of the potentially significant impacts of dams on the hydrology and functions of wetlands downstream.

The analysis phase of the risk assessment first addressed the possible threats posed by the existing land and water uses and their potential effects on the wetlands of the Daly basin. The focus is on threats to the water regime of the wetlands. It then addressed the existing and forecast extent of land use in the Daly basin by utilising a hypothetical Land Use Concept Plan, and the GIS to determine and predict the wetlands that are most at risk from various pressures and the activities that pose most threat. As the Land Use Concept Plan has no official status it is stressed that the risk assessment refers to projected land and water uses and is used to project scenarios that should be considered when an official plan is developed.

The existing and projected land uses in the Daly basin that pose the most risk to the wetlands in terms of altering the water regime are agricultural activities such as crop farming, horticulture, pastoral activities utilising improved pastures, and urban development. These activities are, and will continue to be, developed around two major areas: the Katherine region and the region comprising the majority of the Douglas River, Stray Creek and Fergusson River catchments. Unless appropriately managed, the pressures associated with these activities have the potential to result in negative impacts to a range of wetlands on both a local and basin-wide scale. Dam construction, if it were to proceed, would represent a major threat to many of the downstream channel and channel-dependent wetland habitats in the Daly basin. However, of more immediate concern are potential adverse effects due to land clearance and surface and groundwater extraction for intensive agriculture and urban expansion.

The wetlands most at risk are those directly associated with the land uses that pose most risk. Under the scenario presented by the Land Use Concept Plan, 5 to 15% of the major wetlands of the Daly basin would be likely to experience altered water regimes due to land clearance and/or water extraction. In general, less than 20% of the wetlands of the Daly basin would be contained within conservation reserves and therefore very unlikely to experience altered water regimes. Although a much larger proportion of the river channels would exist within conservation areas, they represent the wetland type that is most vulnerable to threatening activities elsewhere in their catchments and may actually be at greater risk. Approximately 10 to 50% of the major wetlands would possibly experience altered water regimes due to land clearance and water extraction, while the remaining wetland habitats would be unlikely to be affected. Floodplain and river channel habitats within the catchments for which large dams have been proposed would be likely to experience altered water regimes due to water impoundment, while those downstream of these catchments would possibly be affected. Wetlands independent of channel flows would be very unlikely to experience altered water regimes due to water impoundment.

A number of information gaps and uncertainties prevented a more comprehensive assessment of the land and water uses posing the greatest risk and the wetlands at greatest risk in the Daly basin. The most important area in which the understanding of wetlands in the Daly basin needs to be improved is the need for reliable hydraulic information on hydroperiod, frequency and depth of inundation for floodplain wetlands and detailed analyses of the water regime requirements of wetland fauna and flora. There is a parallel need to better understand land management systems which are relevant to the Daly basin, with emphasis on water balance, so that the impacts of development proposals can be more accurately predicted.

In a study that concerns itself with environmental flows and the future allocation of water in the Daly basin, it is recommended that wetlands outside the Daly basin become incorporated into the overall investigation and that the category 'environment' is declared under the NT Water Act (2000) as the beneficial use of all wetlands in the basin. It is concluded that land management of the highest order will be required if the basis for land and water utilisation is to be rationally determined and that a concerted effort will need to be made by land planners to tailor wetland utilisation to suit the functions and values of the wetland concerned.

Finally, with the future need for monitoring in mind, it is recommended that the collection of baseline data focusing on habitat diversity for aquatic organisms, and marginal and riparian vegetation is commenced as soon as possible along a range of representative reaches of the Douglas, Katherine and Daly Rivers.

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