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Wetlands

 

Identifying and Monitoring Change in Wetland Inundation and Vegetation Patterns, Alligator Rivers Region, Northern Territory

Final Report

A.K. Milne, D. Williamson, G. Horn, M. Finlayson and B. Bayliss
Land and Water Resources, Research and Development Corporation
Report for Environment Australia, 2000


1. Introduction

1.1 Background

The wetland areas of the Alligator Rivers Region (ARR) in the Northern Territory are of national and international significance. They form part of Kakadu National Park, a listed world heritage area which is recognised as the most important natural, cultural, recreational and tourist resource in the area. In addition, these wetlands are recognised under the Ramsar Convention for Internationally Important Wetlands, which defines wetlands as;

areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, excluding areas of marine water, the depth of which at low tide does not exceed six meters.

Most of the rivers in the northern Australian region drain from broad uplands and low plateaus towards the coast. Stream flow is seasonal but can be prolonged well into the dry season in many river systems, particularly in the near tidal reaches, by the reverse flow of water from lakes and waterholes with water stored from the wet season. Collectively systems which retain and then release water or which are permanently flooded after the wet season are referred to as freshwater wetlands as opposed to the salt water and tidal wetlands of the coastal littoral zone. The work in this project covers both freshwater and saline wetlands.

Figure 1 Monsoonal freshwater wetlands are found in the generalised region to the north of the line shown on this map of Northern Australia

Figure 1 Monsoonal freshwater wetlands are found in the generalised region to the north of the line shown on this map of Northern Australia (Finlayson et al., 1991)

Ecosystems developed in these freshwater floodplain environments of the ARR are complex, dynamic and resilient, often undergoing annual changes in water depth ranging from being completely dry to being covered 3-4 months of the year by 2-3m of water. Vegetation communities present include forests, woodlands, scrub and heath, sedge and grasslands and extensive areas of macrophytes and submerged heathlands, all of which relate and respond the hydrologic regimes of permanent or seasonal flooding. The length of time surface water is available in a landscape affects which species grow, determines the length of vegetative growth and is influential in deciding the migratory pathways of birds and animals in the environment.

In the near coastal areas of some river systems significant vegetation changes are taking place as a result of the degradation of fresh water uplands. These changes include the extension of mangroves inland along tidal creeks, the death of large areas of freshwater paperbark or melaleuca forests, and the invasion of freshwater wetlands by aquatic weeds especially salvina molesta and mimosa pigra.

There is no detailed inventory of wetlands in northern Australia although considerable effort is currently devoted to the collation and consolidation of records and information (Storrs and Finlayson, 1997). Datasets are held by the Parks and Wildlife Commission of the Northern Territory (PWCNT), the Power and Water Authority (PAWA) and the Department of Primary Industries and Forestry (DPIF).

1.2 Project objectives

Currently there are no maps which show the distribution of wetlands and no data on the drying-out phase which would permit seasonally inundated and permanent wetlands to be discriminated. The objectives of this project are to use remote sensing methodologies to characterise the components of the wetland landscapes and to assess changes in the landscape that are related to the hydrological regime which is dominated by rainfall and tides.

Four aims were stated at the commencement of this project:

  1. Use synthetic aperture data (SAR) to monitor seasonal changes in the temporal and spatial pattern of inundation in order to delineate freshwater from tidal wetland areas in the ARR. (Section 3.1)
  2. Monitor and assess changes in wetland environments over time, including vegetation response and adaptation; wetland loss and degradation and the success of wetland conservation practices. (Section 3.2)
  3. Establish a GIS relational database at ERISS to incorporate the results of ongoing hydrological, geomorphic, ecological and biochemical studies related to the management of wetlands including the development of a predictive modelling capability to assess the impact of future change. (Section 4)
  4. Develop routine procedures for mapping the distribution and ongoing monitoring of wetlands across Northern Australia. (Section 5)

Airborne and satellite remotely sensed data have been used to map the pattern and changing processes of the wetlands landscape. SAR data have been the main source of data, although some optical data and aerial photos have also been used to support analysis and aid interpretation.

1.3 Current status of project

This project commenced in late 1997. Progress reports were submitted on 30 September 1997 and 26 October 1998. This work depended on the acquisition of Radarsat data and its timely delivery from Canada through the ADRO Program. The satellite data required for monitoring seasonal changes in water level were not received until late 1998 and consequently the processing and analysis of annual seasonal change has been delayed. Considerable work has been completed on each objective. Details of further analysis are provided in the following sections. This will be completed in the next 12 months.

1.4 The Alligator Rivers Region (ARR)

The region comprises the catchments of the East Alligator, South Alligator and West Alligator rivers that contain approximately 195,000 ha of freshwater floodplains (Figure 2). Over 225 plant species have been found on the floodplains. Their presence and distribution vary annually with water depth and the period of inundation being the determining factors (Finlayson, 1993).

Figure 2 Rivers and geomorphology of the Alligator Rivers region

Figure 2 Rivers and geomorphology of the Alligator Rivers region

The conservation importance of wetlands found in the ARR has been recognised with the listing of Kakadu National Park as a World Heritage Area. This recognition is based on the natural heritage value of its freshwater wetlands and floristic diversity.

The ARR has ongoing research programs in aquatic biology, plant biology and physiology, environmental chemistry and geomorphology. Much of this work is being undertaken by the Environmental Research Institute of the Supervising Scientist (ERISS ), located in Jabiru.

The initial analyses have been undertaken around the South Alligator River and the mouth of the West Alligator River. Some of these are small test sites or areas of particular significance for wetland processes. The mapping has been extended over wetlands within ARR using AIRSAR, RADARSAT and JERS-1 SAR.

1.5 Remote sensing

Wetlands' monitoring in Australia has been hampered by technical and logistical problems in obtaining, interpreting and reporting data. Remote sensing is a tool that can overcome some of the problems related to lack of access, cloud cover and large spatial areas. Phinn et al (1998) provide a comprehensive review of remote sensing instruments suited to wetlands mapping.

The research undertaken in this project has concentrated on SAR imagery that can record land surface condition through cloud and is sensitive to structural elements of vegetation canopies such as leaves, branches and trunks. SAR images are particularly well suited to wetland inventory and monitoring because of their ability to detect flooding beneath vegetation canopies. Both airborne and satellite SAR have been used in this investigation.

Satellite SAR sensors are currently limited to single frequency, single polarisation systems, either C band (5.6cm), or L band (23.5cm) and transmit and receive in the same polarisation, either horizontally (HH) or vertically (VV). Airborne SAR also operate at X band (3 cm) and P band (65 cm) and can transmit and receive in alternate polarisations (HH, HV, VV, VH). Several satellite SARs are due for launch in the next two years which will record multi-polarised data (ALOS 2002, ENVISAT 2001).

Hess (1997) mapped the wetland communities of the Magela Creek floodplain in the Northern Territory using multi-frequency HH and HV polarised SIR-C data acquired in April and October 1994 using both wet season and dry season data. She classified this wetland environment into five broad classes. This research demonstrated the potential of multi- frequency and multi polarised SAR and provided information on the backscatter characteristics that were used in this current work which aims to map floodwater recession patterns; analyse the wetland landscape processes and extend land cover mapping over a larger area.

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