Assessing the Extent of Saltwater Intrusion in A Tropical Coastal Environment Using Radar and Optical Remote Sensing

Abstract

The Alligator River Region (ARR) of the Northern Territory, Australia, has been identified as being particularly susceptible to saltwater intrusion. The Parks and Wildlife Commission of the Northern Territory (PWCNT) have attempted to restrict the intrusion and preserve the natural environment by creating physical earth barrages. A consequence of the intrusion is the conversion of freshwater paperbark (Melaleuca spp.) swamps into extensions of the mangrove margin. During this process stands of dead Melaleuca result and juvenile mangroves (primarily Avicennia marina) invade and establish in the affected areas. The use of Airborne Polarimetric Synthetic Aperture Radar (AirSAR) fused with Thematic Mapper (TM) is investigated as a means for mapping Melaleuca dieback as a measure of the extent of the saltwater intrusion. Individually the sensors cannot map areas of salt affected Melalecua as there is confusion with other cover types in the study area. The combined data set were evaluated for the most useful components of the optical and AirSAR data in resolving the overlap of land cover types and a map of Melalecua dieback was produced. A total of 0.4% of the Study Area or 2.25 square km was identified as being affected by recent saltwater intrusion with an aerial assessment yielding 100% accuracy. The individual areas range from 100 to 3100 square metres and are typically located along tidal creek lines or at the edge between floodplain and woodland. The high identification accuracy using this methodology may allow the use of Melalecua dieback as a bio-indicator of environmental change in tropical floodplain systems.