Catchment Polygons, National Geochemical Survey of Australia, 2011

Maintenance and Update Frequency: asNeeded | Statement: The Centre for Resource and Environmental Studies (CRES - now the Fenner School of Environment and Society) at the Australian National University extracted catchments with an average area of approx. 5000 km2 from an unpublished, interim version of a nested catchment framework (Stein, 2006). Main stem sub-catchment units less than 2500 km2 in area were combined with the upstream tributary catchment, and closed (internally draining) basins of combined area less than 10,000 km2 were iteratively aggregated with a lower neighbouring catchment via the lowest point on the drainage divide. The catchments were derived using a multi-flow extension of an early version of the GEODATA 9 second Flow Direction Grid (ANZLIC unique identifier: ANZCW0703012015) associated with the GEODATA 9 second Digital Elevation Model (DEM) Version 3 (Hutchinson et al. 2008; ANZLIC unique identifier: AW0703011541). Geoscience Australia then used ESRI ArcGIS® software (v.9.1) (ESRI, 2007) and ArcHydro® Tools (v.1.2 beta) (Maidment and Djokic, 2000) to generate a higher resolution catchment coverage of Australia (threshold area 500 km2) from the 9 second DEM. This high-resolution coverage was used to guide the disaggregation of over-sized polygons in the CRES coarse-scale catchment coverage and bring them within an optimal area range of 4000-6000 km2. Catchment polygons with areas less than this range were, where appropriate, aggregated with neighbouring polygons to again achieve an optimal area. Decisions on which polygons to aggregate or disaggregate were guided by information obtained from these datasets: - NATMAP Raster Premium 2005 (includes 1:250,000 scale topographic and cultural information for the whole of Australia), along with a Landsat 7 national mosaic (Geoscience Australia, 2007); - Shuttle Radar Topography Mission (SRTM) 3 second (~90 m) v.2 digital elevation model (NASA, 2007). Applying the method outlined above, an iterative series of edit, clean and build processes in ArcINFO® resulted in the final national catchment coverage. ArcHydro® Tools (v.1.2 beta) was then used to generate an outlet point for each catchment. These points were used as target sites for field sampling of catchment outlet sediments. Point location adjustments were made manually, if necessary, following consultation with State/Territory Geological Surveys and/or assessment against other environmental datasets. Adjustments were made to account