School of Environmental Systems Engineering, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia

Centre for Water Research, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia

now at: Department of Geography and Department of Civil & Environmental Engineering University of Illinois at Urbana-Champaign, Urbana, Illinois, IL 61801, USA