1. Corresponding author. Environmental Change Institute, Oxford University, UK. Present address: Ninham Shand Consulting Engineers, 81 Church Street, Cape Town 8001, South Africa. Tel: +27 21 481-2459. Fax: +27 21 424-5588. E-mail: James.cullis@shands.co.sa
2. Centre for Ecology and Hydrology (CEH), Maclean Building, Crowmarsh Gifford, Wallingford OX10 8BB, UK. E-mail: dpo@ceh.ac.uk