1. Centre for Environmental Technology and Engineering, Massey University, Private Bag 11222, Palmerston North, New Zealand a.n.shilton@massey.ac.nz
2. School of Civil Engineering, University of Leeds, Leeds LS2 9JT, UK d.d.mara@leeds.ac.uk
3. National Institute for Water and Atmospheric Research, PO Box 11115, Hamilton, New Zealand r.craggs@niwa.co.nz