1. Assoc. Prof., Dept. of Hydr. and Envir. Engrg., Norwegian Univ. of Sci. and Technol., N-7491 Trondheim, Norway. E-mail: Nils.R.Olsen@ bygg.ntnu.no
2. Res., Dept. of Statistics and Modelling Sci., Univ. of Strathclyde, 26 Richmond St., Glasgow G1 1XH, U.K.
3. Sr. Res., Inst. of Freshwater Ecology, The Ferry House, Far Sawry, Ambleside, Cumbria LA22 0LP, U.K.