1. Dept. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616-8627; formerly, Center for Research in Water Resources, Univ. of Texas, Austin, TX 78712.
2. Center for Research in Water Resources, Univ. of Texas, Austin, TX 78712 (corresponding author).
3. School of Civil and Environmental Engineering, Cornell Univ., Ithaca, NY 14853.