1. Hydrologist, U.S. Geological Survey, 10 Bearfoot Rd., Northborough, MA 01532 (corresponding author).
2. Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of Massachusetts, Amherst, 219 Marston Hall, Box 35205, Amherst, MA 01003-5205.
3. Hydrologist, U.S. Geological Survey, retired.