1. U.S. Geological Survey, Water Resources Division, 3215 Marine Street, Boulder, Colorado 80303, U.S. Geological Survey, Water Resources Division, 6000 J Street, Placer Hall, Sacramento, California 95819-6129, National Water Research Institute, 867 Lakeshore Road, Burlington, Ontario, Canada L7R 4A6, and Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1