1. Department of Environmental Sciences, University of California, Riverside, California 92521, Soil Physics and Pesticides Research Unit, George E. Brown Jr. Salinity Laboratory, Agricultural Research Service, U.S. Department of Agriculture, 450 West Big Springs Road, Riverside, California 92507, and North Central Soil Conservation Research Laboratory, Agricultural Research Service, U.S. Department of Agriculture, 803 Iowa Avenue, Morris, Minnesota 56267