1. Department of Agricultural and Resource Economics, University of Maryland, College of Agriculture, College Park, Maryland 20742, Department of Economics, Hubbard Hall, Bowdoin College, Brunswick, Maine 04011, Quality of the Environment Division, Resources for the Future, 1616 P Street, NW, Washington, DC 20036, and North Carolina State University, Campus Box 8109, Raleigh, North Carolina 27695-8109