NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competencies and with regard for appropriate balance.

This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.

The project was supported by the Department of Energy under grant number DE-FG01-90FE62072, the Department of the Interior under contract number 14-01-0001-89-C-39, the State of Arizona Salt River Project Agricultural Improvement and Power District under grant number VN08016CAS, the Environmental Protection Agency, Chevron, and the United States Department of Agriculture under grant number 91-G-018.

B-065

Front cover: Computer-simulated picture of natural and current average conditions for Shenandoah National Park, Virginia. Relative humidity is the same for the simulations (Trijonis, J., W. Malm, M. Pitchford, and W.H. White, 1990).