Reconnaissance study of the thickness of the unsaturated zone in the western conterminous United States
Bedinger, M.S., and Langer, W.H., 1989, Reconnaissance study of the thickness of the unsaturated zone in the western conterminous United States: U.S. Geological Survey Hydrologic Investigations Atlas HA-715, 1 sheet, scale 1:2,500,000.