Keetch, John J.; Byram, George M. (1968). "A Drought Index for Forest Fire Control". Res. Pap. Se-38. Asheville, Nc: U.S. Department of Agriculture, Forest Service, Southeastern Forest Experiment Station. 35 P. 038. USDA Forest Service Southern Research Station. Retrieved August 11, 2016. (Date: 1968) Res. Paper SE-38. 32 pp. Asheville, NC: U.S. Department of Agriculture, Forest Service