SPI standard precipitation index
- T. B. McKee, N. J. Doesken and J. Kleist: THE RELATIONSHIP OF DROUGHT FREQUENCY AND DURATION TO TIME SCALES. Eighth Conference on Applied Climatology, 17-22 January 1993, Anaheim, California
- T. B. McKee, N. J. Doesken and J. Kleist, “Drought Monitoring with Multiple Time Scales,” Proceedings of the 9th Conference on Applied Climatology, 15-20 January 1995, Dallas, TX, American Meteorological Society, pp. 233-236.