Precipitation Index


Devised by McKee, N. J. Doesken and John Kleist, 1993.

Numerical Value of Index Description
2.0 and higher extremely wet
1.5 to 1.99 very wet
1.0 to 1.49 moderately wet
− 0.99 to 0.99 near normal
− 1.0 to −1.49 moderately dry
−1.5 to −1.99 severely dry
−2 and less extremely dry

1. N. J. Doesken, John D. Kleist and Thomas B. McKee.
Use of the Palmer Index and Other Water Supply Indexes for Drought Monitoring in Colorado.
Climatology Report No. 83-3.
Fort Collins, CO: Colorado  Climate Center, Colorado State University, March 1983.

Page ix: "The Palmer Index, developed in the 1960's, has become a credible tool for monitoring drought and assessing drought severity on the national scale. It reasonably depicts soil moisture conditions using a simple hydrologic balance accounting for precipitation, evapotranspiration, run off and soil moisture recharge. However, experiences of the WATF [Colorado Water Availability Task Force] have revealed that the Palmer Index values, currently generated weekly by the National Climatic Data Center for 5 climatic divisions in Colorado, were only marginally useful for drought monitoring. The regions were too large and climatically diverse, and input temperatures and precipitation data were not adequately controlled to produce consistent and meaningful results.

"With the encouragement and cooperation of the WATF this project was undertaken to adapt the Palmer Index model to Colorado. The original program was brought to Colorado, that state was broken down into 25 climatically similar regions, and a simple routine for adjusting input data to correct for missing data and station moves was implemented. The existing model was then used to generate 30 years of monthly Palmer Index values for all 26 regions of the state.

"A thorough examination of these new Palmer Indexes has been performed. Comparisons with the original indexes show noticeable differences and considerable small scale detail which previously could not be resolved."

Available on the web at



NOAA provides maps showing the SPI at different time scales:

N. B. Guttman.
Comparing the Palmer Drought Index and the Standardized Precipitation Index.
Journal of the American Water Resources Association, volume 34, number 1, pages 113-121 (1998).

N. B. Guttman.
Accepting the Standardized Precipitation Index: A calculation algorithm.
Journal of the American Water Resources Association, volume 35, number 2, pages 311-322 (1999).

T. B. McKee, N. J. Doesken and J. Kleist. 
The relationship of drought frequency and duration to time scales.
Eighth Conference on Applied Climatology (Jan 17-23, 1993, Anaheim CA, ).
Boston, MA: American Meteorological Society, 1993.
Pages 179-186.

T. B. McKee, N. J. Doesken and J. Kleist. 
Drought monitoring with multiple time scales. 
Ninth Conference on Applied Climatology (Jan 15-20, 1995, Dallas TX).
Boston, MA: American Meteorological Society, 1995.
Pages 233-236


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