Paper No. 97156 of the Journal of the American Water Resources Association. Discussions are open until December 1, 1999.
ACCEPTING THE STANDARDIZED PRECIPITATION INDEX: A CALCULATION ALGORITHM1
Article first published online: 8 JUN 2007
JAWRA Journal of the American Water Resources Association
Volume 35, Issue 2, pages 311–322, April 1999
How to Cite
Guttman, N. B. (1999), ACCEPTING THE STANDARDIZED PRECIPITATION INDEX: A CALCULATION ALGORITHM. JAWRA Journal of the American Water Resources Association, 35: 311–322. doi: 10.1111/j.1752-1688.1999.tb03592.x
- Issue published online: 8 JUN 2007
- Article first published online: 8 JUN 2007
- Palmer Index;
- Standardized Precipitation Index;
- precipitation probability distributions
ABSTRACT: The Palmer Drought Severity Index (PDSI) has been calculated for about 30 years as a means of providing a single measure of meteorological drought severity. It was intended to retrospectively look at wet and dry conditions using water balance techniques. The Standardized Precipitation Index (SPI) is a probability index that was developed to give a better representation of abnormal wetness and dryness than the Palmer indices. Before the user community will accept the SPI as an alternative to the Palmer indices, a standard method must be developed for computing the index. Standardization is necessary so that all users of the index will have a common basis for both spatial and temporal comparison of index values. If different probability distributions and models are used to describe an observed series of precipitation, then different SPI values may be obtained. This article describes the effect on the SPI values computed from different probability models as well as the effects on dry event characteristics. It is concluded that the Pearson Type III distribution is the “best” universal model, and that the reliability of the SPI is sample size dependent. It is also concluded that because of data limitations, SPIs with time scales longer than 24 months may be unreliable. An internet link is provided that will allow users to access Fortran 77 source code for calculating the SPI.