Source Code: Drought Indices in Fortran (SPI, PDSI)
NOAA's National Centers for Environmental Information (NCEI)
NOAA's National Centers for Environmental Information (NCEI) developed Fortran code to calculate Standardized Precipitation Index (SPI) and the Palmer Drought Severity Index (PDSI). This code is not an exact copy of what is run on NCEI's internal servers, but represents general use version approved for public release.
This code is furnished "as is." The authors, the United States Government, its instrumentalities, officers, employees, and agents make no warranty, express or implied, as to the usefulness of the software and documentation for any purpose. They assume no responsibility (1) for the use of the software and documentation; or (2) to provide technical support to users.
There is not formal documentation on this code, although there are internal 'in-line' comments inside the files.