The National Integrated Drought Information System (NIDIS) highlights freely available code used to create the drought products commonly used within the drought community. Please refer to the specific licensing information for each product to understand the use and redistribution policy for each codebase.
Climate and Drought Indices Code in Python
Most up-to-date version of a drought-related climate indices software implemented in Python, including SPI, SPEI and PET calculations. Maintained by James Adams and has a BSD-3 license.
Drought Indices Code in Fortran
Legacy Fortran code for Palmer (PDSI) and SPI indices from NOAA's National Centers for Environmental Information (NCEI)