This repository contains the soil moisture index (SMI) Fortran program developed at the Dept. Computational Hydrosystems at the Helmholtz Centre for Environmental Research - UFZ. This sofware was developed initially for Samaniego et al. (2013, https://doi.org/10.1175/JHM-D-12-075.1). Features have been added for subsequent publications. These include Thober et al. (2015, https://doi.org/10.1175/JHM-D-15-0053.1), Zink et al. (2016, https://doi.org/10.1088/1748-9326/11/7/074002), and Samaniego and Thober et al. (2018, https://doi.org/10.1038/s41558-018-0138-5). This version is also used in Rakovec et al. (2022) "The 2018–2020 multi-year drought sets a new benchmark in Europe", Earth's Future, 10, e2021EF002394. In particular, it was used in the creation of the associated dataset "The 2018-20 multi-year drought sets a new benchmark in Europe" (https://zenodo.org/record/5801249). It is also used in the second version of the German Drought Monitor Boeing et al. (2022, https://doi.org/10.5194/hess-2021-402). Continuous development of the program is located in the git repository: https://git.ufz.de/chs/progs/SMI and its mirror https://github.com/mhm-ufz/SMI.