The Surface Water Supply Index was developed in Colorado in the early 1980s to complement the Palmer Index, which wasn't designed for mountainous terrain. The SWSI includes snowpack, reservoir storage, streamflow, precipitation and other measurements. It is calculated separately for each river basin. The Natural Resources Conservation Service has a good collection of SWSI monitoring sites.

Each state computes the Surface Water Supply Index for its own river basins, and each state displays the results differently.