Revised Surface-Water Supply Index for Western United States

The surface-water-supply index (SWSI) was introduced in Colorado in the early 1980s as a better indicator of water availability in the western United States than is the Palmer drought index. Similar indexes have been subsequently developed in Oregon and Montana. These indexes have found great usefulness in drought monitoring and in triggering specific drought-related activities by state governments. Two conceptual weaknesses exist in the current SWSIs: (1) Subjective assignment of values to coefficients; and (2) obscured statistical properties of the index. Revisions to overcome these weaknesses include a specific definition of surface-water-supply, use of streamflow volume forecasts, and appropriate handling of data to achieve the desired statistical properties of the index. It is also suggested that indexes for individual hydrologic components be developed to provide supporting information to the SWSI. An example of the development of the revised SWSI is given for the Flathead River basin in Montana.