Surface Water Supply Index Explanation
A Surface Water Supply Index (SWSI) is a predictive indicator of the surface water available in a basin compared to historic supply. The SWSI is calculated by summing the two major sources of irrigation water supply; reservoir carryover and spring and summer runoff. These two sources, March 31 reservoir storage and April-September streamflow, are analyzed together when determining the total surface water supply available for the season.
SWSI uses non-exceedance probabilities to normalize the magnitude of water supply variability between basins. The non-exceedance values are then rescaled to range from +4.1 (extremely wet) to -4.1 (extremely dry). A SWSI value of 0.0 indicates a median water supply as compared to historic occurrences.
NRCS calculates the SWSI by adding the estimated or actual March 31 reservoir storage to the April-September streamflow forecast. This value is then used in the results table to determine the non-exceedance probability and corresponding SWSI value.
The SWSI can also be used to determine if the current year will have a shortage or surplus of water for the irrigation season. For example, an adequate water supply for Magic Reservoir water users is around 270,000 to 290,000 acre-feet. This adequate water supply equals a SWSI value of -1.2. When the SWSI value is less than -1.2, water supply shortages may be expected. Since the SWSI is based on projected reservoir and streamflow values, the SWSI value may change from month to month as the projected reservoir and streamflow forecasts change. Three exceedance streamflow forecasts are included in the results table: the 90%, 50% (Most Probable), and 10% Chance of Exceeding Forecast. These additional forecasts are included to statistically illustrate the possible outcomes based upon the current streamflow forecasts.