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SWSI Calculation Procedure

Following is a description of the column definitions in the results table and calculation procedure. The April results table is used January-April and the May results table is used in May. Some southern Idaho basins use indexes that start in February or March and use the pre-runoff reservoir storage (carryover) and spring and summer streamflows. To calculate the SWSI each month, NRCS uses the respective results table with the first month streamflow forecast and reservoir storage.

1st column - Rank - Number of years used in the analysis.

2nd column - Water Year - Last two digits of each water year.

3rd column - Streamflow Period analyzed.

4th column - End of Month Reservoir Storage, prior to runoff.

5th column - Sum of Streamflow + Reservoir columns - columns 3 and 4. Table is sorted in descending order by this column.

6th column - Non-exceedance Probability - calculated by the formula (1-(Rank/n+1)) where the rank is column 1 and n equals total number of years analyzed.

7th column - SWSI Value - calculated by the formula: (((non-exceedance probability X 100)-50)/12) or (((column 6 X 100)-50)/12). Fifty is subtracted from the non-exceedance probability to center the value around zero. Dividing by 12 scales the values from +4.1 to -4.1

Steps to Calculate Monthly SWSI Values:

1. At the beginning of January, February and March, NRCS receives or projects the pre-runoff reservoir storage.

2. Add the projected reservoir storage, March 31 in most basins, to the April-September streamflow forecast. In April, add the actual storage on March 31 to the April-September streamflow forecast published in April. In May, add the actual storage on April 30 to the May-September streamflow forecast published in May.

3. Use the Streamflow + Reservoir Column to determine where the sum of the current streamflow forecast and reservoir storage fits into the descending order of column 6. The SWSI value can be determined by following this line across to column 7 (SWSI) and splitting the difference, or use the formulas above to determine the non-exceedance probability and SWSI value.

The SWSI values can also be plotted as a time series graph. This allows the user to graphically visualize the values from year to year and see how the current year's values change from year to year and to also compare the current year with historic years of similar water supply conditions. It is our hope to make these graphs available in the future. The actual SWSI value can be calculated at the end of each water year by using archived reservoir and streamflow data along with the adjustments as stated in the Streamflow Adjustment and Reservoir Definitions.