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	SURFACE WATER SUPPLY INDEX         

IDAHO SURFACE WATER SUPPLY INDEX (SWSI) as of February 1, 2013
            
The Surface Water Supply Index (SWSI) is a predictive indicator 
of surface water availability within a watershed for the spring 
and summer water use season. The index is calculated by combining 
pre-runoff reservoir storage (carryover) with forecasts of spring 
and summer streamflow. SWSI values are scaled from +4.0 (abundant 
supply) to -4.0 (extremely dry), with a value of zero indicating 
a median water supply as compared to historical occurrences. The 
SWSI analysis period is from 1981 to present.

SWSI values provide a more comprehensive outlook of water 
availability by combining streamflow forecasts and reservoir 
storage where appropriate. The SWSI index allows comparison of 
water availability between basins for drought or flood severity 
analysis. Threshold SWSI values have been determined for some 
basins to indicate the potential for agricultural irrigation 
water shortages.

                                             Agricultural
                              Most Recent    Water Supply
                      SWSI    Year with      Shortage may
Basin or Region       Value   Similar SWSI   Occur when SWSI
                                Value        is Less Than 
Northern Panhandle     1.0      2008/2000    NA
Spokane               -1.5      1995/1998    NA
Clearwater             0.0      2000         NA
Salmon                -0.3      2010         NA
Weiser                -0.5      2003/2000    NA
Payette                0.5      2008         NA
Boise                 -0.3      2010        -1.6
Big Wood               0.5      2012         0.4
Little Wood            0.8      2012        -1.5
Big Lost               1.5      2011         0.5
Little Lost            0.3      2006         1.3
Teton                 -0.8      2005        -3.8
Henrys Fork           -0.3      2011        -3.0
Snake (Heise)         -1.0      1989/1990   -1.5
Oakley                -1.3      1981/1987    0.1
Salmon Falls          -1.3      2008/2000   -1.0
Bruneau               -0.3      2010         NA
Owyhee                 0.5      2005        -3.3
Bear River             0.5      2001        -3.5

 SWSI SCALE, PERCENT CHANCE OF EXCEEDANCE, AND INTERPRETATION
 -4     -3     -2     -1      0      1      2      3      4
 |------|---- --|------|------|------|------|------|------|
 99%    87%    75%    63%    50%    37%    25%    13%    1%
 ----------------------------------------------------------
 |Much  | Below    |     Near Normal     |  Above  | Much |
 |Below | Normal   |    Water Supply     |  Normal | Above|
 ----------------------------------------------------------
NA = Not Applicable   Note: The Percent Chance of Exceedance is 
an indicator of how often a range of SWSI values might be 
expected to occur. Each SWSI unit represents about 12% of the 
historical occurrences. As an example of interpreting the above 
scale, the SWSI can be expected to be greater than -3.0, 87% of 
the time and less than -3.0, 13% of the time. Half the time, the 
SWSI will be below and half the time above a value of zero. The 
interval between -1.5 and +1.5 described as "Near Normal Water 
Supply," represents three SWSI units and would be expected to 
occur about one-third (36%) of the time.


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Snow Survey Office
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Boise, ID 83709-1574
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