United States Department of Agriculture
Natural Resources Conservation Service
National Water and Climate Center Go to Accessibility Information
Skip to Page Content
	SURFACE WATER SUPPLY INDEX         

IDAHO SURFACE WATER SUPPLY INDEX (SWSI) as of March 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     0.0      2007         NA
Spokane               -1.8      2007         NA
Clearwater            -1.3      2004         NA
Salmon                -0.5      2003         NA
Weiser                -1.0      2005         NA
Payette               -0.5      2010         NA
Boise                 -1.3      2002        -1.6
Big Wood               0.0      2010        -0.1
Little Wood            0.8      2012        -1.9
Big Lost               0.5      2009         0.4
Little Lost           -0.3      2012         1.1
Teton                 -2.0      2002        -3.9
Henrys Fork           -0.8      2005        -3.2
Snake (Heise)         -1.8      2007        -1.6
Oakley                -0.3      2009        -0.4
Salmon Falls          -1.3      2000        -1.1
Bruneau               -0.3      2008         NA
Owyhee                -0.5      2012        -3.4
Bear River             0.5      2001        -3.3

 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.


Return to the Table of Contents

E-mail Questions or Comments

or Write:
Snow Survey Office
Natural Resources Conservation Service
9173 West Barnes Drive, Suite C
Boise, ID 83709-1574
(208) 378-5740