What is a Surface Water Supply Index?

The Surface Water Supply Index (SWSI) is a predictive indicator of  total surface water availability within a watershed for the  spring and summer water use seasons.  The index is calculated by combining pre-runoff reservoir storage (carryover) with forecasts of spring and summer streamflow which are based on current snowpack and other hydrologic variables.  SWSI values are scaled from +4.1 (abundant supply) to -4.1 (extremely dry) with a value of zero (0) indicating media water supply as compared to historical analysis.  SWSI's are calculated in this fashion to be consistent with other hydroclimatic indicators such as the Palmer Drought Index and the Precipitation index.  
 
Utah Snow Surveys has also chosen to display the SWSI as a PERCENT CHANCE OF NON-EXCEEDANCE.  While this is a very cumbersome name, it has the simplest application.  It can be best thought of as a simple scale of 1 to 99 with 1 being the drought of record (driest possible conditions) and 99 being the flood of record (wettest possible conditions) and a value of 50 representing average conditions. This rating scale is a percentile rating as well, for example a SWSI of  75% means that this years water supply if greater than 75% of all historical events and that only 25% of the time has it been exceeded.  Conversely a SWSI of 10% means that 90% of historical events have been greater than this one and that only 10% have had less total water supply.  This scale is far more intuitive for most people and is totally comparable between basins: a SWSI of 50% means the same relative ranking on watershed A as it does on watershed B, which may not be strictly true of the +4 to -4 scale.
 
In general terms, the scale is divided into 3 parts: LOW - AVERAGE - HIGH.  1 to 33 means below normal water supply, 34 to 66 means average conditions and 67 to 99 means above average water supply amounts.
 
Another benefit to the SWSI is in the calculation table or the actual ranking of each individual year.  The current SWSI can be directly compared to the years it most closely resembles. In other words, is the current year similar to the familiar floods of 1983 or the drought of 1977.  Managers can then refer to records from those particular years in determining strategies for dealing with the current years water supply.  Also evident is whether the streamflow component or the reservoir component is the predominant driving force at any given time.
 
SWSI's can be an excellent water management tool in determining over all risk and management strategies. It gives the water user and manager more information than simply streamflow or reservoir level alone.