Online Drought Indicators and Indices

Surface Water Supply Index (SWSI)


Index name: Surface Water Supply Index (SWSI).

Ease of use: Yellow.

Origins: Developed by Shafer and Dezman in 1982 to directly address some of the limitations identified in PDSI.

Characteristics: Takes into account the work done by Palmer with PDSI but adds additional information including water supply data (snow accumulation, snowmelt and runoff, and reservoir data), and is calculated at the basin level. SWSI identifies the approximate frequency of mild drought occurrence at 26%–50%, moderate drought occurrence at 14%–26%, and severe drought occurrence at 2%–14%. Extreme drought occurs approximately less than 2% of the time.

Input parameters: Reservoir storage, streamflow, snowpack and precipitation.

Applications: Used to identify drought conditions associated with hydrological fluctuations.

Strengths: Taking into account the full water resources of a basin provides a good indication of the overall hydrological health of a particular basin or region.

Weaknesses: As data sources change or additional data are included, the entire index has to undergo a recalculation to account for these changes in the inputs, making it difficult to construct a homogeneous time series. Since calculations may vary between basins, it is difficult to compare basins or homogeneous regions.

Resources: Calculations and an explanation of the methodology are provided in the references below.


Doesken, N.J. and D. Garen, 1991: Drought Monitoring in the Western United States using a Surface Water Supply Index. Preprints, Seventh Conference on Applied Climatology, Salt Lake City, UT. American Meteorology Society, 266–269.

Doesken, N.J., T.B. McKee and J. Kleist, 1991: Development of a Surface Water Supply Index for the Western United States. Climatology Report 91-3, Colorado Climate Center,

Shafer, B.A. and L.E. Dezman, 1982: Development of a Surface Water Supply Index (SWSI) to Assess the Severity of Drought Conditions in Snowpack Runoff Areas. Proceedings of the Western Snow Conference, Colorado State University, Fort Collins, CO, 164–175.

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