The SWSI, pronounced swazee, was developed by Shafer and Dezman (1982) to address limitations of the Palmer indices and incorporate water supply data, such as snow accumulation and melt, which are important in the western United States. The index is based on four components: snowpack, streamflow, precipitation, and reservoir storage. Monthly data for each component are analyzed according to probabilities of occurrence, combined into an overall index, and weighted according to their relative contributions to surface water in the basin. A modified SWSI (Garen, 1993) provides stronger statistical foundations to the index, with drought categories and cumulative frequencies as follows:
Drought Category Cumulative Frequency (approx.)
-2.00 to 0.00 -3.00 to -2.00 -4.00 to -3.00 Below -4.00
Mild drought Moderate drought Severe drought Extreme drought
An advantage of the SWSI is that it represents water supply conditions unique to each hydrological area, such as regions heavily influenced by snowpack. Limitations are that changing data sources or water supply sources require that the entire index be recalculated to account for changes in the frequency distributions and the weights of each component. For instance, discontinuing any station means that new stations need to be added to the system and new frequency distributions need to be determined for that component. Thus, a homogeneous time series of the index is difficult to maintain. If extreme events are beyond the historical time series, the index will also need to be recalculated. Further, because the index is unique to each basin, comparisons among basins or regions are limited (Doesken et al., 1991).
The following sections provide guidance in the development, implementation, and evaluation of these common indicators. Although the purpose of this chapter is not to review all possible indicators and triggers, these key examples will nonetheless illustrate important and more general concepts. For additional details on specific indicators and definitions, see, for example, Dracup et al. (1980), Fisher and Palmer (1997), WMO (1992), and Heim (2002).
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