Is drought defined as a certain % of basal flow of designated streams in relation to the 30 year average basal flow of these streams? Is there a numerical scale associated with the terms; exceptional, extreme, severe etc.?

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Dan Israel

MIKE MOSS SAYS:      Dan,      Drought is not quite that neatly defined due to the varying interactions between rainfall, evaporation, water storage and demand, and in fact the drought categories that are shown on the U.S. Drought Monitor are somewhat subjectively assigned by a committee of scientists working in concert with local and regional officials such as State Climate Office personnel. This group does make use of a series of objective, though imperfect, measures of drought, such as the Palmer Drought Severity Index, Keetch-Byram Drought Index, Soil and Crop Moisture Indices, Rainfall and Evaporation anomalies, Streamflow and Groundwater measurements and so on, in order to assess the type (hydrologic and/or agricultural) and severity of drought around the country. Several of those indices do have numerically delineated categories with similar wording to the final drought categories, and the Drought Monitor committee combines several of those with other inputs to synthesize the final map.

For some additional background on the process, see the following addresses:

Regarding your question about streamflow in particular, as mentioned above that is considered as part of the overall process. Rather than flow as a percentage of an average, however, streamflow values are usually presented in the form of percentile values, that is a given flow on a particular day is greater than X percent of historical readings for that date, with percentiles between 25 and 75 percent considered "normal." While these aren't based on 30-year averages per se, they usually compute the percentiles only for locations where at least 30 years of data are available.


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