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Abstract
Given the large number of locations within the study watershed (Washington, USA) where instream flow information is required, a GIS based stratification procedure using multivariate data analysis techniques was developed to extrapolate either instream flow recommendations or habitat availability curves (weighted usable area curves) upon which instream flow recommendations could be developed from study sites to other sites where no site‐specific information is available. One‐hundred‐seventy‐two drainages were divided into 21 hydrologically and geomorphologically similar subgroups. Mean annual flow versus drainage area and 60% exceedence flow versus drainage area relationships were also developed for the study area. Monthly instream flow recommendations were then extrapolated to the unmeasured streams within the hydrologically and geomorphologically similar subgroups by scaling them by a percentage of the mean annual flow; weighted usable area curves were extrapolated to the unmeasured streams within the same subgroups by scaling the flows by a percentage of the 60% exceedence flow. Extrapolated monthly instream flow recommendations ranged between 0 and 76 percent but were generally within about 20 percent or less when compared to recommendations developed from site‐specific analyses. Extrapolated weighted usable area relationships had an average root mean square error less than about 17% and reflected the basic functional relationships when compared to site specific analyses. This study demonstrates that either instream flow recommendations or weighted useable area relationships derived from site‐specific data can be extrapolated to unmeasured sites for use within the context of strategic watershed planning based on the development and application of two extrapolation methodologies.