The Number of Transects Required to Compute a Robust PHABSIM Habitat Index

The value of the number of transects within a segment of an instream flow study that will generate a stable relationship between the PHABSIM habitat index and discharge is examined. Over 600 instream flow studies were reviewed to determine the common practice standard, showing a median number of eight transects per four mile reach. Since the number of study transects varies by length of reach and purpose of study, transects per mile were computed as a more standardized value, with a median result of 2.32 transects per mile for 552 reaches. Studies conducted for water rights claims (255) had a median of 1.30 transects per mile and those conducted for project evaluations (259) had a median of 4.17 transects per mile. Several high transect number data sets were also systematically subsampled to determine how many transects are required to produce a robust habitat index function. This analysis showed that approximately 18-20 transects will in most cases yield a result nearly identical to one obtained using 40-70 transects per segment. When used for time series analysis and comparison of flow alternatives, remaining small differences in the functions would be inconsequential.If used for standard setting purposes, however, the differences could affect flow recommendations and more transects are likely to be necessary.