Generalized Low‐flow Frequency Relationships for Ungaged Sites in Massachusetts1

ABSTRACT: Regional hydrologic procedures such as generalized least squares regression and streamflow record augmentation have been advocated for obtaining estimates of both flood‐flow and low‐flow statistics at ungaged sites. While such procedures are extremely useful in regional flood‐flow studies, no evaluation of their merit in regional low‐flow estimation has been made using actual streamflow data. This study develops generalized regional regression equations for estimating the d‐day, T‐year low‐flow discharge, Q d, t , at ungaged sites in Massachusetts where d = 3, 7, 14, and 30 days. A two‐parameter lognormal distribution is fit to sequences of annual minimum d‐day low‐flows and the estimated parameters of the lognormal distribution are then related to two drainage basin characteristics: drainage area and relief. The resulting models are general, simple to use, and about as precise as most previous models that only provide estimates of a single statistic such as Q 7,10 . Comparisons are provided of the impact of using ordinary least squares regression, generalized least squares regression, and streamflow record augmentation procedures to fit regional low‐flow frequency models in Massachusetts.