By Kernell G. Ries, III and Paul J. Friesz
WaterResources Investigations Report 004135
ABSTRACT
Methods and computer software are described in this report for determining flowduration,lowflow frequency statistics, and August median flows. These lowflow statistics can be estimated for unregulated streams in Massachusetts using different methods depending on whether the location of interest is at a streamgaging station, a lowflow partialrecord station, or an ungaged site where no data are available. Lowflow statistics for streamgaging stations can be estimated using standard U.S. Geological Survey methods described in the report. The MOVE.1 mathematical method and a graphical correlation method can be used to estimate lowflow statistics for lowflow partialrecord stations. The MOVE.1 method is recommended when the relation between measured flows at a partialrecord station and daily mean flows at a nearby, hydrologically similar streamgaging station is linear, and the graphical method is recommended when the relation is curved.Equations are presented for computing the variance and equivalent years of record for estimates of lowflow statistics for lowflow partialrecord stations when either a single or multiple index stations are used to determine the estimates.>The drainagearea ratio method or regression equations can be used to estimate lowflow statistics for ungaged sites where no data are available. The drainagearea ratio method is generally as accurate as or more accurate than regression estimates when the drainagearea ratio for an ungaged site is between 0.3 and 1.5 times the drainage area of the index datacollection site.Regression equations were developed to estimate the natural, longterm 99, 98, 95, 90, 85, 80,75,70, 60, and 50percent duration flows; the 7day, 2year and the 7day, 10year low flows;and the August median flow for ungaged sites in Massachusetts. Streamflow statistics and basin characteristics for 87 to 133 streamgaging stations and lowflow partialrecord stations were used to develop the equations. The streamgaging stations had from 2 to 81 years of record, with a mean record length of 37 years. The lowflow partialrecord stations had from 8 to 36 streamflow measurements, with a median of 14 measurements.All basin characteristics were determined from digital map data. The basin characteristics that were statistically significant in most of the final regression equations were drainage area, the area of stratifieddrift deposits per unit of stream length plus 0.1, mean basin slope, and an indicator variable that was 0 in the eastern region and 1 in the western region of Massachusetts.The equations were developed by use of weightedleastsquares regression analyses, with weights assigned proportional to the years of record and inversely proportional to the variances of the streamflow statistics for the stations. Standard errors of prediction ranged from 70.7 to 17.5 percent for the equations to predict the 7day, 10year low flow and 50percent duration flow, respectively. The equations are not applicable for use in the Southeast Coastal region of the State, or where basin characteristics for the selected ungaged site are outside the ranges of those for the stations used in the regression analyses.
A World Wide Web application was developed that provides streamflow statistics for datacollection stations from a data base and for ungaged sites by measuring the necessary basin characteristics for the site and solving the regression equations. Output provided by the Web application for ungaged sites includes a map of the drainagebasin boundary determined for the site,the measured basin characteristics, the estimated streamflow statistics, and 90percent prediction intervals for the estimates.An equation is provided for combining regression and correlation estimates to obtain improved estimates of the streamflow statistics for lowflow partialrecord stations. An equation is also provided for combining regression and drainagearea ratio estimates to obtain improved estimates of the streamflow statistics for ungaged
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Streamflow statistics web application

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