PROGRAMS AND PLANSLow-Flow Frequency Estimation at Partial-Record Sites In Reply Refer To:

In Reply Refer To:
WGS-Mail Stop 415                                    December 16, 1985







Subject:  PROGRAMS AND PLANS—Low-Flow Frequency Estimation at Partial-

                             Record Sites


Instructions and guidelines on how to estimate low-flow statistics at partial-record sites are provided in several documents.  In all cases, the recommended approach is to make base-flow measurements at the partial-record site and, in some way, relate them to concurrent daily flows at a nearby index station.  The D-day, T-year low flow at the partial-record site is estimated using this relationship and low-flow statistics at the index station.  Techniques of Water-Resources Investigations (TWRI) Book 4, Chapter B1, recommends using graphical correlation techniques for establishing this relationship and transferring D-day, T-year low flows from the index station to the partial-record site.  Surface Water Branch (SWB) Technical Memorandum No. 79.06 expands upon guidelines in TWRI, Book 4, Chapter B1, but still recommends the same graphical technique for estimating low-flow statistics at the partial-record site.  Furthermore, SWB Technical Memorandum No. 79.06 provides an evaluation of the accuracy of low-flow statistics estimated in this way.  Copies of Open-File Report 85-95 “Low-Flow Frequency Estimation Using Base-Flow Measurements” by J. R. Stedinger and W. O. Thomas, Jr., were distributed with SWB Technical Memorandum 85.09.  This report recommended estimating low-flow statistics at partial-record sites by (1) estimating the mean and variance of the annual D-day low flows at the partial-record site and (2) using those statistics to estimate the D-day, T-year low flow.  Finally, Office of Surface Water (OSW) Technical Memorandum No. 85.17 recommended the continued use of the graphical correlation techniques described in TWRI Book 4, Chapter B1.


The purpose of this memorandum is to provide further guidance on estimating low-flow statistics at partial-record sites.  As the use of computers became more prevalent, many analysts began to use linear regression analysis to transfer low-flow statistics to partial-record sites in lieu of using the graphical correlation technique.  In Open-File Report 85-95, Stedinger and Thomas point out that the linear regression approach of transferring low-flow statistics is biased.  Attached is a brief summary and clarification of the important technical points and conclusions presented in that report.  Based on the attached summary and Open-File Report 85-95, the following recommendations are made:


    1.  The linear regression approach should not be used to transfer

        low-flow statistics from an index station to a partial-record



    2.  If a mathematical technique is utilized, then either the

        moment approach suggested by Stedinger and Thomas or the

        maintenance of variance extension technique (MOVE.1) should

        be utilized.  Approximately 10 base-flow measurements should

        be available to use either of these mathematical techniques.


    3.  The graphical correlation technique described in TWRI Book 4,

        Chapter B1, can be utilized but the analyst should give more

        weight to the lower data points in the base-flow measurement/

        daily mean flow relationship.  This graphical correlation

        technique may be most appropriate when there are fewer than 10

        base-flow measurements available or when there is a nonlinear

        relationship between base-flow measurements at the partial-

        record site and concurrent daily mean flows at the index



Estimates of low-flow statistics at partial-record sites should be entered in the Streamflow/Basin Characteristics File.  These estimates should be qualified with a code of SL (synthetic low flows) as described in the WATSTORE User’s Guide Volume 4, Chapter II.


Any questions about the attached summary or Open-File Report 85-95 should be referred to Will Thomas in the Office of Surface Water.





                                 Verne R. Schneider

                                 Acting Chief, Office of Surface Water




WRD Distribution: FO, SL