PROGRAMS & PLANS--Policy on Providing Low-Flow Information

In Reply Refer To:                                         September 20, 1985
WGS-Mail Stop 415


Subject:  PROGRAMS & PLANS--Policy on Providing Low-Flow Information

Water Resources Division (WRD) offices of the U.S. Geological Survey (USGS)
receive many requests for low-flow information at both gaged and ungaged
sites.  Guidelines for determining low-flow information are given in TWRI
Book 4, Chapter B1, "Low-Flow Investigations," by H. C. Riggs, and Surface
Water Branch (SWB) Technical Memorandum 79.06 (dated July 24, 1979).  In the
Southeastern Region the Southeastern Region Policy Memorandum 74.18 contains
additional guidelines.  Other regions may have adopted similar guidelines.
None of these documents, however, address the approval procedure for providing
low-flow information.  The only reference to approval procedures is in the
"WRD Publications Guide" (page 434) which states that data reports that con-
tain low-flow statistical parameters that require assumptions about type of
frequency distribution, independence of events, and adequancy of sample are
interpretive and must receive Director's approval.  This memorandum is in-
tended to be a supplement to the "WRD Publications Guide" to cover instances
when a routine low-flow determination is needed at only one or two sites.

A low-flow workshop was held in the Southeastern Region in Atlanta,
February 26-27, 1985, and was attended by several District personnel, the
Southeastern Region Surface Water Specialist, and two members of the Office
of Surface Water including the Acting Chief of the Office of Surface Water.
The approval policy for low-flow information and other low-flow guidelines
were discussed at length.  Subsequently, the approval policy was discussed
in an Office of Surface Watr meeting and reviewed by the Surface Water
Specialist in each Region.  As a result of those meetings and reviews, the
following guidelines were developed and should be considered policy:

Methods--Estimates of low-flow statistical data for specific requests should
be made by one or more of the following methods and generally in the following
order of priority:

1.  From published reports describing low-flow characteristics at specific

2.  From actual low-flow data, either continuous record or partial record.
    Techniques for frequency analysis and correlation analysis as described
    in TWRI Book 4, Chapter B1, and SWB Memorandum 79.06 should be followed.
    If previously released information is updated based on new data or new
    methodology, this fact should be made known to the user.

3.  From published regional equations which relate low-flow characteristics
    to basin, geologic, and climatic characteristics.  Standard errors of
    estimate, if available, should be given with estimates of this type.

4.  If a low-flow profile or seepage-run is available on a channel, estimates
    at intermediate main stream sites or of tributary low flows may be made.

5.  If low-flow statistical data have been defined by measurements and records
    in a region of similar hydrologic characteristics and indicate little
    variability in unit low-flow characteristics in that region, estimates
    may be made by interpolation or by drainage-area ratio for other streams
    in that region that are known to be unaffected by regulation, diversions,
    or return flow.  An alternate procedure would be to show the range in a
    low-flow characteristic in cubic feet per second per square mile for
    nearby sites and state that the flow at the requested site may be within
    this range.

6.  If many measurements in a similar hydrologic region during a 10-year or
    lesser drought show that practically all streams below A square miles
    drainage area (or other variable such as recession index) had no flow,
    then an estimate of zero would be appropriate for an ungaged stream of
    less than A square miles drainage area.

7.  If none of the above are available for making an estimate, do not make
    an estimate.  Suggest that base-flow measurements could be made at the
    requested site to define low-flow characteristics.  Likewise, if an
    estimate is given that may be uncertain or have a high potential for
    significant error, then suggest that more base-flow measurements be

Documentation--A systematic file of all low-flow statistical information
released should be maintained.  The USGS downstream numbering system is
preferred for identification of sites; in some cases, the site may fall
between numbers and 9th and 10th digits will be needed.  This file should
be checked before responding to any request to avoid giving out two
different estimates of lwo flow for a site.

Coordination--Before releasing low-flow estimates and during the planning
stages of new interpretative reports, contacts should be made with other
Districts or agencies that may have information at the site, or for the
area of interest.

Approval--Published low-flow statistical data and estimating techniques have
already received Director's approval and obviously need no further approval
for dissemination to the public and cooperators.

New estimates made for one, or a few, individual sites may be provided to
the requestor without Director's approval provided they are fairly routine
and are made according to the above guidelines.  If unusual conditions or
anomalous estimates appear to exist, the Regional surface water specialist
should be consulted before releasing the data.  The need for further review
and approval will be decided at that time.

For requests involving more than a few sites, or for regional studies and
development of low-flow equations and other estimating techniques, Director's
approval must be obtained via the usual reports process.  It is recommended
that available low-flow information be organized, analyzed, and published to
expedite the estimating procedure and that the program to obtain base-flow
measurements and other information be accelerated to provide more complete
coverage.  Regulated streams are a special case that deserve much more

                                      Verne R. Schneider
                                      Acting Chief, Office of Surface Water

WRD Distribution:  A, FO, PO

This memorandum is a supplement to SWB Memorandum 79.06, the WRD Publications
Guide, and TWRI Book 4, Chapter B1.