PUBLICATIONS--Guidelines for Publication of Streamflow Statistics in the Annual State Data Reports 

In Reply Refer To:                                   March 4, 1992
WGS-Mail Stop 415


SUBJECT:  PUBLICATIONS--Guidelines for Publication of Streamflow 
                        Statistics in the Annual State Data 

Changes in the format of the Water Resources Division (WRD) 
annual State data reports were described in Office of Surface 
Water Technical Memorandum No. 91.12.  Guidelines for revising the 
format of these reports are contained in WRD Memorandum No. 92.04.  
The purpose of this memorandum is to revise, reemphasize, and 
supplement existing guidelines, particularly those provided in WRD 
Memorandum No. 92.04, relative to the publication of the summary 
streamflow statistics.

The U.S. Geological Survey (USGS) publishes daily mean discharges 
for many regulated streams--that is, streams where streamflow has 
been altered by human activities, such as the construction of 
reservoirs or diversions, modification of stream channels, or 
urbanization of watersheds.  Streamflow data are needed for 
regulated as well as unregulated streams to make various water-
management decisions.  

Generally, reservoirs and diversions account for most of the major 
changes in a watershed that significantly affect streamflow 
statistics.  However, stream channel modification such as 
straightening and paving, or watershed modifications such as an 
increase in impervious area in a watershed as a result of 
urbanization, major loss of vegetation caused by major 
forest fires, land-surface disruption caused by surface mining, or 
a change in agricultural practices or irrigation diversions can 
also significantly affect streamflow statistics.  Publication of 
streamflow statistics for a watershed without documenting the time 
periods for when streamflow conditions have changed abruptly can 
be misleading and inappropriate.  Changes in watershed conditions 
that alter streamflow should be documented briefly in the Remarks 
paragraph of the annual State data report.

The following guidelines are intended to improve the consistency 
of streamflow statistics published in the annual State data 
reports across the Nation.  

     1.  Guidelines described in WRD Memorandum No. 92.04 state 
that at least 5 years of data should be used to define the annual 
streamflow published in the annual State data reports.  This is 
consistent with the traditional guidelines used for publishing the 
mean annual streamflow which has been considered an estimate of 
the long-term mean.  With the concurrence of the Assistant Chief 
Hydrologist, Scientific Information Management, this criterion for 
computation and publication of mean annual streamflow in the 
summary of streamflow statistics in annual State data reports is 
herein rescinded.  The reason for this change in the guidelines is 
that the mean annual streamflow for a designated time period and 
that for the current-year are provided primarily for comparison 
statistics purposes.  Therefore, upon receipt of this memorandum, 
no restriction on the length of record used for computing the mean 
annual streamflow or any other statistic for any designated time 
period is to be applied.  

The summary of streamflow statistics published in the annual data 
reports is provided mainly for comparison of streamflow statistics 
between two periods.  The values are not intended to be estimates 
of long-term streamflow statistics or of streamflow population 
values because no inference of frequency or probability of 
exceedance or nonexceedance is specified.  Statements that the 
summary of streamflow statistics represents any specific watershed 
condition should be avoided.  

    2.  WRD Memorandum No. 92.04 states that streamflow statistics 
for periods of unregulated or regulated periods flow conditions or 
both can be published in the annual State data reports.  This 
option is reemphasized in this memorandum.  The selection of the 
appropriate period(s) of regulation or nonregulation should be 
determined by District personnel who are most knowledgeable about 
watershed conditions.  As noted above, the time period of 
regulated streamflow should be identified in the Remarks paragraph 
of the data report.  In the context of this memorandum, regulated 
streamflow is used generically to mean regulation of streamflow by 
reservoirs, diversions, or stream channel and (or) watershed 

        3.      Different streamflow statistics are affected to 
varying degrees by regulation.  For example, reservoir storage, 
without accompanying diversions, might not significantly affect 
annual total streamflow or annual and monthly mean streamflow but 
might significantly affect peak flow, low-flow, and daily-flow 
duration statistics.  Under these conditions, it might be 
advisable to publish only the annual and monthly statistics.  In 
summary, District personnel are responsible for deciding what 
streamflow statistics should be published.  

        4.      WRD Memorandum No. 92.04 also states that the 
publication of peak flows above base is optional but that the 
entry of these data into the Peak Flow File is mandatory.  This 
memorandum reemphasizes the requirement to enter peak flow above 
base into the Peak Flow File.  A canvass of cooperating agencies 
in 1975 and 1981 indicated a continuing demand for peak flow above 

base.  Even if the District decides not to publish peaks above 
base information.  Therefore, even if the District decides not to 
publish these data, quality assurance procedures should be 
implemented to ensure that accurate data are entered into the Peak 
Flow File in a timely manner.  
In summary, the above guidelines are intended to reemphasize, 
revise, and supplement existing guidelines, such as those in WRD 
Memorandum No. 92.04.  The basic objectives are to publish 
meaningful statistics representative for either regulated or 
unregulated streamflow conditions (or both), to avoid publishing 
statistics that could be misleading or incomplete, and to avoid 
making any interpretive statements regarding streamflow statistics 
that represent any specific watershed condition.  The publication 
of these summary statistics should provide the public useful data 
for comparing current-year streamflow statistics against those for 
a previous designated period.

                                  Charles W. Boning
                                  Chief, Office of Surface Water