COMPUTATIONS--Automatic Entry of Streamflow Statistics into the Streamflow/Basic Characteristics (SBC) File





In Reply Refer To:                                          February 6, 1986
WGS-Mail Stop 415




OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 86.03

Subject:  COMPUTATIONS--Automatic Entry of Streamflow Statistics into the
                        Streamflow/Basic Characteristics (SBC) File

The Daily Values (DV) and SBC Files are two of the data files contained in
the National Water Data Storage and Retrieval system (WATSTORE).  Procedures
are available for retrieving data from the DV File, performing certain
statistical analyses, and then storing these statistics in the SBC File.
One series of programs for doing this is known as Daily Value Statistics
(programs A969 and A193).  In this procedure, the daily flows are retrieved
from the DV File using the standard retrieval program, annual D-day high- or
low-flow values are complated by program A969 and these annual values are
passed to program A193, and the logarithms of these annual values are fit to
a Pearson Type III distribution (WATSTORE User's Guide, Volume 1, Chaper IV,
Section G).  Program A193 creates an output file of flow statistics (e.g.,
1-, 3-, 7-day, 2-, 5-, 10-year low flows, etc.) that can be used in a subse-
quent job step to update the SBC File.  Program L156 reads the output from
program A193 and formats these flow statistics for entry into the SBC File
using program E772 (WATSTORE User's Guide, Volume 4, Chapter II, Section A).
The retrieval of data, computation of flow statistics, and entry of these
statistics into the SBC File can be accomplished in one job execution.

Prior to January 10, 1986, program A193 produced an incorrect output file of
flow statistics for entry into the SBC File when there were zero values in
the sample.  This problem was identified and documented in Surface Water
Branch Technical Memorandum No. 82.11.  Program A193 has now been modified
(effective January 10, 1986) to produce a correct output file of flow
statistics for entry into the SBC File.  The program is applicable to all
sites including those that have zero values in the data series.

The determination of a log-Pearson Type III frequency curve for a sample
with zero values requires special computational procedures because it is
not possible to take the logarithm of zero.  Conditional probability is
used to adjust for the number of zero values in the sample and to define the
adjusted (unconditional) frequency curve.  The computational procedures used
in program A193 are described in Surface Water Branch Memorandum No. 70.07
and in a paper by M. E. Jennings and M. A. Benson in Water Resources Research,
volume 5, number 1, 1969, pages 276-280.

The flow statistics and computed frequency curve should be carefully reviewed
before entry into the SBC File.  The analyst should determine if the logarithms
of the data fit a Pearson Type III distribution and if the conditonal proba-
bility adjustment for zero flow values is appropriate.  Program A193 will



OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 86.03                      2

produce line printer plots of the data and the frequency curves if the analyst
requests them.  If the computed frequency curve does not adequately fit the
data, as described by the Weibull plotting positions, then the frequency
curve should be drawn graphically using the plotting position.  In this case,
the flow statistics must be entered manually into the SBC File.

The Districts are responsible for updating the contents of the SBC File
(Water Resources Division Memorandum No. 74.127, Surface Water Branch
Technical Memorandum No. 76.03).  This file should be updated on a regular
basis because it is a valuable asset in regional studies of streamflow
characteristics.

Any questions about the modifications to programs A969/A193 should be directed
to George Dempster in the Branch of Computer Technology, Scientific Information
Management.  Questions concerning the computation of a frequency curve may be
directed to your appropriate Regional Surface Water Specialist.




                                       Verne R. Schneider
                                       Acting Chief, Office of Surface Water

WRD Distribution:  A, B, S, FO, PO

This memorandum supersedes Surface Water Branch Memorandum 82.11.