Using Program HYDCOMP, Version 1.0
A
method used by the U.S. Geological Survey for quality control in computing daily
value flow records is to compare hydrographs of computed flows at a station
under review to hydrographs of computed flows at a selected index station. The
hydrographs are placed on top of each other (as hydrograph overlays) on a light
table, compared, and missing daily flow data estimated. This method, however,
is subjective and can produce inconsistent results, because hydrographers can
differ when calculating acceptable limits of deviation between observed and
estimated flows. Selection of appropriate index stations also is judgemental,
giving no consideration to the mathematical correlation between the review
station and the index station(s).
To
address the limitation of the hydrograph overlay method, a set of software
programs, written in the SAS macrolanguage, was developed and designated
Program HYDCOMP. The program automatically selects statistically comparable
index stations by correlation and regression, and performs hydrographic
comparisons and estimates of missing data by regressing daily mean flows at the
review station against -8 to +8 lagged flows at one or two index stations and
day-of-week. Another advantage that HYDCOMP has over the graphical method is
that estimated flows, the criteria for determining the quality of the data, and
the selection of index stations are determined statistically, and are
reproducible from one user to another.
HYDCOMP will load the most-correlated index stations into another file containing the “best index stations,” but will not overwrite stations already in the file. A knowledgeable user should delete unsuitable index stations from this file based on standard error of estimate, hydrologic similarity of candidate index stations to the review station, and knowledge of the individual station characteristics. Also, the user can add index stations not selected by HYDCOMP, if desired.
Once
the file of best-index stations is created, a user may do hydrographic
comparison and data estimates by entering the number of the review station,
selecting an index station, and specifying the periods to be used for
regression and plotting. For example, the user can restrict the regression to
ice-free periods of the year to exclude flows estimated during iced conditions.
However, the regression could still be used to estimate flow during iced
conditions.
HYDCOMP
produces the standard error of estimate as a measure of the central scatter of
the regression and R-square (coefficient of determination) for evaluating the
accuracy of the regression. Output from HYDCOMP includes plots of percent
residuals against (1) time within the regression and plot periods, (2) month
and day of the year for evaluating seasonal bias in the regression, and (3) the
magnitude of flow. For hydrographic comparisons, it plots 2-month segments of
hydrographs over the selected plot period showing the observed flows, the
regressed flows, the 95 percent confidence limit flows, flow measurements, and
regression limits. If the observed flows at the review station remain outside
the 95 percent confidence limits for a prolonged period, there may be some
error in the flows at the review station or at the index station(s). In
addition, daily minimum and maximum temperatures and daily rainfall are shown
on the hydrographs, if available, to help indicate whether an apparent change
in flow may result from rainfall or from changes in backwater from melting ice
or freezing water.
HYDCOMP
statistically smooths estimated flows from non-missing flows at the edges of
the gaps in data into regressed flows at the center of the gaps using the
Kalman smoothing algorithm. Missing flows are automatically estimated by
HYDCOMP, but the user also can specify that periods of erroneous, but
nonmissing flows, be estimated by the program.
By Curtis L. Sanders, Jr.
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