Guidelines for Frequency of Discharge Measurements during Ice-effected Periods
In Reply Refer To: January 26, 1994
Mail Stop 415
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 94.03
SUBJECT: Guidelines for Frequency of Discharge Measurements
during Ice-effected Periods
The purpose of this memorandum is to provide guidelines for the
frequency of discharge measurements during periods of ice-affected
streamflow at gaging stations. Office of Surface Water (OSW)
technical reviewers have noted during District reviews that the
number of discharge measurements during ice-affected streamflow
conditions has gradually declined over the past several decades.
There is concern that this situation has progressed to the point
where the integrity of many winter records has fallen below
desirable levels.
Determination of daily mean discharge values during winter ice
periods is based on discharge measurements and in most cases on
comparison of hydrographs for nearby stations. The accuracy of
discharge estimates during ice-affected periods is, therefore,
directly related to the number and quality of discharge
measurements. About one half of all ice-affected discharge
measurements are considered "poor," according to Janice Fulford's
recent investigation of measurement accuracy reported in
U.S. Geological Survey (USGS) Open-File Report 92-493
"Characteristics of U.S. Geological Survey Discharge Measurements
for Water Year 1990." There are limitations in improving the
accuracy of ice-affected discharge measurements, but more frequent
measurements can be made.
Several USGS publications provide guidance on discharge-
measurement frequency. Water Supply Paper 337 "The Effects of Ice
on Streamflow" by W.G. Hoyt, 1913, (p. 67) states: "The number of
current-meter measurements necessary to determine the winter flow
of a stream depends so largely on local conditions that no
definite rule can be given, but the axiom applicable to open-water
measurements--the greater the number the greater the accuracy--
applies with equal force to measurements made when ice is present.
If the control section of a station is always clear of ice or if
the flow is affected only by anchor ice, no more measurements are
necessary than are necessary to determine the open-water flow at
the same station. If surface ice is present at stations on
streams uncontrolled by storage, the number of discharge
measurements will depend almost entirely on the temperature; when
conditions of temperature and runoff are uniform, one measurement
a month may be sufficient to insure good records; when thaws are
frequent the number increases rapidly, and during certain periods
in the winter, it may be necessary to make measurements almost
daily."
Water Supply Paper (WSP) 2175, Vol. 2, by S.E. Rantz (p. 360)
states: "Because of the variability of the backwater effect,
discharge measurements should be made as frequently as is feasible
when the stream is under ice cover, particularly during periods of
freeze-up and thaw when flow is highly variable. (Procedures for
making measurements under ice cover are described in the section
in chapter 5 titled, "Current-Meter Measurements from Ice Cover.")
In midwinter the frequency of measurements will depend on climate,
accessibility, size of stream, winter runoff characteristics, and
required accuracy of the discharge record. As a general rule, two
measurements per month is the recommended frequency. At stations
below powerplants that carry a variable load, it may be necessary
to make two measurements during each winter visit--one at the high
stage of the regulated flow and the other at the low stage. The
backwater effects may be markedly different at the two stages. In
very cold climates where winter ice-cover persists and winter
discharge shows a relatively smooth recession, fewer winter
measurements are needed than in a climate that promotes the
alternate freezing and thawing of river ice."
In the 1987-88 study of winter discharge record accuracy described
in USGS Open-File Report 90-554, "Evaluation of Selected Methods
for Determining Streamflow During Periods of Ice Effect" by
N.B. Melcher and J.F. Walker, discharge measurements were made at
1-5 day intervals during periods of ice effect on three Iowa
streams. These measurements were used to establish a base record
which was compared with the results of a variety of computational
techniques based on discharge measurements made at a 6-week
frequency. Statistical evaluation of results from the commonly
used "Hydrographic-and-climate comparison" technique indicated
that 60 percent of estimated daily discharges had relative errors
greater than 10 percent, and 9 percent had relative errors greater
than 50 percent.
An acceptable measurement frequency during winter ice periods may
vary significantly from stream to stream depending on the relative
significance of winter flows to annual discharge, accessibility
during winter months, the economics of frequent measurements, and
the accuracy goals for given stations. Although judgment that
considers these constraints must be exercised in winter data
collection, Districts should be knowledgeable of the potential
reduced accuracy of discharge estimates with less frequent
measurements. In general, the OSW endorses the recommendations
given in WSP 337 and 2175 and adds the following guidance:
1. For streams whose surfaces typically stay frozen for all or
most of the winter period but respond to temperature changes
with change in streamflow, a discharge measurement should be
made as soon as possible after the ice sheet formation is
safe, and no less frequently than once every few weeks
thereafter until breakup occurs.
2. For streams whose surfaces typically stay frozen for the
entire winter period and that are characterized by gradual
streamflow recessions, a discharge measurement should be
made as soon as possible after the ice sheet formation is
safe, and at least once every 6-weeks thereafter until
breakup. Particular attention is needed to obtain a discharge
measurement at minimum flow, which often occurs just prior to
breakup.
3. For streams whose surfaces stay frozen for a period, break
up and then refreeze, at least one discharge measurement
should be made during each frozen period.
4. For streams whose records have good hydrographic comparison,
discharge measurements at least every four weeks are
recommended and additional discharge measurements should be
obtained at the best (index) station, particularly during
winter temperature extremes to improve definition of the
discharge hydrograph.
Districts are encouraged to carefully analyze the quality of ice-
affected gaging station records, taking into consideration the
number and quality of discharge measurements, groupings of
stations used for hydrographic comparisons, applicability of
weather records, and to downgrade the accuracy of published
records that are derived from limited basic information sources.
In execution of data programs, it also is recommended that the
Districts' quality assurance plan address winter discharge
records, specifying the accuracy goal for various stations or
groups of stations in the District. Such accuracy goals may be
stated in terms of reduced measurement frequency for those
stations.
Charles W. Boning, Chief
Office of Surface Water
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