Policy Statement on Stage Accuracy
In Reply Refer To: December 4, 1992
Mail Stop 415
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 93.07
SUBJECT: Policy Statement on Stage Accuracy
The purpose of this memorandum is to generalize and clarify USGS
policy on accuracy goals for collection of surface-water stage
(water-level) or gage height data. For a number of years, USGS
practice in stage data collection has been guided by statements in
USGS Water-Supply Paper (WSP) 2175, page 63, and USGS Techniques
of Water-Resources Investigations, Book 3, Chapter A-7, (TWRI
3A7), page 24, that an accuracy of 0.01 foot usually is needed for
stage records used in computation of discharge. These statements
are expressed as an accuracy goal and policy for stage data
collection in OSW TM 89.08. In all cases, the accuracy has been
expressed as an absolute magnitude of 0.01 ft, independent of the
stage being measured. Widespread use of stage sensors other than
floats in stilling wells and increased concern for assurance of
record quality have led to a need for reassessment, explanation,
and generalization of this policy.
Although stage data are collected for various purposes, the
predominant use is for computation of discharge from a stage-
discharge relation. Because the uses to which stage data may be
put cannot be predicted, it is OSW policy that surface water stage
records at stream sites be collected using instruments and
procedures that provide sufficient accuracy to support computation
of discharge from a stage-discharge relation, unless higher
accuracy is required. A specific numerical accuracy criterion
is given in a following paragraph. At non-stream (reservoir,
lake, estuary) sites, the same numerical accuracy goal is to be
used unless higher accuracy is required. Higher accuracy may be
required for computation of storage changes in reservoirs or for
computation of discharge using slope ratings or unsteady-flow
models; in such cases, the instruments and procedures needed to
achieve the required accuracy should be used. When field
conditions such as high velocities, wave action, or channel
instability make it impossible to collect accurate stage data or
to define an accurate stage-discharge relation, stage data should
be collected with the greatest accuracy feasible, using
instruments and methods appropriate for the field conditions.
The accuracy of surface water discharge records depends on the
accuracy of discharge measurement, the accuracy of rating
definition, and the completeness and accuracy of the gage-height
record. Accuracies of discharge records for individual days
commonly are about 5 to 10 percent. Individual discharge
measurements seldom are better than 2 percent. Stage discharge
relations commonly have slopes of about 3 on logarithmic plots in
which discharge is plotted as a function of effective stage (gage
height minus offset, where offset commonly is approximately equal
to gage height of zero flow). This implies that a 1 percent error
in the effective stage input to the rating would translate into a
3 percent error in the computed discharge.
The total uncertainty in discharge computed from a stage discharge
relation is the square root of the sum of squares of this error
and other unavoidable errors and approximations in the flow
measurement and rating development procedures. Examination of the
equation x = \r(z2 + 7y2) shows that improvement in the stage-
accuracy component (z) much beyond the combined accuracy of the
other error sources (y) will have rapidly diminishing effect on
the improvement of the overall accuracy (x). Thus, although 0.01
ft stage accuracy may be needed at low stages and discharges, that
degree of accuracy is not essential for accurate determination of
discharge at high stages.
An acceptable balance between stage-measurement accuracy and other
components of discharge-record accuracy can be achieved by using
instruments capable of sensing and recording stage with an
accuracy of either 0.01 ft or 0.2 percent of the effective stage
being measured, whichever is less restrictive. For example, the
required accuracy would be 0.06 ft at 30 ft effective stage, 0.02
ft at 10 ft, and 0.01 ft at all effective stages less than 5 ft.
In this context, effective stage is the height of the water
surface above the orifice or other point of exposure of the sensor
to the water body; the instrument should be installed in the
field with the orifice only sightly below the zero-flow stage.
When evaluating instrument accuracy specifications, it should be
noted that many instruments are rated in terms of full-scale
percentage accuracy. An instrument with 50-ft range and 0.2-
percent full-scale accuracy has an absolute error tolerance of
0.10 ft, applicable throughout the range of stage, and thus would
not have sufficient accuracy at low stages.
Realistic evaluation of instrument accuracy requires a combination
of specialized laboratory testing and field trials under
controlled conditions. The laboratory tests evaluate instrument
accuracy by comparisons with known measurement standards over a
range of specified stages, temperatures, and other conditions;
the field tests evaluate the instrument's ability to operate
reliably and maintain its accuracy with time under a range of
field operating conditions. One of the major functions of the
USGS Hydrologic Instrumentation Facility (HIF) is the performance
of laboratory and field evaluations, which are carried out through
the Test and Evaluation Section. Results of laboratory and field
tests of pressure sensor systems for stage measurement regularly
are distributed to WRD offices in the HIF newsletter (WRD
Instrument News) and in the INSTRUMENTS continuum on the QVARSA
node of the USGS DIstributed Information System (DIS). When
selecting stage-measurement instrumentation, Districts should
refer to the HIF newsletter and the INSTRUMENTS continuum, and
should consult with the HIF and other appropriate sources for
information on accuracy and field performance of any instruments
under consideration.
Accurate stage measurement requires not only accurate
instrumentation but also proper installation to ensure that the
stage of the water body is accurately transmitted to the sensor.
In addition, continual monitoring of the performance of all system
components is necessary to ensure that accuracy does not
deteriorate with time. The standard methods for stage measurement
described in WSP 2175 and TWRI 3A7 were developed for this
purpose; these methods include frequent reading of independent
reference gages, comparison of inside and outside gages,
observation of high water marks, redundant recording of peaks and
troughs by use of max/min indicators, use of crest stage gages,
and regular maintenance of gage datums by levels. These checks
should be augmented as appropriate for unusual field conditions
and instrument types not discussed in the standard references.
Hydrographers should notice and keep records of instrument
performance, including comparisons of recorded stages with
reference gage readings, and any corrections applied. These
records should be considered in evaluations of instrument
suitability, in maintenance of District quality assurance plans,
and in planning of future operations.
Charles W. Boning, Chief
Office of Surface Water
This memorandum supersedes Office of Surface Water Technical
Memorandum No. 89.08.
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