Policy Statement on Stage Accuracy

In Reply Refer To:                                December 4, 1992
Mail Stop 415


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.