Distribution of Open-File Report 92-144, "Determination of Error in Individual Discharge Measurements,"



In Reply Refer To:                                  March 11, 1993
Mail Stop 415



OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 93.14

Subject:  Distribution of Open-File Report 92-144, "Determination 
          of Error in Individual Discharge Measurements," by 
          V.B. Sauer and R.W. Meyer

The purpose of this memorandum is to make an initial distribution 
of the subject report and to provide guidelines for using the 
technique described therein for estimating discharge-measurement 
error.

Open-File Report (OFR) 92-144 provides methods of calculating or 
estimating the magnitude of errors that occur from many, but not 
all, sources of error encountered in making measurements using 
Price-type current meters.  By summing the square of these errors 
and taking the square root of the result, the technique may be 
used to estimate the standard error for an individual measurement.  
Based on statistical theory for normally distributed data, the 
standard error represents the upper and lower bounds of error that 
include approximately 68 percent of the measurements made under 
those conditions (same velocity, streambed type, etc.).

Because not all sources of error are considered by this estimation 
technique, the resulting error should be considered a probable 
lower value; the actual value could be significantly higher under 
some circumstances.  Sources of error not considered include 
excessive proportions of flow in a small number of sections, 
suspended debris (such as moss, leaves, and twigs) that catches on 
the meter, non-typical velocity distributions caused by an 
upstream partial obstruction such as vegetation or a rock ledge, 
and excessive turbulence or agitation.

It is standard U.S. Geological Survey (USGS) practice for 
hydrographers to assign qualitative ratings of accuracy to 
individual discharge measurements--excellent, good, fair, and 
poor.  These ratings are based on subjective evaluation of 
objective factors affecting measurement accuracy.  The factors 
include: number and distribution of verticals, average velocity, 
uniformity of flow, regularity and firmness of channel bottom, 
steadiness of stage and discharge during the measurement, and 
presence or absence of ice or debris in the flow.

The error estimation technique presented in OFR 92-144 is a 
quantitative rating of accuracy based on computations reflecting 
those sources of error that can be quantified.  For measurements 
to which it is applicable, the new technique should be more 
consistent among hydrographers than the subjective rating.  The 
Office of Surface Water, therefore, encourages the use of the 
technique and comparison of the results with those of the 
qualitative technique.

Using the new technique requires only one additional observation: 
that of streambed conditions as described in OFR 92-144 (p. 5).  
The error may be computed by hand or by a computer program that is 
presently available from the authors for Prime computers.  If 
there is sufficient interest, the Office of Surface Water will 
also compile the program on Data General work stations and 
personal computers.  The second release of NWIS II is planned to 
have space to enter the estimated error (calculated elsewhere), as 
well as the old subjective rating.  Plans now call for a 
subsequent release to include the algorithms for calculating the 
estimated error within NWIS II.

Experience and subsequent investigation will reveal how useful the 
new technique will be.  Users should keep in mind that it cannot 
be applied blindly.  The technique may not be appropriate for use 
with measurements made under adverse conditions not included in 
the equations for estimating error.  Examples of these conditions 
include significant effects from boundary conditions, ice, flow 
obstructions, wind, improper procedures, and large changes in 
stage.  In these cases, the hydrographer should apply the old 
subjective rating of accuracy to the measurement and consider a 
calculated estimate of error as being "at least this high."  For 
the majority of measurements, however, the new technique should 
provide an objective and consistent method of rating discharge 
measurement accuracy.

The new error estimation technique, like the traditional 
subjective accuracy rating, requires that the hydrographer observe 
and record the condition of the channel bottom and the 
characteristics of the flow field as they affect measurement 
accuracy.  In addition, to permit assignment of accuracy ratings 
when the new method is not applicable and to permit evaluation of 
the performance of the new method, hydrographers should continue 
to assign accuracy ratings to all discharge measurements based on 
qualitative evaluation of the measurement conditions.  Thus, 
hydrographers should continue to follow standard USGS procedures 
for observing, documenting, and evaluating conditions affecting 
the accuracy of all discharge measurements.

Additional copies of OFR 92-144 may be obtained from the Office of 
Surface Water:

                    U.S. Geological Survey
                    415 National Center
                    Reston, Virginia 22092
                    703/648-5301




                                  Charles W. Boning, Chief
                                  Office of Surface Water

Attachment

WRD DISTRIBUTION:  A, B, FO, PO