Abstract

The uncertainty, or standard error, for individual measurements of stream discharge is computed based on a root-mean-square error analysis of the individual component errors. The component errors include errors in the measurement of width, depth, and velocity, and in computation procedures. This analysis can be used to evaluate the uncertainty for most discharge measurements made with the vertical axis, cup-type current meter. The procedures do not apply to other types of current meters or other methods such as dilution gaging or ultrasonic methods. The study indicates that standard errors for individual discharge measurements can range from about 2 percent under ideal conditions to about 20 percent when conditions are poor and shortcut methods are used. Most measurements will have standard errors ranging from about 3 percent to 6 percent. Some conditions, such as wind, ice, boundary effects, flow obstructions, improper equipment, as well as incorrect measuring procedures and carelessness, can result in larger errors than indicated by the error analysis. A computer program is available for making the error computations.

Posted May 2010