Comparison of estimates of uncertainty of discharge at US geological survey index-velocity gages on the Chicago sanitary and ship canal, Illinois

Estimates of uncertainty of discharge at time scales from 5 minutes to 1 year were obtained for two index-velocity gages on the Chicago Sanitary and Ship Canal (CSSC), Ill., instrumented with acoustic velocity meters (AVMs). The velocity measurements obtained from the AVMs are corrected to a mean channel velocity by use of an index velocity rating (IVR). The IVR is a regression-derived relation between the AVM velocity estimates and those obtained using acoustic Doppler current profilers (ADCPs). The uncertainty estimation method is based on the first-order variance method, but the AVM velocity error is estimated from an empirical perspective, using the statistics of the IVR regression. Some uncertainty exists regarding whether to include the standard error of the IVR regression (????2) in the discharge uncertainty. At the 5-minute time scale when ?? ??2 is included, it has the dominant contribution to the discharge uncertainty, and the discharge uncertainty (expressed as the standard deviation of the discharge estimate) is about 5 m3/s at one gage and 8 m3/s at the other, independent of discharge. When ????2 is not included, the discharge uncertainty at the 5-minute time scale is much smaller (about 0.5 m3/s) and depends more strongly on discharge. For time scales one day or greater and when ????2 is not included, the uncertainty of the IVR parameters dominates the discharge uncertainty. The value of the discharge uncertainty is about 0.4 m3/s for one gage and 0.5 m3/s for the other gage at long time scales.