It is often difficult to obtain discharge ratings at urban gaging stations. This report presents the results of using an automated constant-rate dyedilution technique to measure discharge at two sites in Madison, Wisconsin. The sites selected have well-defined stage-discharge ratings, developed during previous hydrologic studies. One site has a 60-inch diameter concrete storm sewer and the other a concrete-lined open trapezoidal channel.
Equipment selected to automate the method included a portable automatic water sampler, a four-channel event recorder, and a variable-speed peristaltic pump for dye injection. An electrical switching circuit was designed to coordinate the operation of this equipment.
During the data-collection period of 1 year, a total of 20 storms were monitored.
The analysis showed that the discharges measured by dye-dilution techniques were generally within ± 10 percent of the discharges determined from ratings established by current-meter measurements. Larger differences were noted at the start of and on the rising limb of four hydrographs. Of the 20 storms monitored, dilution measurements on 17 were of acceptable accuracy. Peak discharges from the open-channel site ranged from 0 to 12 percent departures from the existing rating whereas the comparison of peak discharge at the storm sewer site ranged from 0 to 5 percent departures from the existing rating.
