1 In Search of Easy - to - Use Methods for Calibrating ADCP ’ s for Velocity and Discharge Measurements Kevin

Acoustic Doppler current profilers (ADCP’s) have become a common tool for measuring streamflow and profiles of water velocity. Despite their widespread use, no standard procedure has been adopted or accepted for calibration of ADCP’s. Limitations of existing facilities for testing point-velocity meters, the complexity of ADCP instruments, and rapid changes in ADCP technology are some of the reasons that a standard procedure has not been adopted. This paper outlines various methods for calibrating ADCP’s, discusses the advantages and disadvantages of these methods, and presents a simple, cost-effective procedure for calibrating an ADCP in the field. Standard methods for the calibration of current meters involve towing a meter in a tow tank at various known speeds. This method has also been used to calibrate ADCP's. Disadvantages to this method include lack of adequate and uniform backscattering material, lack of flowing water in the testing facility, and inability to use the ADCP’s internal flux-gate compass. Use of flumes for ADCP calibration is not practical for many ADCP’s due to width and depth restrictions associated with the instruments. ADCP’s and conventional methods for measuring velocity and discharge have also been compared. However, these field comparisons are costly and conventional velocity and discharge measurements may be subject to relatively large uncertainties. The USGS is investigating a new method for ADCP calibration. This method requires the use of differential global positioning system (DPGS) with sub-meter accuracy and standard software for collecting ADCP data. The method involves traversing a long (400 – 800 meter) course at a constant compass heading and speed, while collecting simultaneous DGPS and ADCP data. This process is repeated several times and the ratio of the course length measured by means of the ADCP to the course length measured by means of DGPS is computed. When this ratio is less than 0.995, measurements made with RD Instruments’ Rio Grande ADCP most likely have a negative bias error and when it is greater than 1.003 the ADCP most likely has a positive bias error. It is estimated that this procedure can be completed in 2 hours or less, and can be done by anyone with access to a sub-meter DGPS.