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Technical Note: Difficult Lessons Learned in Measuring Stage and Flow Rate on Small Watersheds

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Applied Engineering in Agriculture. 27(6): 933-936. (doi: 10.13031/2013.40626) @2011
Authors:   D. M. Endale, D. S. Fisher, M. B. Jenkins, H. H. Schomberg
Keywords:   Discharge, Flow measurement error, Flume, Hydrology, Water quality, Water quantity, Sensors

Submersible pressure transducers integrated with data loggers are commonly used to measure stage (water level) and estimate flow rate from established stage-discharge relationships; however, proper data quality assurance is important to reduce uncertainty in resulting estimates. For 2 yr, USDA-ARS researchers in Watkinsville, Georgia, continuously monitored flow rate from a spring using a flume fitted with a pressure transducer and data logger and performed periodic manual flow rate checks. Initially, during a 162-day period in 2003 with 72 comparisons, the automated system underestimated the mean flow rate by approximately 17% due to presumed drift and lack of frequent calibration. After calibrating the transducer to adjust for drift with the data logger program, 66 comparisons during a 349-day period in 2004 showed that the difference in mean flow rate between the manual and automated estimates was < 5%. During 235 days late in the monitoring period, the automated system overestimated the mean flow rate by approximately 29% because of another apparent drift. In a similar set-up at a pond outflow, a drift of 32 mm in transducer output caused underestimation of pond outflow by 27% during a 30-day period. Users should not assume that pressure transducer-based automated water level measurement systems continuously record correct data. Routine verification of transducer output is crucial to avoid inaccurate (and possibly useless) stage and flow rate estimates.

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