Box 4 Measurement Errors in Synoptic Surveys
Standard stream gauging techniques introduce errors that range from 2% to 20% (Sauer and Meyer, 1992). A good reference for stream gauging methods is the USGS publication by Turnipseed and Sauer (2010). They suggest flow measurement errors usually average between 3 and 6%. When evaluating whether observed streamflow changes are meaningful, gains or losses must be outside of individual measurement error ranges. For example, if streamflow measurements contained an error of +/- 6%, and the upstream discharge measurement was 30 m3/s (+/- 1.8 m3/s) and the downstream measurement was 28 m3/s (+/- 1.7 m3/s), the net groundwater exchange computed by difference (assuming no other inputs or outputs within the reach) would be -2 m3/s. If the 6% error is applied to the stream gauging measurements the upstream measurement could be as high as 31.8 m3/s and the downstream measurement as low as 26.3 m3/s. That means that for a measurable difference in streamflow to be observed the value obtained would have to exceed the difference of the discharges with the 6% error applied (31.8 m3/s -26.3 m3/s = 5.5 m3/s). Unfortunately, the error ranges of inflow and outflow measurements did not allow differentiation of gain or loss when a difference of 2 m3/s was observed. Depending on site conditions, increasing the distance separating the measurements may resolve the issue if sufficient additional streamflow gains/losses occur. Roberts and Warren (1999) noted that additional errors are introduced if flow conditions are changing during the period of measurement.