Estimation of average stream velocity

Stream tracer studies provide information supporting diverse applications in environmental research and management. Average stream velocity through a reach is often estimated from tracer temporal profiles. This Technical Note addresses the calculation of average reach velocity. It is shown here that, under steady flow, average reach velocity over a fixed distance equals the spatial harmonic mean velocity. Similarly, the average reach velocity from the point of tracer injection to a fixed downstream measurement site is equal to downstream distance divided by the harmonic mean tracer time-of-travel, rather than the commonly used temporal centroid.