Tracers, such as fluorescein dye, are widely employed to measure overland flow speeds by time-of-travel along measured flow paths. Among several disadvantages of this method are the involvement of human reaction time when using stopwatches, and the relatively long travel path that is consequently needed for reliable timing. Long flow paths mean that local variability along the flow path cannot be detected. This paper describes a new optical tachometer that overcomes these limitations, as well as offering other advantages. It is based on the use of a small floating reflector target that is carried on the surface tension film, and which passes between two reflective sensors mounted above the flow. The new device allows virtual 'spot' measurements of surface flow speed over a path as short as I cm, and eliminates the influence of human reaction time. The new device is battery powered and portable, and provides an improved alternative to dye timing in many field and laboratory applications. Its use will allow the collection of more refined data than have hitherto been easily achievable. Copyright (C) 2003 John Wiley Sons, Ltd.
