Extending streamgauging thanks to low-cost velocity-head rods

Hydrological data are limited spatially and temporally due to the limited resources of national and local hydrological services. The lack of streamflow data may be critical in some regions of the world, headwater or urban catchments, or ephemeral streams for instance. Low-cost, easy-to-operate streamgauging techniques may help involve less specialized operators, including professionals or simple citizens, to extend the range of streamflow observations. The velocity-head rod has been revisited since the beginning of this century and proves to be an efficient low-cost tool for discharge measurements in wadable streams with velocity greater than 20 cm/s over most of the cross-section. We have validated the velocity rating of the literature and discharge measurements within 10% of the reference. We have also brought a number of practical improvements (while keeping the manufacturing cost around 100€), proposed an estimation of velocity measurement uncertainties, and issued a field procedure and spreadsheet for velocity and flow measurements. We have trained and helped operators from diverse backgrounds in different countries. The technique is quick and easy but possible operator effects must be minimized. The velocity-head rod is useful for rapid discharge estimation, teaching or demonstration, citizen-science programmes and cooperation with resource-limited services.