Surface velocity is traditionally measured with invasive techniques such as velocity probes (in shallow rivers) or Acoustic Doppler Current Profilers (in deeper water). In the last years, researchers have developed remote sensing techniques, both optical (e.g. image-based velocimetry techniques) and microwave (e.g. Doppler radar). These techniques can be deployed from Unmanned Aerial Systems (UAS), which ensure fast and low-cost surveys also in remote locations. We compare the results obtained with a UAS-borne Doppler radar and UAS-borne Particle Image Velocimetry (PIV) in different rivers, which presented different hydraulic conditions (width, slope, surface roughness and sediment material). The Doppler radar was a commercial 24 GHz instrument, developed for static deployment, adapted for UAS integration. PIV was applied with natural seeding (e.g. foam, debris) when possible or, with artificial seeding (woodchips) in the stream where the density of natural particles was insufficient. PIV reconstructed the velocity profile with high accuracy typically in the order of a few cm/s in all investigated rivers, whilst UAS-borne radar was only successful in locations with high water roughness. However, UAS integration of Doppler radar is complex because of vibrations, large instrument sampling footprint, large required sampling time and difficult-to-interpret quality indicators.
