Measuring the world's rivers with videos from Space

The accurate measurement of volumes of water flowing in the world's rivers is of critical importance for people, for nature and for industry. Our planet's rapidly changing climate is increasing this need, as water becomes scarcer as a resource and more dangerous as a hazard. Additionally, many river monitoring networks globally are inadequate and declining. To date, satellite-based methods used observations of river width, water surface height, and water surface area, but did not include the critical parameter of water flow speed. Here, we present a significant advance by demonstrating a method for determining water flow speed with a high degree of accuracy using video imagery obtained by a constellation of low-earth-orbit satellites. The very high resolution of the video imagery also allows observations to be made in rivers as narrow as 70 m wide. Through a programme of ground-based validation measurements, we have demonstrated agreement in discharge measurements better than 5% at a range of river sites around the world. This development can herald a step change in capabilities for the measurement of rivers globally, allowing observations in remote locations, and during extreme events such as floods, with no need for people or equipment to be on site.