This refers to the flow rate of water (the volume of flowing water) in a given watercourse. It is important to note that the flow rate at a site will vary throughout the different periods of the year. For example, the watercourse’s flow rate will increase dramatically during periods of heavy rainfall. It is therefore necessary to measure flow rates over the course of a year to get an accurate picture of the total amount of energy that will be available for the generation of electricity.
Measuring the flow of a watercourse is a long and complicated process, so if you are planning on installing a hydro system, it is a good idea to hire a hydrologist who can carry out these calculations for you. The hydrologist will produce a flow duration curve for the watercourse, which shows how the flow is distributed over a given period of time and which is necessary to determine the appropriate size of turbine for the site. This will also give you an idea of how much electricity is likely to be generated by a hydro installation.
A useful preliminary resource that can be consulted is the government-funded Centre for Ecology & Hydrology (CEH), whose website has an archive which contains flow data (annual hydrographs and flow duration curves) for rivers in the United Kingdom: