Put simply, head is the vertical difference in water level between the points at which the system takes in and discharges water. Head is usually measured in meters. The greater the head is, the greater the water pressure and total system output will be. Run-of-river hydro sites located on steep hills are therefore ideal.
At some sites, there may be some measures that can be taken to slightly increase the head measurement (and in turn the system output), such as clearing silt from the riverbed or raising sluices in order to increase the water level. Caution should be taken here, however, as this can cause unwanted effects such as flooding.
Head has an impact on the type of system that will need to be used - which also affects costs. Turbines at sites with a higher head measurement usually, though not always, cost less than those at sites with lower ones (more on costs).
A head measurement of 10 meters or less is usually considered ‘low head’, 10-50 meters ‘medium head’, and 50 meters plus is usually called ‘high head’.