Flow rate, sometimes represented by Q, is the number of cubic meters of water that pass through a hydroelectric power plant per second. Thus, the unit for flow rate is meters cubed per second (). Understanding the flow rate of the stream or river used for a hydropower generation plant is vitally important to predicting the available amount of hydropower. This, along with the head drop of the stream or river are the two values necessary to calculate the available power.
Flow rate simply represents the volume of water that can be captured and then re-directed by a dam to flow across a turbine generator to move it and generate electricity. The larger the flow - meaning the larger flow rate value - the more energy is available to be converted to electricity.
Both a high flow rate and a high head aren't necessary for a hydroelectric power plant to be viable. If there is a high flow rate, the head doesn't need to be as high as the sheer amount of water flowing through the dam is sufficient to move turbines. Conversely, if the water is falling a large distance it isn't as important for a large volume of water to flow to the turbines.