In hydroelectric dams, the tail race is at a much lower level than the height of the reservoir behind the dam. This difference in height corresponds to the amount of hydropower that can be obtained from the water, and the height difference is known as the hydraulic head.

What do you mean by tail race?

: a race for conveying water away from a point of industrial application (such as a waterwheel or turbine) after use.

What is head race and tail race?

The used water is released through the tail race. The difference between head race and tail race is called gross head and by subtracting the frictional losses we get the net head available to the turbine for generation of electricity.