Hydropower in one line
The power in falling water is P = rho g Q H: water density (1000 kg/m³) times gravity (9.81 m/s²) times flow in cubic meters per second times head in meters. A turbine converts most of it, so the result is multiplied by efficiency.
In this teaching model, flow rises in proportion to gate opening up to 150 m³/s, and efficiency peaks near 94 percent at about 85 percent opening, dropping off on either side. Real units have measured efficiency hills that also depend on head.
Try it: set the head to 300 m and the gates to 85 percent. Output passes 350 MW. Now close the gates to 30 percent and watch both the flow and the efficiency fall.
Where Francis fits
Francis turbines cover a wide middle range of heads, from tens of meters to several hundred. Lower heads favor propeller and Kaplan turbines, and very high heads favor Pelton wheels, which use free jets instead of a full pressure case.