can u please tell me, what does a hydrologist mean by dam efficiency.
I believe what the question is asking is how efficient a hydroelectric dam is at converting the potential energy of the water stored in the dam to electric power. A tall dam with more hyrdraulic head would have more power to turn turbines, than a lower one with less head. Hydraulic head is the height of the water column that in turn generates water pressure resulting in a stronger flow. For instance, if you have water towers in your neighborhood, the height of the water is what generates the water pressure at your tap. That is why water pressure drops in the morning when everyone is taking baths or showers.
In a dam the head provides the potential energy to turn turbines and the stronger the flow the more and larger turbines it can turn to generate more energy.
The kinetic energy of the water is converted to electrical energy by the turbine.
Swiss mathematician Leonhard Euler showed in 1754 that the torque on the shaft is equal to the change in angular momentum of the water flow as it is deflected by the turbine blades and the power generated is equal to the torque on the shaft multiplied by the rotational speed of the shaft.
Hydroelectric Power Generation Efficiency
Hydroelectric power generation is by far the most efficient method of large scale electric power generation. See Comparison Chart. Energy flows are concentrated and can be controlled. The conversion process captures kinetic energy and converts it directly into electric energy. There are no inefficient intermediate thermodynamic or chemical processes and no heat losses.
The conversion efficiency of a hydroelectric power plant depends mainly on the type of water turbine employed and can be as high as 95% for large installations. Smaller plants with output powers less than 5 MW may have efficiencies between 80 and 85 %.
It is however difficult to extract power from low flow rates.