Power Systems
The advantage of hydro electric power station over thermal power station is

 hydro electric power station can supply the power throughout the year
 the initial cost of hydro electric power station is low
 the operation cost of hydro electric power station is low
 hydro electric station can be constructed at the place where the energy is required

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Power Systems
An hydro-graph indicates

 the average run off during the period.
All of these
 the discharge at any time during the period under consideration.
 the maximum and minimum run off during the period.

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