The main parameters considered to design turbines are the water head and the flow rate of the water. Each type of turbine has its own ranges of water head and flow rate.
Figure turbine selection curve
The other factor to be considered is the efficiency of the turbine. Turbine efficiency differs according to the specific speed and its capacity. It also changes according to the load percentage.
Figure turbine efficiency curve
Depending upon the way that the machine is to be loaded suitable turbine can be selected depending upon the efficiency.