A surge tank is a standpipe or storage reservoir at the downstream end of a closed aqueduct or feeder pipe. A surge tank is used to absorb sudden rises of pressure and to supply water quickly if there is a drop in pressure. A surge tank, also known as a surge drum, is an open tank to which the top of a surge pipe is fitted, to avoid the loss of water during a pressure surge. A surge tank is an additional reservoir or storage space, placed in between the main storage reservoir and the powerhouse, and as near to the powerhouse as possible. Surge tanks are generally used in high or medium head plants, where there is considerable distance between the power unit and the water source.