Water storage refers to the process of holding water in some holding area for a period of time. There are multiple locations on Earth where water is stored, and the water stays there for variable periods of time depending on where exactly it is being stored.
It is important to investigate water storage from several different viewpoints. In the hydrologic cycle, these water storage locations vary drastically and include any natural storage area in the atmosphere, on the surface of the Earth, or below ground. For hydroelectricity, water storage is important as it includes lakes and hydroelectric reservoirs, which are utilized to generate electricity. Water storage can also include any man-made method of holding water, although these generally don't hold much of the overall volume of water on Earth, and include rain barrels or larger scale water towers for towns.
The storage of water in a variety of places on the Earth is a vital component of the hydrologic cycle. The movement of water around the world and its storage in a variety of location has contributed to the relatively constant amount of water that has existed on Earth.
The storage of water on Earth can be separated into three main locations: the atmosphere, on the surface of the Earth, and underground. Specifically these water storage areas are known as reservoirs and include oceans, glacier ice, groundwater, lakes, soil moisture, living organisms, the atmosphere, and rivers. Collectively, all water storage areas make up the hydrosphere. Most water on earth is found in oceans and seas, then in glaciers and groundwater. ~97% of the world's water is stored in the oceans as saltwater. Because the overwhelming majority of water is stored here, this can be seen as the start and end point of the hydrologic cycle. Water that starts here evaporates up into the atmosphere, where the majority of it falls back into the ocean as rain, or a much smaller amount falls onto land. For a more in-depth look at where Earth's water is stored, see Figure 1 below.
Water moves from reservoir to reservoir through a variety of different transportation mechanisms, but this water can stay in storage for varying lengths of time. The movement water from place to place can be fairly erratic, but there tends to be trends for how long certain types of reservoirs maintain their water for. The average length of time that water stays in any reservoir in the hydrologic cycle is known as the residence time. Water the water in the atmosphere is renewed every 15 days, while soil moisture lasts a couple of months. Lakes replenish their water every 50 to 100 years, while groundwater can last anywhere from 100 to 10 000 years. Finally, ice caps hold water the longest with residence times of up to 200 000 years.
The type of storage that occurs on the land surface and under the ground largely depend on the geologic features related to the types of soil and the types of rocks present at the storage locations.
Water storage is also important in the area of hydroelectricity. A hydroelectric reservoir is a type of water storage, and these reservoirs are extremely important in the generation of electricity from falling water. A hydroelectric reservoir is a large collection of water behind a hydroelectric dam that makes use of potential energy of water for generating electricity.This water is held back by the dam, and is allowed to fall to generate electricity when it is needed.
The water held in the reservoir of a hydroelectric facility is at a higher elevation than the rest of the dam, giving this water potential energy that is converted into mechanical energy of moving turbine blades as it falls. The height that this water is at is known as the hydraulic head, and is one of the major factors in determining how much electricity can be generated.