Water storage refers to holding water in a contained area for a period of time. Water storage can be natural or artificial. Natural water storage occurs in all parts of the hydrologic cycle. Water can be stored in the atmosphere, on the surface of the Earth, or below ground. Artificial water storage is done for a variety of reasons and is done on small and large scales. Water storage locations are commonly referred to as reservoirs.
Each stage of the hydrologic cycle involves the storage of water (Figure 1). The storage of water on Earth can be separated into three main natural locations: above, at, and below the surface of the Earth. Water can be stored in the atmosphere, on the surface of the Earth, or underground. These water storage areas are most commonly known as reservoirs. Natural reservoirs include oceans, glaciers and other bodies of ice, groundwater, lakes, soil moisture, wetlands, 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. The overwhelming majority of water is stored here, so oceans can be seen as the start and end point of the hydrologic cycle. Water from the ocean evaporates into the atmosphere, then falls back to Earth's surface as precipitation. Most precipitation falls back into the ocean but some precipitation falls onto land.
Water moves from reservoir to reservoir through a variety of different transport mechanisms such as evaporation, condensation, etc. Each type of reservoir (e.g. atmospheric, aquifers, lakes) has a different residence time. The residence time is how long the water stays in a reservoir. Water in the atmosphere stays there for an average of 15 days, while soil moisture lasts a couple of months. Lakes replenish their water every 50 to 100 years, while groundwater can reside in the reservoir for 100 to 10 000 years. Ice caps have the longest residence times, going 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 of the area and is related to the types of soil and the types of rocks present at the storage locations.
Artificial water storage is done for commercial and private reasons. Artificial water storage ranges in size from rain barrels and household water tanks to urban infrastructure and industrial reservoirs. Some of the smaller types of artificial storage include water towers, tanks, and rain barrels for use by people in times of drought or in areas that do not have easy access to water sources. Commercial uses for artificial water storage tend to be larger in scale, such as holding ponds and dam reservoirs. Some examples of industries that use water storage include various types of mining, agriculture (for irrigation and livestock), and energy generation.
An important use of artificial water storage is in hydroelectricity. A reservoir of water is built up behind a hydroelectric dam. The water in the reservoir is at a higher elevation than the water on the other side of the dam. The dam then converts the potential energy of the water into mechanical energy. This is done by having the water drop through the dam and push turbine blades as it falls. The height that this water is at is known as the hydraulic head, and it is one of the major factors in determining how much electricity can be generated.