The planet’s water is constantly moving, changing form, and traveling across the globe in a process known as the Water Cycle. This continuous circulation occurs on, above, and below the Earth’s surface, linking all water sources. Within this vast system, a reservoir serves as a temporary or permanent holding location for water. Reservoirs are fundamental to the global distribution and availability of water.
Defining Water Reservoirs and Storage
Reservoirs are the physical locations where water is stored. The water contained within these locations is referred to as storage. This concept is distinct from the fluxes, which are the processes that move water between reservoirs.
Fluxes include evaporation, precipitation, runoff, and infiltration, representing the rate of water movement. Storage represents the total volume held at any given moment and is necessary for the cycle to operate. The storage capacity of reservoirs helps stabilize the planet’s climate and sustains ecosystems by regulating water supply over time. A large majority of the Earth’s total water is in long-term storage rather than actively moving through fluxes.
The Earth’s Primary Water Storage Compartments
The oceans are the largest reservoir, containing approximately 96.5% of all water on Earth. This water is saline and not directly usable for human consumption. This immense volume of seawater stores heat and moisture, driving much of the global climate system.
The remaining water is freshwater, and most of this is locked away in solid form. Ice caps and glaciers represent the largest freshwater reservoir, holding about 68.5% of the planet’s total freshwater supply, primarily in the Antarctic and Greenland ice sheets. This water only re-enters the active cycle through melting.
Beneath the surface, groundwater constitutes the second-largest freshwater reservoir, making up about 30% of the total freshwater. This water is stored in aquifers and saturated zones within the Earth’s crust. While some groundwater is accessible and actively used, deep groundwater can be nearly isolated from the surface cycle.
Surface water, which includes lakes, rivers, and swamps, is the most visible and accessible to human populations. Rivers contain only a tiny fraction of the total water, but they are a primary source for cities and agriculture. Lakes hold the majority of liquid surface freshwater, with the North American Great Lakes alone containing about one-fifth of the world’s liquid surface freshwater.
The atmosphere and biosphere represent the smallest reservoirs, but they are crucial to the continuous cycle. Water vapor, clouds, and water held within living organisms are constantly cycling through the atmosphere and the terrestrial environment. Although minute in volume, the atmospheric reservoir is the source for all precipitation, making it a highly dynamic component of the system.
Understanding Water Residence Time
The concept of Residence Time introduces a temporal element to the water cycle, representing the average length of time a water molecule remains in a specific reservoir. This measurement is calculated by dividing the total volume of water in a reservoir by the rate at which water flows into or out of it. Residence time varies dramatically between different water storage compartments, revealing how quickly water is exchanged.
Reservoirs like the atmosphere and rivers have short residence times, often measured in days. Water vapor in the atmosphere, for example, cycles out in approximately 10 days. Similarly, water in a river may only reside for a few days before flowing into a larger body.
In contrast, large reservoirs have long residence times. Water in the deep ocean can remain there for thousands of years, and water locked in major ice caps can be stored for hundreds of thousands of years. Deep groundwater can have a residence time stretching into millions of years, as it is largely isolated from the active surface cycle.
Understanding residence time is important because it dictates how a reservoir responds to changes and pollution. Reservoirs with short times, like rivers, flush contaminants quickly. Those with long times, such as large lakes or deep aquifers, accumulate pollutants over long periods and take much longer to clean up. This temporal perspective provides context for managing water resources.