Water exists across the planet in several distinct natural storage areas, known as reservoirs. The constant movement of water between these five main sources—oceans, ice caps, groundwater, surface water, and the atmosphere—defines the global water cycle, or hydrologic cycle. Driven by solar energy and gravity, this cycle ensures a continuous circulation of water, revealing how the planet’s supply is distributed and made available for all life.
Oceans: The Largest Water Reservoir
The oceans represent the largest reservoir of water on Earth, holding approximately 97% of the total global supply and covering over 70% of the planet’s surface. The vast majority of this water is saline, meaning it contains high concentrations of dissolved salts. This high salt content, mainly sodium chloride, makes the water unsuitable for direct human consumption or agricultural irrigation without extensive treatment. Removing the salt through processes like desalination is energy-intensive and costly, limiting the use of this massive reservoir. The oceans are also integral to the water cycle, serving as the source of about 86% of global evaporation, which supplies the atmosphere with freshwater vapor.
Frozen and Subsurface Freshwater Storage
The two largest freshwater reservoirs on Earth are locked away beneath the surface and in solid form. These sources account for the vast majority of the planet’s freshwater, but their accessibility varies greatly.
Ice Caps and Glaciers
Ice caps and glaciers constitute the largest reservoir of fresh water, storing approximately 68.7% of all freshwater globally. This frozen supply is primarily concentrated in the massive ice sheets covering Antarctica and Greenland. These ice masses represent water that has been stored for thousands of years, accumulated from snowfall that has been compressed into dense ice. While the sheer volume of water is enormous, its frozen state and remote location mean it is largely inaccessible for current human use.
Groundwater
Groundwater is the second largest reservoir of fresh water, accounting for about 30.1% of the global freshwater supply. This water is stored underground in permeable rock formations called aquifers, where it saturates the pores and fractures below the water table. Groundwater is an important resource, supplying drinking water to over two billion people and supporting global agricultural irrigation. Many of the world’s largest aquifers are being depleted faster than natural processes can replenish them. Excessive pumping can also lead to issues like land subsidence and, in coastal areas, the intrusion of saltwater into freshwater reserves.
Atmospheric and Surface Water Systems
The final two sources are characterized by their high accessibility and dynamic movement within the hydrologic cycle, despite holding the smallest amounts of water.
Surface Water (Lakes and Rivers)
Surface water includes all the visible fresh water found in lakes, rivers, reservoirs, and streams. While this is the most easily accessible source for human use, it makes up a minute fraction of the planet’s total water—less than 1% of all freshwater. Rivers and lakes are essential for municipal water supplies and maintaining diverse ecosystems. Ease of access has made surface water the primary source for human settlements throughout history. Its small volume, however, makes it highly vulnerable to pollution and fluctuations in climate.
Atmospheric Water
Atmospheric water exists as water vapor, clouds, and precipitation. Although it holds the smallest volume of water at any given time, it is the most dynamic reservoir, acting as the superhighway for water movement around the globe. Water vapor rises into the atmosphere through evaporation from oceans and land, and transpiration from plants. The atmosphere’s ability to hold water vapor is directly linked to temperature, and this moisture condenses to form clouds, which release water back to the land and oceans as precipitation.