The continuous cycling of water between the Earth’s surface and atmosphere is known as the hydrologic cycle. Understanding where water resides and how it moves is fundamental to appreciating the planet’s dynamic systems. The distribution of water is highly uneven, with certain locations holding massive quantities while others contain only a small, rapidly changing fraction.
Defining Storage and Movement in the Water Cycle
The places where water accumulates on Earth are referred to as reservoirs, which are distinct from the processes that move water between them. A reservoir is simply a natural storage location, such as an ocean, a glacier, or an underground aquifer. The movement of water between these storage areas is called a flux, which includes processes like evaporation, precipitation, and surface runoff.
The rate at which water moves through a reservoir is measured by its residence time, which is the average period a water molecule spends in that particular location. Reservoirs with a long residence time, like deep groundwater or ice sheets, hold water for hundreds or thousands of years, making them relatively static. Conversely, the atmosphere has a very short residence time, meaning water vapor cycles through it in approximately nine days.
The Dominant Reservoir: Global Oceans
The global ocean system is the largest water reservoir. Oceans and seas contain the overwhelming majority of the planet’s water, accounting for approximately 97.2% of the total volume. This immense body of saline water covers about 71% of the Earth’s surface and is the primary source of all water movement in the cycle.
The ocean holds roughly 330 million cubic miles of water. Since this water has an average salinity of about 35 parts per thousand, it is unusable for direct human consumption or agricultural irrigation without costly desalination. Water molecules in this reservoir have a very long residence time, estimated to be over 3,000 years. Evaporation from the ocean surface is the primary flux that feeds moisture into the atmosphere, driving global weather patterns.
The Largest Freshwater Reservoir
While the oceans hold the most water overall, the largest reservoir of freshwater is located in the cryosphere, which consists of ice caps, glaciers, and permanent snow. This frozen storage accounts for nearly 70% of all freshwater on the planet. The two main ice sheets, Antarctica and Greenland, store the bulk of this volume, making them the world’s largest frozen freshwater reserves.
This vast frozen supply is largely inaccessible for human use due to its remote location and solid state. The water trapped within major ice sheets can have residence times spanning thousands of years, as movement only occurs through slow glacial flow or melting at the edges. The long-term stability of this reservoir is a significant factor in sea level regulation, and changes in its volume have a profound effect on the global climate system.
Storage in Groundwater and Surface Water
After the ice caps, the next most significant freshwater reservoir is groundwater, which is stored in subterranean layers of rock and sediment called aquifers. Groundwater is the largest reservoir of liquid freshwater, accounting for about 30.1% of the total freshwater supply. Although much of this water is deep and some is saline brine, the shallow, potable portion is a foundational resource for human and ecological use, especially in arid regions.
Water in aquifers has a highly variable residence time. Surface water, which includes lakes, rivers, and wetlands, constitutes a much smaller fraction of the total water supply. Despite its small volume, this surface reservoir is the most visible and dynamic part of the cycle. Water stays in rivers for only weeks and in lakes for months to centuries.