Water on Earth exists in the hydrosphere, encompassing all water in its various forms. This water constantly moves through the water cycle, cycling between oceans, land, and the atmosphere. Main sources are called reservoirs, where water is stored in liquid, solid, or gaseous states. While the total volume of water is vast, the amount readily available for human use is surprisingly small.
Oceans The Vast Saline Reservoir
The single largest reservoir of water on Earth is the global ocean, which covers more than 70% of the planet’s surface. Approximately 96.5% to 97.2% of all the water on Earth is contained within these marine environments. This immense volume is what gives Earth its characteristic “blue planet” appearance from space.
The defining characteristic of ocean water is its high salinity, which renders it unusable for most direct human applications, such as drinking and crop irrigation. Average ocean salinity is about 35 parts per thousand, meaning every kilogram of seawater contains approximately 35 grams of dissolved salts. These dissolved minerals, predominantly sodium chloride, are the residue of runoff from continents. Although desalination technology exists, the energy and infrastructure required to treat this volume means the oceans remain largely untapped as a direct source of consumable water.
Frozen and Underground Reserves
Following the oceans, the next two largest reservoirs hold the majority of the world’s freshwater supply. Freshwater accounts for only about 2.5% to 3% of the total water on Earth. This small fraction is unevenly distributed between solid ice and subsurface liquid water.
The largest store of freshwater is locked up in the planet’s cryosphere, consisting of ice caps, glaciers, and permanent snow. These frozen sources hold approximately two-thirds, or about 68% to 78%, of all freshwater. The Antarctic and Greenland ice sheets alone contain the vast majority of this ice. This water is largely unavailable for use due to its distant location and solid state, though its melting contributes significantly to rising sea levels.
The second largest freshwater reservoir is groundwater, which is stored beneath the surface in the pore spaces of soil and rock, forming saturated zones called aquifers. This source makes up about 0.61% of Earth’s total water, but represents roughly 30% of the world’s total freshwater supply. Groundwater is often considered the largest reservoir of usable freshwater because it is sheltered from rapid evaporation and surface pollution.
However, the accessibility of groundwater varies significantly based on the aquifer’s geology. Water moves slowly through the subsurface, a process governed by the rock’s permeability. While groundwater is a primary source for irrigation and public supply globally, extraction rates in many regions currently exceed the natural rate of recharge, leading to unsustainable depletion of these reserves.
Dynamic Surface and Atmospheric Sources
The remaining sources of water are categorized as dynamic, meaning they are constantly in motion and represent the smallest fractions of the global supply. These include water visible to humans, such as lakes, rivers, and atmospheric moisture. Though a tiny percentage of the total, these sources are the most actively utilized for human needs.
Lakes and rivers, the most convenient sources of surface freshwater, hold a minute proportion of the world’s water. Lakes contain the majority of the liquid surface freshwater, with rivers, despite their constant flow, holding the smallest volume of all major reservoirs. These surface bodies are prone to rapid changes in volume from season to season, making them highly responsive to weather patterns and human management.
The atmosphere itself is a temporary reservoir, containing only about one-thousandth of one percent (0.001%) of the total water on the planet. This water exists primarily as water vapor, clouds, and precipitation. Though the volume is small, the atmospheric moisture has a short residence time, meaning it cycles quickly through the system, constantly renewing the surface and subsurface sources through precipitation.