Freshwater is defined by its low concentration of dissolved salts. Despite the planet being covered mostly by water, this low-salinity water is remarkably scarce compared to the vast oceans. Understanding where this limited supply is stored is important for resource management and appreciating the scale of the global water cycle.
Freshwater vs. Global Water Supply
The planet holds an immense volume of water, but over 97% of this global supply is saline, residing in the oceans. This leaves only about 2.5% of the world’s water as freshwater. The vast majority of this freshwater, nearly 69%, is not in a readily usable liquid state. Instead, this portion is frozen solid, locked away in ice caps, glaciers, and permanent snow cover, primarily in Greenland and Antarctica.
The remaining liquid water is the supply that actively moves through the environment and is accessible for biological and human use. This liquid component includes water found underground, in surface bodies, and suspended in the atmosphere. The distribution of this liquid resource reveals a clear dominance by a source that is often out of sight.
The Largest Liquid Reservoir: Groundwater
The majority of all liquid freshwater on Earth is found beneath the surface, stored as groundwater. This reservoir holds approximately 98% of the world’s unfrozen freshwater supply. Groundwater accumulates in aquifers, which are layers of porous rock, sand, or gravel saturated with water below the water table. The water table marks the upper boundary of this saturated zone, and its depth can vary from near the surface to hundreds of meters down.
This water fills the microscopic pore spaces and fractures within the geological material, rather than being held in underground rivers or lakes. Because of this structure, groundwater moves extremely slowly. This slow movement contributes to its longevity as a reservoir, but also means its recharge rate is often slow, making it vulnerable to over-extraction. Its subterranean location makes it challenging to monitor and protect from contamination.
Visible and Accessible Surface Water
The liquid freshwater visible on the Earth’s surface makes up only a tiny fraction of the total supply. Lakes, rivers, and wetlands account for less than two percent of the total liquid freshwater. Despite their small volume, these sources are disproportionately important because of their high accessibility and role in the hydrological cycle. Rivers and lakes are often the primary source for municipal water supplies, agriculture, and industry.
Of all the surface liquid freshwater, the majority is contained within lakes, which can hold up to 87% of this supply. Rivers and streams, while dynamic and constantly flowing, contain the least amount of water at any single moment. These surface bodies are constantly replenished by precipitation and groundwater discharge. However, they are also highly susceptible to rapid changes from drought, evaporation, and human diversion, making them a less stable storage medium compared to groundwater.
Transient Liquid Sources (Soil and Atmosphere)
Soil moisture and atmospheric vapor are highly transient reservoirs of liquid freshwater. Soil moisture is the water held in the unsaturated zone immediately beneath the surface, distinct from the saturated zone that defines groundwater. Although it represents a minute quantity of the global total, soil moisture is biologically important as the direct water source for nearly all terrestrial vegetation and agriculture.
Atmospheric water vapor, including clouds and humidity, is an even smaller component of the liquid freshwater cycle. This moisture is the mechanism by which water is transported across the globe, eventually condensing and falling as precipitation to replenish the larger reservoirs. The short residence time of water in the atmosphere and in the topsoil means these are temporary holdings, constantly in motion between the major storage areas of groundwater and surface water.