Earth’s water supply is immense, estimated at over 332 million cubic miles, but most of it is not readily available for human use. A water reservoir is a natural storage area where water accumulates. Understanding global water distribution reveals that most of the planet’s water is held in reservoirs that are either chemically unsuitable or physically inaccessible. This makes the small portion of usable freshwater especially valuable.
The Dominant Reservoir: Saline Oceans
The single largest repository of water on Earth is the global ocean, containing approximately 97% of the entire water supply. This volume is distributed across the Pacific, Atlantic, Indian, Southern, and Arctic ocean basins. The primary characteristic making this reservoir unsuitable for human consumption is its salinity. Ocean water typically contains about 35 grams of dissolved salt, predominantly sodium chloride, per liter, resulting in an average salinity of about 35 parts per thousand (ppt). This high salt concentration is toxic to most terrestrial life and requires energy-intensive processes like desalination to become potable.
Primary Freshwater Storage: Ice and Glaciers
Moving beyond the salty oceans, the next largest reservoir holds the majority of the planet’s freshwater in a frozen state. Ice caps, glaciers, and permanent snow collectively lock up roughly 68.7% of all freshwater on Earth. This makes the cryosphere the second-largest water reservoir overall, holding about 2.0% to 2.1% of the total global water.
This volume of ice is concentrated primarily in the ice sheets covering Antarctica and Greenland, with a smaller portion found in mountain glaciers and ice caps worldwide. The water stored here is considered inaccessible because recovering it would require immense energy and create environmental instability. The frozen state of this reservoir means it is essentially removed from the active water cycle, only contributing to liquid reserves through slow melt processes.
Hidden Liquid Reserves: Groundwater
The largest source of liquid freshwater is stored underground in the form of groundwater, which accounts for about 30.1% of the total freshwater supply. This reserve is held in geological formations called aquifers, which are bodies of rock or sediment saturated with water. Groundwater is widely distributed and is the most heavily used liquid reserve for human consumption, irrigation, and industrial needs.
The capacity of an aquifer to store and transmit water depends on two properties: porosity and permeability. Porosity is the measure of the empty space within the material, such as the tiny voids between sand grains, which determines how much water the aquifer can hold. Permeability describes how well these pore spaces are interconnected, dictating how easily water can flow through the rock or sediment. Materials like well-sorted sand and gravel have high values for both, making them excellent aquifers, while clay may have high porosity but low permeability due to poorly connected pores.
Active, Accessible Sources: Surface Water and Air
The remaining portion of the global water supply, though the most visible, represents only a tiny fraction of the total. This category includes surface water bodies like lakes, rivers, and wetlands, as well as atmospheric water vapor and biological water. Collectively, these sources account for less than 0.3% of the total Earth’s water, or about 1.2% of the world’s freshwater.
Rivers and lakes are the primary sources of water for most human populations and ecosystems. Although small in volume, these reservoirs are highly dynamic, meaning water moves through them quickly with a high turnover rate. For instance, the atmosphere holds a mere 0.001% of total water, yet its rapid cycling as vapor, clouds, and precipitation is fundamental to the entire water cycle and weather patterns.