A water reservoir is any natural or artificial container where water is stored within Earth’s hydrosphere. These reservoirs are components of the global water cycle, holding water for periods ranging from days to thousands of years. Understanding the distribution of water across these different stores is fundamental to grasping the planet’s available water resources.
The Global Water Budget: Dominance of Saline Water
The overwhelming majority of water on our planet is saline, meaning it contains a high concentration of dissolved salts. Approximately 97.2% to 97.5% of the total global water supply resides in the oceans and seas. This enormous volume establishes the saltwater ocean as the undisputed largest water reservoir on Earth.
The remaining fraction, a mere 2.5% to 3%, constitutes all the freshwater on the planet. This small percentage includes all the water locked in ice, stored underground, and found in surface bodies like rivers and lakes. The sharp distinction between the massive saline volume and the limited freshwater supply underscores the scarcity of usable water for human consumption and terrestrial ecosystems. This limited freshwater pool is where the second largest water reservoir is found.
Glaciers and Ice Caps: Earth’s Second Largest Water Reservoir
The second largest reservoir of water overall, and the largest store of freshwater, is the frozen water mass of glaciers and ice caps. This store, often referred to as the cryosphere, holds approximately 2.0% to 2.15% of all water on Earth. Its significance is magnified when considering freshwater, as it accounts for nearly 68% to 70% of the planet’s total freshwater supply. This massive frozen reserve is geographically concentrated in two major ice sheets.
More than 99% of the world’s land ice is contained within the Antarctic Ice Sheet and the Greenland Ice Sheet. The Antarctic Ice Sheet alone holds the vast majority of this frozen water, representing an enormous volume of freshwater that has accumulated over millennia. Smaller ice caps, mountain glaciers, and permanent snow cover in high-altitude regions make up the rest of this significant reservoir.
The frozen state of this reservoir means it is largely inaccessible for direct human use, yet it plays a profound role in regulating global sea levels and climate. The reflective white surface of the ice sheets, known as albedo, helps to reflect solar radiation, keeping the planet cooler. As the Earth’s climate changes, the stability of this enormous frozen reservoir becomes a major concern due to its potential contribution to sea level rise.
Completing the Picture: Other Significant Water Stores
Below the oceans and the cryosphere, the next most voluminous water store is groundwater. Groundwater is the water found beneath the surface in the pore spaces of soil and rock, a layer known as the saturated zone. This reservoir holds roughly 0.61% to 0.76% of the Earth’s total water, representing nearly 30% of all freshwater.
Groundwater is particularly important because it is the largest source of liquid, unfrozen freshwater. It is an important resource for human consumption, agriculture, and industry, often feeding rivers and lakes and providing a steady water supply during dry periods. Deeper, older groundwater, sometimes called fossil water, can be non-renewable on a human timescale, making sustainable management of this resource increasingly important.
The remaining fraction of Earth’s water is distributed across several much smaller but highly dynamic reservoirs. These include ground ice and permafrost, which together hold a significant portion of the remaining freshwater.
Minor Reservoirs
The smallest volumes of water are stored in:
- Freshwater lakes, which contain a small but easily accessible amount of water.
- Soil moisture.
- Water vapor in the atmosphere.
- The planet’s rivers and biological organisms.