Earth is known as the Blue Planet because nearly 71 percent of its surface is covered by water. The total volume is distributed across many different storage areas, or reservoirs. A water reservoir is any natural location where water is collected and stored, such as oceans, ice sheets, or underground aquifers. This distribution is highly unequal, with the vast majority of the planet’s water locked away in forms not directly available for human use. Understanding the relative sizes of these reservoirs is fundamental to grasping global water availability and scarcity.
Earth’s Primary Water Storage
The largest reservoir of water on Earth is the global ocean. Oceans, seas, and bays contain approximately 96.5 percent of all the water on the planet, totaling about 1.35 billion cubic kilometers. This enormous volume is the primary driver of the planet’s weather and climate systems. The average depth of the oceans is several kilometers, creating a massive holding capacity.
The defining characteristic of this reservoir is its salinity, with an average of about 35 grams of salt dissolved in every kilogram of seawater. Because of this high salt content, the ocean water is unsuitable for drinking, agriculture, or most industrial applications without costly and energy-intensive desalination processes. Consequently, despite holding nearly all of the Earth’s water, this largest reservoir is largely inaccessible for sustaining human and terrestrial life.
The Vast Reserves of Frozen Water
The second largest overall reservoir, and the largest storage of fresh water, is found in ice caps, glaciers, and permanent snow. This frozen water accounts for approximately 68.7 percent of all freshwater on Earth, representing about 1.74 percent of the planet’s entire water volume.
The majority of this ice is concentrated in the massive ice sheets covering Antarctica and Greenland. These two locations alone hold more than 99 percent of the ice on Earth’s surface. While this water is fresh, its frozen state renders it largely static and unavailable for use in the short term, as it is locked away in remote polar regions. This large, frozen mass acts as a long-term buffer in the global water cycle.
Dynamic and Accessible Freshwater
The remaining freshwater, which is most relevant to human and ecological systems, is distributed across several smaller, more dynamic reservoirs. The largest component of this readily usable supply is groundwater, stored beneath the Earth’s surface in aquifers and soil pore spaces. Groundwater makes up about 30.1 percent of the world’s total freshwater, making it the largest source of liquid freshwater.
This underground supply is crucial, as it is approximately 60 times more plentiful than the water found in all surface lakes and streams combined. Groundwater can be broadly divided into shallow, renewable sources that are frequently recharged by precipitation and deep, non-renewable aquifers that hold water accumulated over thousands of years. The shallow sources are often connected to surface water, helping to sustain rivers and wetlands even during dry periods.
In contrast, surface water—including lakes, rivers, and swamps—accounts for a very small fraction of the total freshwater supply, representing only about 1.2 percent of the world’s freshwater. Lakes hold the largest portion of this surface water, with rivers containing the smallest amount. Although rivers and lakes are the most visible and heavily utilized sources for human consumption and irrigation, their volume is minuscule compared to the underground and frozen reservoirs.
Other minor, highly active reservoirs include soil moisture and atmospheric vapor. Soil moisture, which is essential for plant life and agriculture, is a significant component of the near-surface water cycle. Atmospheric vapor, while only accounting for a tiny percentage of total water, plays a disproportionately large role by facilitating the transport of water across the globe through precipitation.
Quantifying the Global Water Supply
A numerical summary highlights the stark disparity in the Earth’s water distribution. The oceans hold approximately 96.5 percent of all water, leaving only about 3.5 percent as freshwater. Of that small freshwater total, nearly 68.7 percent is locked away in ice caps and glaciers.
The next largest component is groundwater, which accounts for about 30.1 percent of the freshwater supply. This means that the combined total of all liquid surface water—including lakes, rivers, and swamps—represents a fraction of a single percent of the Earth’s entire water inventory. This perspective underscores the reality that the water readily available to sustain life and civilization is a remarkably small and precious resource.