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For humans, water is the most essential component of Earth's natural environment. However, its distribution is highly uneven: the majority of Earth's total water volume is concentrated in the oceans, with only a small fraction (3.5%) existing as freshwater. Most of this freshwater is stored in glaciers and underground reservoirs, while a very small portion is found in surface waters, such as lakes, swamps and rivers. Water on Earth exists in three states, solid, liquid and gas, and is continuously cycled through the hydrological cycle. Water exchange primarily occurs above the ocean or in regions with closed basins on land, a process known as the small hydrological cycle. When water circulates between the world’s oceans and land, it is referred to as the large hydrological cycle.
