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Out of all the water on Earth, saline water in oceans, seas and saline groundwater make up about 97% of it. Only 2.5–2.75% is fresh water, including 1.75–2% frozen in glaciers, ice and snow, 0.5–0.75% as fresh groundwater and soil moisture, and less than 0.01% of it as surface water in lakes, swamps and rivers.
The United Nations estimates that only 200,000 cubic kilometers of the total 1.4 billion cubic kilometers of water on Earth is freshwater available for human consumption. A mere 0.014% of all water on Earth is both fresh and easily accessible. [34] Of the remaining water, 97% is saline, and a little less than 3% is difficult to access.
Water resources are natural resources of water that are potentially useful for humans, for example as a source of drinking water supply or irrigation water. These resources can be either freshwater from natural sources, or water produced artificially from other sources, such as from reclaimed water or desalinated water (). 97% of the water on Earth is salt water and only three percent is fresh ...
The lower mantle of inner earth may hold as much as 5 times more water than all surface water combined (all oceans, all lakes, all rivers). [19] The amount of water stored in the Earth's interior may equal or exceed that in all of the surface oceans. [20] Some researchers proposed the total mantle water budget may amount to tens of ocean masses ...
Oceans are Earth's deepest and most extensive basins with average depths of the abyssal plains being about 4 km beneath sea level. Gravity will naturally move and transfer materials from land to the ocean, with the ocean becoming the end-repository. [ 28 ]
As groundwater levels drop, rivers may lose direct contact with groundwater. [59] In coastal regions, more saltwater may mix into freshwater aquifers due to sea level rise and more intense storms. [11]: 16 [4] This process is called saltwater intrusion. Warmer water in lakes, oceans, reservoirs and rivers can cause more eutrophication.
From shallow waters to the deep sea, the open ocean to rivers and lakes, numerous terrestrial and marine species depend on the surface ecosystem and the organisms found there. [28] The ocean's surface acts like a skin between the atmosphere above and the water below, and harbours an ecosystem unique to this environment.
Ocean currents are important in the study of marine debris. [15] [16] Plankton are dispersed by ocean currents. Upwellings and cold ocean water currents flowing from polar and sub-polar regions bring in nutrients that support plankton growth, which are crucial prey items for several key species in marine ecosystems. [17]