Water on Earth is in constant movement from place to another. Water evaporates from the oceans forming water vapor; part of this vapor is transported inland and condenses as precipitation in the form of rainfall, snow, and ice. The rest returns to the oceans. Rainfall flows as runoff to rivers and then to oceans and lakes. Some of this water also infiltrates the ground and forms aquifers or groundwater deposits. Snow also melts into water and it flows to rivers or lakes, oceans, etc. This process occurs in continuous process known as the hydrologic cycle. Virtually all energy needed to evaporate water from the oceans and other water bodies comes from the sun.
According to the U.S. Geological Survey the total amount of water on Earth’s in all its forms is approximately 1,385 million cubic kilometers (km3). Of this total amount about 96% of the world’s water is in the oceans as saline water. Fresh surface water (i.e., lakes, rivers) constitutes about 93,100 km3 or about 0.007% and fresh groundwater is about 10.6 million km3 or about 0.76%, and “most of the water people and life on earth need every day comes from surface-water sources”. However, most of the fresh water on Earth, approximately 1.74%, is stored as a solid in the polar ice caps, glaciers, and as snow. Hence, the freshwater resources we need and use every day are very limited and are especially vulnerable to human impacts.
Most of water on Earth is found in the oceans in the form of saline water. Ocean’s salinity varies from about 33.5 grams/liter to 36.4 grams/liter of dissolved salts. To put this in perspective, most potable water has less than 0.5 grams/liter of dissolved salts. This makes seawater unsuitable for direct human consumption, crop irrigation, and many other uses. Hence, the vast majority of water in our planet is present in the land as surface water, snow, and ice and as groundwater below the Earth’s surface and in the atmosphere as a water vapor.
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