Water on Earth is distributed among air, oceans, lakes, rivers, glaciers, underground, and swamps. All of them are the hydrosphere that is foundation of life on Earth. The hydrosphere has existed for about 4 billion years, but the seafloor spreading and continental drift result in its constantly changing shape.
Saline Water in Oceans
71% of Earth's surface is covered by oceans whose average depth is about 3800 meters. If Earth were a standard solid sphere that has neither mountains nor valleys, its surface would be covered by 2700 meters water. For convenience, global oceans are divided into four parts: the Pacific, Atlantic, Indian, and Arctic Oceans. Pacific Ocean is the largest, and Arctic Ocean is perennially covered by floating ice, even in summer.
Their most notable characteristic is the salty and bitter taste. The total dissolved salts in 1 kilogram seawater is called salinity that is usually expressed in parts per thousand (‰). Average salinity of global seawater is 35‰. Were these salts extracted and spread over land, the entire land would be elevated by about 150 meters. The most abundant salt is sodium chloride that accounts for about 85%. The remaining are divided among magnesium, calcium and potassium salts. Saltiness comes from sodium ions, while magnesium ions contribute to bitterness.
However, differences in evaporation and precipitation cause slight variations in salinity. When evaporation exceeds precipitation, seawater taste saltier. This is why salinity near equator and poles is relatively low.
Terrestrial Water
Some evaporated seawater is carried over land by air currents where under certain conditions it condenses and falls back to ground as precipitation. Terrestrial saline water is primarily found in underground and lakes. Freshwater is in glaciers, underground, lakes, soil and rivers.
Freshwater in Rivers
Although they only account for about 0.013% of Earth's total water, they are the easiest accessible freshwater. Precipitation from atmosphere is their main source. Aside from evaporation, most precipitation flows from high to low areas to gather into small streams which then converge into larger rivers. Another portion seeps into ground and soil. Most rivers in the world are replenished by rainwater, such as tropical Nile and Amazon rivers.
Rainwater plays a secondary role in some rivers, while melting snow from high mountains is major source, such as Yangtze and Yellow Rivers originating from Qinghai-Tibet Plateau. In summer, not only does glacier melting peak, but the warm and humid monsoon from Pacific also brings abundant rainfall to cause floods during this time. Remarkable floods have even altered the courses of Yangtze and Yellow Rivers in history. This trouble has been mostly resolved recently, although occasional small floods still occur. Ocean is the final destination of most rivers that are known as exorheic rivers. Some are endorheic, like the Volga River draining into Caspian Sea and the Tarim River vanishing in Xinjiang desert
Fresh and salt water in Lakes
Precipitation collecting in the depressions on land forms lakes. They are enclosed water bodies that is surrounded by land completely, though some may connect to seas via rivers. The salinity of freshwater lakes is less than 1‰. Many cities and villages rely on them as primary freshwater resources. They are relatively young or have rivers that carry salts to the nearest sea. Russia's Lake Baikal is notable not only for its depth and volume but also for its clear water and biodiversity. Freshwater marine life, such as sharks and seals, are among its most fascinating species.
Saline lakes, as the name implies, contain more dissolved salts. Their salinity usually exceeds 1‰, some are even as salty as seawater. Various minerals are introduced by rainwater that flows over the ground surface. If there’re no rivers to carry the slightly salty water away, becoming salty is the ultimate fate of freshwater lakes. This is especially true in arid regions where rapid evaporation accelerates salinization process. Biodiversity here is reduced significantly, mainly because the salt poses substantial survival challenges for many species. Only a few can thrive in such harsh environments.
Glaciers as the Largest Freshwater Reservoirs
In cold high-latitude and mountainous areas, snow is the most common precipitation. Perennial snow compacts and recrystallizes into glacier ice that slowly flows from high to low areas under their own weight. The glacier tongue often appears at the terminal which it is the most active locations that melting water, ice crevasses, ice caves and ice stalactites are often discovered here. Sometimes, they become sea floating icebergs to threaten navigation.
Glaciers and permanent snow store 70% of Earth's freshwater. They are the largest freshwater reservoirs on land. Meltwater not only nourishes some large rivers and plateau lakes but also irrigates desert oases. Currently, humans can’t directly use glacier freshwater but can only indirectly obtain them from rivers and lakes.
The intensifying global greenhouse effect is causing them to disappear. If they all melted, sea levels would rise by more than 60 meters. Major international cities like Shanghai, Hong Kong, Tokyo, and New York, along with low-lying countries like UK and Netherlands, would vanish.
Atmospheric Water
Water in the atmosphere mainly exists as vapor. Vapor originates from evaporation of surface water bodies, plant transpiration, and industrial emissions. Atmospheric water condenses into clouds and fog, eventually returning to ground as precipitation, such as rain, snow, and hail.