Only 0.3 percent of Earth's fresh water sits in lakes and rivers
Earth holds an estimated 1.386 billion cubic kilometres of water, yet 97.5 percent is salty. Of the small fresh share, most is locked in ice or hidden underground. Just 0.3 percent of it lies in the lakes, reservoirs and rivers that people can easily reach.
The hydrosphere is all the water on, beneath and above a planet's surface: oceans, rivers, glaciers, soil moisture, groundwater down to about 2 kilometres, clouds and vapour, and even the water inside living things. Earth's has existed for roughly 4 billion years, though seafloor spreading and drifting continents keep reshaping it. Its mass is about 0.023 percent of the planet's, and ocean covers some 71 percent of the surface, with an average salinity of around 35 grams of salt per kilogram of seawater. The word entered English in 1887, translating a German term coined by Eduard Suess.
Sunlight and gravity drive water from one reservoir to another. Each year about 577,000 cubic kilometres evaporates, most of it from the oceans, and the same amount falls back as rain and snow. Land receives more precipitation than it loses to evaporation, and that surplus, roughly 44,800 cubic kilometres, returns to the sea through rivers and groundwater, supplying most of the fresh water that life and economies depend on.
Residence times vary enormously. Hydrologist Igor Shiklomanov estimated that oceans take about 2,500 years to renew completely, permafrost and ice around 10,000 years, deep groundwater and mountain glaciers 1,500 years, lakes 17 years and rivers just 16 days. The writer Marq de Villiers argues that the planet's total water has hardly changed since geological times: it can be polluted or misused but only moves around.
Human pressure is growing. Dams and diversions redirect flows, and only about a third of rivers still run freely. The Ogallala Aquifer, which irrigates American farmland, is being drained far faster than it refills, and its exhaustion would remove more than 20 billion dollars of food and fibre from world markets. Shiklomanov found that by 1998 some 35 percent of people already faced very low or catastrophically low water supplies.
Source: Hydrosphere
