Water cycle
The processes that drive these movements are evaporation, transpiration, condensation, precipitation, sublimation, infiltration, surface runoff, and subsurface flow.
The ocean plays a key role in the water cycle as it is the source of 86% of global evaporation.
Activities such as deforestation, urbanization, and the extraction of groundwater are altering natural landscapes (land use changes) all have an effect on the water cycle.
Research has shown that global warming is causing shifts in precipitation patterns, increased frequency of extreme weather events, and changes in the timing and intensity of rainfall.
The water molecule H2O has smaller molecular mass than the major components of the atmosphere, nitrogen (N2) and oxygen (O2) and hence is less dense.
A huge concentration of these droplets over a large area in the atmosphere becomes visible as cloud, while condensation near ground level is referred to as fog.
Some precipitation falls as snow, hail, or sleet, and can accumulate in ice caps and glaciers, which can store frozen water for thousands of years.
Some water infiltrates deep into the ground and replenishes aquifers, which can store freshwater for long periods of time.
Water stored in the soil remains there very briefly, because it is spread thinly across the Earth, and is readily lost by evaporation, transpiration, stream flow, or groundwater recharge.
Ice from Antarctica has been reliably dated to 800,000 years before present, though the average residence time is shorter.
An alternative method to estimate residence times, which is gaining in popularity for dating groundwater, is the use of isotopic techniques.
[20] The Earth's ice caps, glaciers, and permanent snowpack stores another 24,064,000 km3 accounting for only 1.7% of the planet's total water volume.
[4]: 85 The IPCC Sixth Assessment Report in 2021 predicted that these changes will continue to grow significantly at the global and regional level.
The water cycle is essential to life on Earth and plays a large role in the global climate system and ocean circulation.
[24] Fundamental laws of physics explain how the saturation vapor pressure in the atmosphere increases by 7% when temperature rises by 1 °C.
Cultural eutrophication of lakes is primarily due to phosphorus, applied in excess to agricultural fields in fertilizers, and then transported overland and down rivers.
Both runoff and groundwater flow play significant roles in transporting nitrogen from the land to waterbodies.
[30] The hydrodynamic wind within the upper portion of a planet's atmosphere allows light chemical elements such as Hydrogen to move up to the exobase, the lower limit of the exosphere, where the gases can then reach escape velocity, entering outer space without impacting other particles of gas.
700 BC), the Greek poet Hesiod outlines the idea of the water cycle: "[Vapour] is drawn from the ever-flowing rivers and is raised high above the earth by windstorm, and sometimes it turns to rain towards evening, and sometimes to wind when Thracian Boreas huddles the thick clouds.
By roughly 500 BCE, Greek scholars were speculating that much of the water in rivers can be attributed to rain.
These scholars maintained the belief, however, that water rising up through the earth contributed a great deal to rivers.
Aristotle correctly hypothesized that the sun played a role in the Earth's hydraulic cycle in his book Meteorology, writing "By it [the sun's] agency the finest and sweetest water is everyday carried up and is dissolved into vapor and rises to the upper regions, where it is condensed again by the cold and so returns to the earth.
[37][38] Much like the earlier Aristotle, the Eastern Han Chinese scientist Wang Chong (27–100 AD) accurately described the water cycle of Earth in his Lunheng but was dismissed by his contemporaries.
The first published thinker to assert that rainfall alone was sufficient for the maintenance of rivers was Bernard Palissy (1580 CE), who is often credited as the discoverer of the modern theory of the water cycle.
