The water cycle, also termed the hydrologic cycle, describes how water moves in all three states (liquid, solid, gas) below the surface, on top of the surface, and in the atmosphere. Approximately 75% of the Earth's surface is covered by liquid and frozen water. Heat from the Sun is the driving force for the temporal and spatial water movement and its physical state. However, it is important to understand the water cycle is complex and water does not move in one continuous direction at one rate. Water can change its physical state and pathway at any time under specific environmental conditions.
Origins of water
Liquid water is unique to planet Earth in our solar system, yet there is evidence of surface features carved in the past on other terrestrial planets, as well as evidence of frozen water on other planets and moons. For years, astronomers have debated two obvious sources of water in our solar system: comets and asteroids. The primary difference between the two is that comets typically have a greater concentration of material that vaporize when heated, yet both comets and asteroids can contain ice. Collisions with Earth by comets and/or asteroids could easily have delivered water to our planet.
However, by comparing the amount of deuterium in comets to Earth's ocean waters, scientists have been collecting more and more evidence that Earth's water probably did not come from comets.  One study suggests that most of Earth's water came from asteroids, not comets.  Scientists have also reported that a study of ancient meteorites shows that liquid water has been on Earth since its formation 4.6 billion years ago,  with Earth and other planets inheriting much of their water from the cloud of gas from which the Sun was born 4.6 billion years ago. 
Precipitation (e.g. rain, snow, and ice) falls unevenly across the Earth’s surface. From there, liquid water moves from where it falls through the systems of Earth - air (atmosphere), land (lithosphere), life (biosphere), and water (hydrosphere). The hydrosphere includes liquid water in flowing and standing bodies of water on the surface, frozen water in glaciers and snow, groundwater in wells and aquifers, and water vapor as clouds and fog. Approximately ~71% of our planet’s surface is covered by liquid water.
Rainfall is one part of the water cycle that varies with intensity and duration over a geographic area. The time of year and atmospheric conditions will also dictate if the precipitation falling is in a liquid or frozen state. Scientists measure rainfall with data collected from satellites (NASA TRIMM and GPM missions). The amount of precipitation that falls in a particular place on Earth depends on the climate of that area, specifically, the season of the year, the latitude of the location, prevailing wind/weather patterns and local physical features.
Liquid water, whether it be from rainfall or snow melt, collects or flows over the surface from higher elevations to lower elevation areas and into watersheds defined by drainage divides. Drainage divides are physical landforms, most commonly mountains or high elevation areas, where water falling on one side of the landform goes into one watershed, and water falling on the other goes into a different watershed. As water flows over the surface it collects into streams, or other moving bodies of water. As water moves down a watershed it gathers in increasingly larger bodies of moving and still water. Stream water may eventually flow into one of Earth’s oceans through a number of large rivers such as the Delaware, Mississippi, Nile or Amazon. Not all of the water flowing in a river or its tributaries will travel all the way to the ocean; some water will evaporate to the atmosphere, enter the groundwater system, or be taken up by plants.
Scientists from the U.S. Geological Survey observe, analyze, and understand the movement and condition of surface water through stream gages placed in rivers across the United States. Stream gages take measurements such as how many cubic feet of water flow past a stationary point per second. The variations recorded in streamflow over time will indicate the volume of water moving through a particular stream location. Smaller streams within a watershed will feed into larger streams, increasing the volume of water moving through the system but not necessarily the velocity at which the water flows. Knowing about temporal and spatial streamflow data and patterns of streamflow can help scientists understand connections between weather events, seasonal changes, and timing of flooding.
Water that falls on, rests on, or travels over the ground is also is absorbed by sediment and soil at the surface and moves down through pore spaces and into the fractures of rocks to become part of the groundwater system. At depths up to a few kilometers, where some porous sedimentary rocks become saturated with water, we define the water table. Water also moves through these water-bearing rocks (called aquifers) to supply natural streams and rivers. Water in the groundwater system flows downhill, just as water on the surface does. On the surface, water movement is changed by physical surface features. Underground the movement of water is altered by the underlying rock layers. This can cause groundwater to collected when it encounters impermeable rock, like granite. It also means that layers of permeable rock that is saturated with water can be found between layers of impermeable rock, creating a contained aquifer.
Groundwater is withdrawn by humans through wells for agricultural and industrial uses, which can cause changes in the depth of the water table over time. The amount of water coming into the groundwater system must equal the withdrawal rate in order for the water table to remain unchanged. Human activities, such as chemical spills, can contaminate groundwater and make wells unusable for water withdrawal for human consumption, agricultural use, etc. The shape of the land, including the underlying layers of rock below the surface (subsurface stratigraphy) are important to consider when deciding to drill a new well in a developed area.
As a result of humans’ need for water, and in particular, the growing industrial and agricultural uses for water, it has become a social issue in human society. Questions about who has rights to water in any location is often negotiated through a political process, and can often lead to inequities and social harm to some groups of people. As the need for fresh water grows, because of increasing population and industrialization, water rights, management and equity issues will become more and more critical.
A new study published in Nature gives evidence that the water cycle is even more complicated than previously thought. It was believed that water contained in soil has been supplying plant transpiration, entering the groundwater system, and contributing to streamflow. Building on prior studies of hydrogen and oxygen isotopes, the data show that ecohydrological separation is taking place. Water used by plants is now documented to be isotopically distinct from that which enters streams, suggesting that hydrological separation of precipitation inputs creates distinct pools of water resources.
Other closely related articles in this wiki include:
- NASA GPM (n.d.). Earth Observatory Water Cycle Overview - Precipitation Education. Accessed December 9, 2015.
- Blake, G.A.; Qi1, C.; Hogerheijde, M.R.; Gurwell, M.A.; and Muhleman, D.O. (March 18, 1999). Sublimation from icy jets as a probe of the interstellar volatile content of comets. Nature 398, 213-216. Accessed August 31, 2015. doi:10.1038/18372
- Altwegg, K., and 31 additional authors. (January 23, 2015). 67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio. Science 347 (6220). Accessed August 31, 2015. doi:10.1126/science.1261952
- Sarafian, A.; Nielsen, S.; Marschall, H.; McCubbin, F.; and Monteleone, B. (October 31, 2014). Early accretion of water in the inner solar system from a carbonaceous chondrite–like source. Science 346(6209). Accessed August 30, 2015. doi:10.1126/science.1256717
- Gibney, E. (September 26, 2014). Earth has water older than the Sun. Nature News. Accessed August 31, 2015. doi:10.1038/nature.2014.16011
- Evaristo, J.; Jasechko, S.; and McDonnell, J. (September 3, 2015). Global separation of plant transpiration from groundwater and streamflow. Nature 525, 91-94. Accessed September 11, 2015. doi:10.1038/nature14983
Relevant external links include:
- The Water Cycle - by NASA Earth Observatory
- Earth's Water Cycle - by NASA Goddard (video)
- The Water Cycle in Gorongosa National Park - by HHMI BioInteractive (animation)
- The Water Cycle for Schools (diagram) - by USGS Water Science School
- Water Cycle - by NASA Science/Oceanography/Ocean & Earth System
- NASA Balances Water Budget with New Estimates of Liquid Assets - by NASA Goddard, July 7, 2015
- Where does water go when it doesn't flow? - National Science Foundation Press Release 15-074, July 9, 2015