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The Water Cycle

The new water cycle diagram is now available.

The water cycle describes where water is on Earth and how it moves. Human water use, land use, and climate change all impact the water cycle. By understanding these impacts, we can work toward using water sustainably. 

•   Water Science School HOME   •   Water Cycle Diagrams   • Glossary of Water Cycle Terms   • Water Pools and Fluxes Data Tables

What is the water cycle? 

The water cycle describes where water is on Earth and how it moves. Water is stored in the atmosphere, on the land surface, and below the ground. It can be a liquid, a solid, or a gas. Liquid water can be fresh or saline (salty). Water moves between the places it is stored. Water moves at large scales, through watersheds, the atmosphere, and below the Earth's surface. Water moves at very small scales too. It is in us, plants, and other organisms. Human activities impact the water cycle, affecting where water is stored, how it moves, and how clean it is.  

Pools store water 

Oceans store 96% of all water on Earth. Ocean water is saline, meaning it’s salty. On land, saline water is stored in saline lakes. The rest of the water on Earth is fresh water. Fresh water is stored in liquid form in freshwater lakes , artificial reservoirs, rivers , and wetlands. Water is stored in solid, frozen form in ice sheets and glaciers, and in snowpack at high elevations or near Earth's poles. Water vapor is a gas and is stored as atmospheric moisture over the ocean and land. In the soil, frozen water is stored as permafrost and liquid water is stored as soil moisture. Deeper below ground, liquid water is stored as groundwater in aquifers. Water in groundwater aquifers is found within cracks and pores in the rock.   

Fluxes move water between pools 

As it moves, water can change form between liquid, solid, and gas. Circulation mixes water in the oceans and transports water vapor in the atmosphere. Water moves between the atmosphere and the surface through evaporation , evapotranspiration , and precipitation . Water moves across the surface through snowmelt , runoff , and streamflow . Water moves into the ground through infiltration and groundwater recharge. Underground, groundwater flows within aquifers. Groundwater can return to the surface through natural discharge into rivers, the ocean, and from springs .   

What drives the water cycle? 

Water moves naturally and because of human actions. Energy from the sun and the force of gravity drive the continual movement of water between pools. The sun’s energy causes liquid water to evaporate into water vapor. Evapotranspiration is the main way water moves into the atmosphere from the land surface and oceans. Gravity causes water to flow downward on land. It causes rain, snow, and hail to fall from clouds.    

Humans alter the water cycle 

In addition to natural processes, human water use affects where water is stored and how water moves. We redirect rivers. We build dams to store water. We drain water from wetlands for development. We use water from rivers, lakes, reservoirs, and groundwater aquifers. We use that water to supply our homes and communities . We use it for agricultural irrigation and grazing livestock. We use it in industrial activities like thermoelectric power generation , mining , and aquaculture . 

We also affect water quality. In agricultural and urban areas, irrigation and precipitation wash fertilizers and pesticides into rivers and groundwater . Power plants and factories return heated and contaminated water to rivers. Runoff carries chemicals, sediment , and sewage into rivers and lakes. Downstream from these sources, contaminated water can cause harmful algal blooms , spread diseases, and harm habitats for wildlife.   

The water cycle and climate change 

Climate change is actively affecting the water cycle. It is impacting water quantity and timing. Precipitation patterns are changing. The frequency, intensity, and length of extreme weather events, like floods or droughts , are also changing. Ocean sea levels are rising, leading to coastal flooding. Climate change is also impacting water quality. It is causing ocean acidification which damages the shells and skeletons of many marine organisms. Climate change increases the likelihood and intensity of wildfires , which introduces unwanted pollutants from soot and ash into nearby lakes and streams.   

What determines water availability? 

Humans and other organisms rely on water for life. The amount of water that is available depends on how much water there is in each pool (water quantity). Water availability also depends on when and how fast water moves (water timing) through the water cycle. Finally, water availability depends on how clean the water is (water quality). By understanding human impacts on the water cycle, we can work toward using water sustainably.    

Read more about the components of the water cycle in more detail: 

Atmosphere   ·  Condensation   ·  Evaporation   ·  Evapotranspiration   ·  Freshwater lakes and rivers   ·  Groundwater flow   ·  Groundwater storage   ·  Ice and snow   ·  Infiltration   ·  Oceans   ·  Precipitation   ·  Snowmelt   ·  Springs   ·  Streamflow   ·  Sublimation   ·  Surface runoff  

• Water Cycle

Weather & Climate

Societal applications, nasa earth science: water cycle.

Earth is a truly unique in its abundance of water. Water is necessary to sustaining life on Earth, and helps tie together the Earth's lands, oceans, and atmosphere into an integrated system. Precipitation, evaporation , freezing and melting and condensation are all part of the hydrological cycle - a never-ending global process of water circulation from clouds to land, to the ocean, and back to the clouds. This cycling of water is intimately linked with energy exchanges among the atmosphere, ocean, and land that determine the Earth's climate and cause much of natural climate variability. The impacts of climate change and variability on the quality of human life occur primarily through changes in the water cycle . As stated in the National Research Council's report on Research Pathways for the Next Decade (NRC, 1999): "Water is at the heart of both the causes and effects of climate change."

Importance of the ocean in the water cycle

The ocean plays a key role in this vital cycle of water. The ocean holds 97% of the total water on the planet; 78% of global precipitation occurs over the ocean, and it is the source of 86% of global evaporation. Besides affecting the amount of atmospheric water vapor and hence rainfall, evaporation from the sea surface is important in the movement of heat in the climate system. Water evaporates from the surface of the ocean, mostly in warm, cloud-free subtropical seas. This cools the surface of the ocean, and the large amount of heat absorbed the ocean partially buffers the greenhouse effect from increasing carbon dioxide and other gases. Water vapor carried by the atmosphere condenses as clouds and falls as rain, mostly in the ITCZ, far from where it evaporated, Condensing water vapor releases latent heat and this drives much of the the atmospheric circulation in the tropics. This latent heat release is an important part of the Earth’s heat balance, and it couples the planet’s energy and water cycles.

The major physical components of the global water cycle include the evaporation from the ocean and land surfaces, the transport of water vapor by the atmosphere, precipitation onto the ocean and land surfaces, the net atmospheric transport of water from land areas to ocean, and the return flow of fresh water from the land back into the ocean. The additional components of oceanic water transport are few, including the mixing of fresh water through the oceanic boundary layer, transport by ocean currents, and sea ice processes. On land the situation is considerably more complex, and includes the deposition of rain and snow on land; water flow in runoff ; infiltration of water into the soil and groundwater ; storage of water in soil, lakes and streams, and groundwater; polar and glacial ice; and use of water in vegetation and human activities. Illustration of the water cycle showing the ocean, land, mountains, and rivers returning to the ocean. Processes labeled include: precipitation, condensation, evaporation, evaportranspiration (from tree into atmosphere), radiative exchange, surface runoff, ground water and stream flow, infiltration, percolation and soil moisture.

Evaporation ("E") controls the loss of fresh water and precipitation ("P") governs most of the gain of fresh water. Scientists monitor the relationship between these two primary processes in the oceans. Inputs from rivers and melting ice can also contribute to fresh water gains. Evaporation minus precipitation is usually referred to as the net flux of fresh water or the total fresh water in or out of the oceans. E-P determines surface salinity of the ocean, which helps determine the stability of the water column. Salinity and temperature determine the density of ocean water, and density influences the circulation. E-P determines surface salinity of the ocean, which helps determine the stability of the water column. Precipitation also affects the height of the ocean surface indirectly via salinity and density.

The ocean surface is constantly being stirred up by wind and changes in density or buoyancy. The ocean naturally has different physical characteristics with depth. As depth increases, temperature decreases because the sun only heats surface waters. Warm water is lighter or more buoyant than cold water, so the warm surface water stays near the surface. However, surface water is also subject to evaporation. When seawater evaporates, water is removed, salt remains, and relatively salty water is left behind. This relatively salty water can float at the surface; for example, in the tropics it floats because is it so warm and buoyant.

At higher latitudes, sea water tends to be salty because of poleward transport of tropical water and to a lesser extent, sea ice formation. When sea ice forms, the salt is not crystallized in the ice, leaving the remaining waters relatively salty. Also, near the poles, the seawater is cold and dense. The interaction between water temperature and salinity effects density and density determines thermohaline circulation, or the global conveyor belt. The global conveyor belt is a global-scale circulation process that occurs over a century-long time scale. Water sinks in the North Atlantic, traveling south around Africa, rising in the Indian Ocean or further on in the Pacific, then returning toward the Atlantic on the surface only to sink again in the North Atlantic starting the cycle again.

Generalized model of the thermohaline circulation: 'Global Conveyor Belt' This illustration shows cold deep high salinity currents circulating from the north Atlantic Ocean to the southern Atlantic Ocean and east to the Indian Ocean. Deep water returns to the surface in the Indian and Pacific Oceans through the process of upwelling. The warm shallow current then returns west past the Indian Ocean, round South Africa and up to the North Atlantic where the water becomes saltier and colder and sinks starting the process all over again.

NASA & The Water Cycle

Water is an integral part of life on this planet, and NASA plays a major role at the forefront of water cycle research. Currently, there are many NASA missions that are simultaneously measuring a myriad of Earth's water cycle variables; Evaporation, Condensation, Precipitation, Groundwater Flow, Ice Accumulation and Runoff. NASA's water cycle research missions can be grouped into 3 major categories; Water Cycle, Energy Cycle, and Water and Energy Cycle Missions. By studying each and every variable of Earth's water and energy cycles, "As Only NASA Can", a crucial understanding of the water cycle's effect on global climate is currently underway.

NASA's goal is to improve/nurture the following global measurements: precipitation (P), evaporation (E), P-E and the land hydrologic state, such as soil-water, freeze/thaw and snow. Through NASA's water cycle research, we can understand how water moves through the Earth system in the hydrological cycle and we will be in a better position to effectively manage this vital renewable resource and help match the natural supply of water with human demands. NASA is the only national agency that has the ability to support a full range of water cycle research, from large-scale remote sensing to in-situ field observations, data acquisition and analysis, and prediction system development.

More NASA Missions and Instruments planned to help better understand the workings of the water cycle. Within the next decade, an experimental global water and energy cycle observation system combining environmental satellites and potential new exploratory missions - i.e. advanced remote sensing systems for solid precipitation, soil moisture, and ground water storage - may be feasible. These proposed new approaches are tantalizing, for knowledge of global fresh water availability under the effects of climate change is of increasing importance as the human population grows. Space measurements provide the only means of systematically observing the full Earth while maintaining the measurement accuracies needed to assess global variability.

Sea Surface Salinity (SSS) is a key tracer for understanding the fresh water cycle in the ocean. This is because whereas some parts of the water cycle increase salinity, other parts decrease it. Global SSS patterns are governed by geographic differences in the "water budget." Like on continents, some latitudes of the ocean are "rainy" whereas others are arid and "desert-like." In general, latitude zones dominated by precipitation have low SSS and those dominated by high evaporation have high SSS. The lowest SSS occurs in temperate latitudes (40 - 50 degrees North and South), near coasts and in equatorial regions and the highest SSS occurs at about 25 - 30 degrees North and South latitude, at ocean centers and in enclosed seas.

To track changes in SSS patterns over time, scientists monitor the relationship between evaporation and precipitation in the oceans. After the launch of Aquarius in 2008, scientists will be able to produce accurate maps of global (E - P). Thus, for the first time we will observe how the ocean responds to variability in the water cycle, from season-to-season and year-to-year.

NASA's Aqua mission contributions to monitoring water in the Earth's environment will involve all six of Aqua's instruments: the Atmospheric Infrared Sounder (AIRS), the Advanced Microwave Sounding Unit (AMSU), the Humidity Sounder for Brazil (HSB), the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E), the Moderate Resolution Imaging Spectroradiometer (MODIS), and Clouds and the Earth's Radiant Energy System (CERES). The AIRS/AMSU/HSB combination will provide more-accurate space-based measurements of atmospheric temperature and water vapor than have ever been obtained before, with the highest vertical resolution to date as well. Since water vapor is the Earth's primary greenhouse gas and contributes significantly to uncertainties in projections of future global warming , it is critical to understand how it varies in the Earth system.

Frozen water in the oceans, in the form of sea ice, will be examined with both AMSR-E and MODIS data, the former allowing routine monitoring of sea ice at a coarse resolution and the latter providing greater spatial resolution but only under cloud-free conditions. Sea ice can insulate the underlying liquid water against heat loss to the often frigid overlying polar atmosphere and also reflects sunlight that would otherwise be available to warm the ocean. AMSR-E measurements will allow the routine derivation of sea ice concentrations in both polar regions, through taking advantage of the marked contrast in microwave emissions of sea ice and liquid water. This will continue, with improved resolution and accuracy, a 22-year satellite record of changes in the extent of polar ice. MODIS, with its finer resolution, will permit the identification of individual ice floes, when unobscured by clouds.

http://science.nasa.gov/earth-science/oceanography/ocean-earth-system/ocean-water-cycle/

Water Cycle

The complete water cycle includes evaporation from oceans and land which is then transported by the atmosphere, together with a small fraction from plant transpiration and from ice sublimation, and precipitates over ocean and land surfaces as rain, snow, freezing rain, sleet, hail, or graupel (graupel is soft hail or snow pellets, precipitation that forms when supercooled water droplets freeze on falling snowflakes). Liquid precipitation over land, joined by melted snow or ice, either penetrates the soil towards aquifers, runs off back to the ocean as rivers, or evapotranspirates back to the atmosphere. A fraction of the water that penetrated the soil also ends up in the ocean as submarine groundwater discharge. Thus the complete water cycle involves the oceans, land, atmosphere, biosphere, and cryosphere. (See below a depiction of Earth’s water cycle; read more about it at NASA’s Earth Observatory website .)

This section focuses on JPL studies of the terrestrial components of the water cycle and highlights how the work of JPL scientists spans various lifecycle stages of satellites missions.

Current Challenges

Water cycle research at the Jet Propulsion Laboratory is geared towards answering the following questions:

• How is Earth’s water cycle changing and what are the implications for hydrologic extremes and water availability?
• To what extent are human activities, especially water management, driving regional and global changes in the water cycle? What are the processes driving water cycle acceleration?
• How does terrestrial water storage modulate heat storage and sea level rise?

We address these challenges by observing and understanding total land water storage, soil moisture, surface water (rivers and lakes), and snow.

Total water storage and GRACE

The Gravity Recovery And Climate Experiment (GRACE) mission measured changes in Earth’s gravity field from 2002 to 2017.  Its successor GRACE Follow-On (GRACE-FO) was launched in 2018 and will provide similar measurements.  Because water is heavy (1000 kg, or one metric ton, per cubic meter!), the movements of water on Earth can be tracked through changes in Earth’s gravity felt from orbit.  Given the extensive 15+ year data record from GRACE, JPL scientists have been able to discover answers to Earth’s water cycle mysteries, including why the increase in sea level appeared to have slowed down: much of the precipitation destined to oceans was temporarily stored on land.  Other JPL studies, also using GRACE, have shown decreased groundwater availability in several regions, and the conditions setting the stage for drought or flood.

Soil moisture and SMAP

The Soil Moisture Active Passive (SMAP) mission was launched in 2015 and successfully measures changes in soil moisture around the world.  Given that the time range of data from SMAP is still relatively short, much effort has gone into interpreting the SMAP data together with measurements on the ground.

Over one-third of the global land area undergoes a seasonal transition between predominantly frozen and non-frozen conditions each year, with the land surface freeze/thaw (FT) state a significant control on hydrological and biospheric processes over northern land areas and at high elevations. Initial data from SMAP’s radar also captured the 2015 spring thaw progression over the Northern Hemisphere, with a thaw front extending from predominantly non-frozen southern latitudes to the still-frozen north (Derksen, Xu, Dunbar, et al., 2017). This kind of work links the water and carbon cycles.

SMAP is also able to measure ocean salinity, which enabled JPL-led studies to focus on both sides of the land/sea continuum.  Fournier, et al. (2016) presented the first such two‐sided analysis, focusing on the May 2015 severe flooding in Texas. Their investigation benefited from simultaneous measurements of land surface soil moisture and sea surface salinity, both from SMAP, as well as ancillary data.

Surface Water and SWOT

The Surface Water and Ocean Topography (SWOT) mission is the first satellite mission specifically designed to observe the rivers and lakes on Earth’s continents.  SWOT is currently expected to launch in late 2021 and JPL scientists are working to help with satellite design decisions in order to guarantee the best return on investment for NASA.    An example of this is an effort to quantify see how fast SWOT data will need to be made available to users so they can track conditions that quickly change, like water moving through Earth’s rivers.

Snow and ASO

Understanding the amount of water stored as snow in mountains is critical for water managers, particularly for the cities that are located downstream of these mountains.  As NASA investigates potential designs for future Earth orbiting satellites to observe snow, JPL scientists are studying potential measurement strategies using airplanes before using spacecrafts.  The Airborne Snow Observatory (ASO, Painter, Bormann et al, 2017) is an example of such a strategy to measure snowpack.

Research Page

Water cycle and precipitation.