Water is essential to life on Earth. It has three phases (solid, liquid, and gas). In these three phases, water ties together the major parts of the Earth’s climate system — air, clouds, the ocean, lakes, vegetation, snowpack offsite link, and glaciers. offsite link The water cycle is often taught as a simple, circular cycle of evaporation, condensation, and precipitation. Although this can be a useful model, the reality of the cycle is much more complicated.
The paths and influences of water through Earth’s ecosystems are extremely complex and not completely understood. NOAA is striving to expand understanding of the water cycle at global to local scales to improve our ability to forecast weather, climate, water resources, and ecosystem health.
What is the water cycle?
The water cycle describes the continuous movement of water within the Earth and atmosphere. Water moves naturally, but humans also have a big role in the modern water cycle. This is a complex process involving pools and fluxes.
Pools: When talking about the water cycle, a pool refers to the many forms and places where water is stored. This includes a "pool" of water like a lake or a swimming "pool," but also refers to things like water vapor in the atmosphere or ice in a glacier.
Fluxes: These are the ways that water moves between the pools, including state changes like evaporation or condensation.
Explore the visual depiction of the water cycle or read about the different ways water is stored and moves below.
Pools: Where and how water is stored.
Water is stored in many places and in different forms, such as liquid water, ice, snow, and water vapor. Water is stored in the atmosphere, at the surface, and underground.
Salt water makes up 97.5% of Earth’s water.
Most of Earth’s water is saline, or salt water. In fact, 97% of Earth’s total water is stored in the ocean. There is also a small amount of salt water stored in other pools, such as in saline lakes.
Freshwater makes up 2.5% of Earth’s water.
Freshwater is found in many different pools. Though lakes and rivers may be the first freshwater pools to come to mind, they actually hold only a small percentage of the freshwater on Earth. Here are some pools in order of how much of Earth’s freshwater they hold.
- Glaciers, ice sheets, and permanent snow offsite link: The water frozen in these pools accounts for approximately 68% of freshwater.
- Groundwater: This liquid water is below ground and accounts for approximately 30% of freshwater.
- Permafrost and ground ice: This water is frozen on or just beneath Earth’s surface and accounts for less than one percent of freshwater.
- Lakes, rivers, and wetlands: Despite accounting for only about one-fourth of a percent of freshwater, surface water like lakes, rivers, and wetlands are the most important sources of water for use by people.
- Other pools: Small percentages of freshwater are also stored in other pools, including in clouds as water vapor or droplets, as soil moisture, accumulated snow offsite link, and in the bodies of living things (including our bodies!).
Use the GOES Image Viewer to visualize parts of the water cycle, such as atmospheric water vapor and clouds.
Fluxes: How water moves.
There are many ways that water moves between pools — liquid water evaporates into water vapor, is transported through the atmosphere, condenses to form clouds, and returns to the surface as precipitation like rain or snow. Water can flow across the land, down into the ground, then be taken up by plants, and back into the atmosphere. Water can even return to the atmosphere from solid ice or snow, skipping the liquid phase!
Here are some of ways that water moves, or fluxes, and the terms we use to describe them:
From liquid to vapor and back again.
- Evaporation: Liquid water can turn into water vapor, like moisture in the atmosphere, through evaporation. Any liquid water can evaporate — from falling rain to water in the ocean or in a puddle on the sidewalk. Evaporation happens when water absorbs energy from the sun, the atmosphere, or even our own body heat.
- Transpiration: Liquid water evaporates from the leaves of plants in a process called transpiration. Much of the water that plants take up through their roots and store eventually leaves the plant this way.
- Condensation: Water vapor can condense into liquid. Water vapor condenses into water droplets to form clouds or to form dew on the ground. Condensation happens when there is more water vapor than the air can hold at the current temperature.
Removing the middleman: From vapor to solid and back again.
- Sublimation: Snow or ice can turn directly into water vapor — without melting — through sublimation.
- Deposition: Water vapor can turn directly into ice without condensing first. This is called deposition and can result in phenomena like hoar frost.
Flowing and falling: Moving from place to place.
- Precipitation: Once water has condensed in the atmosphere, it eventually falls back to Earth through rain, snow, or other types of precipitation.
- Runoff and streamflow: Water can flow over the surface of the land. This may be through lakes, streams, or as ice in glaciers. Water can also flow over impermeable surfaces, like concrete.
- Infiltration: Water on the ground surface can move into the soil.
- Percolation: Water that has infiltrated the soil can continue moving downward into the groundwater.
- Transportation: Water can be transported through the atmosphere as water, ice, or vapor. Clouds moving across the sky are a visual example. Atmospheric rivers are a type of concentrated transportation through the atmosphere.
Monitoring and predicting the water cycle
NOAA and other government agencies collect data on the pools and fluxes of the water cycle using a variety methods and in multiple scales, from satellite imagery to rain gages and stream gages. Scientists use the data and knowledge about the conditions that lead to fluxes in water to make predictions. Just a few parts of the water cycle that NOAA predicts and forecasts include:
- Precipitation probability
- Precipitation amount
- Fog
- Evaporation and transpiration
- Runoff
- River levels and flow rate
- Potential for extreme events like drought or floods.
CoCoRaHS is a community-based network of volunteers (like you!) working together to measure and map precipitation using low-cost measurement tools.
Humans and the water cycle
Describing the water cycle is incomplete without considering the ways that humans interact with, affect, and are affected by the water cycle. Humans use water in many ways — we need water to drink and for purposes like hygiene, growing food, industry, hydropower, and recreation. Through this use, people disrupt the flow and storage of water. Water is also key to climate variability and change, and extreme events like drought and flood.
How humans impact the water cycle.
Humans disrupt the natural flow of water. We change where and how much water is stored through building dams, changing the path of rivers, and other actions. When we use water, we move it between pools. We also affect water quality, which impacts its availability for human use and ecosystem health.
What does it mean to “waste” water?
Sometimes people wonder how we can waste water if water cycles. Water isn’t available for people to use in every part of the cycle and some sources, like groundwater, take a long time to replenish. If we use water from our freshwater sources faster than it can be restored, we risk diminishing, or even depleting, our water supply, which disrupts the water cycle. We also need to clean our water to use it safely, which takes time and energy.
Understanding the water cycle helps us protect our lives and livelihoods.
We use our knowledge about the water cycle to predict water supply and weather, as well as monitor extreme events like drought and floods.
Often, these predictions require scientists to consider many sources of data, and to work together across agencies. Here are a few examples:
- Predicting whether a river may flood includes many factors, such as precipitation and runoff potential, knowledge about the geology and vegetation surrounding the river, and the rate that groundwater feeds into the river.
- Improving hurricane prediction offsite link requires information from NASA and NOAA satellites, NOAA Hurricane Hunters, and weather and ocean data.
- Water supply forecasts, which are critical for farmers, require information about snow, streamflow, precipitation, temperature, and other climatic variables. These forecasts are a shared responsibility between state and federal agencies including the United States Department of Agriculture and the National Weather Service which regularly coordinate to share information.
We can also use our knowledge of the water cycle to predict larger changes that could have far-reaching impacts, such as sea level rise, glacier retreat, and changes to water-related climate patterns.
The National Weather Service Office of Water Prediction provides outlooks and data on precipitation, drought, flooding, and more, including an annual National Hydrologic Assessment. These services help people across the U.S. make water-related decisions.
The water cycle and our changing climate
Warming global temperatures are affecting where, when, and how much water is available. Water that is stored as ice in pools, like glaciers and sea ice, melt and move to liquid pools, like the ocean. Warmer temperatures lead to more water being stored in the atmosphere, and that influences extreme weather events such as droughts, heavy precipitation, and hurricanes. These events are expected to increase as climate changes.
How can warmer temperatures cause both drought and heavy rainfall?
Warmer air causes more evaporation and can hold more water vapor before it is saturated and condenses into precipitation. This means there can be longer intervals between rainfalls, and rainfalls may be more intense.
These intervals and heavier rains can lead to weather extremes. Warm ocean water is one of the ingredients for hurricanes and more moisture in the air can lead to bigger and stronger hurricanes. On the other hand, greater time between rainfall events can cause drought, and even wildfires.
When water evaporates and then condenses, it takes in and gives off heat. This affects weather, helps form rainfall, and plays an important role in Earth's climate and energy balance.
Heat and atmospheric water vapor: A positive feedback loop.
More water vapor in the atmosphere can also lead to warming temperatures because water vapor traps energy. The resulting warmer temperatures can in turn lead to more evaporation and enables the atmosphere to hold more water. This relationship between water vapor and heat is called a positive feedback loop.
EDUCATION CONNECTION
The water cycle impacts ecosystems, economies, and our daily lives. The resources in this collection help teachers guide their students beyond the classic water cycle diagram and through the complex social and environmental issues that surround water. The water cycle provides the opportunity to explore the nature of science using models and empirical evidence.