Water cycle

Water is essential to life on Earth. In its three phases (solid, liquid, and gas), water ties together the major parts of the Earth’s climate system — air, clouds, the ocean, lakes, vegetation, snowpack, and glaciersoffsite link

The water cycle shows the continuous movement of water within the Earth and atmosphere. It is a complex system that includes many different processes. Liquid water evaporates into water vapor, condenses to form clouds, and precipitates back to earth in the form of rain and snow. Water in different phases moves through the atmosphere (transportation). Liquid water flows across land (runoff), into the ground (infiltration and percolation), and through the ground (groundwater). Groundwater moves into plants (plant uptake) and evaporates from plants into the atmosphere (transpiration). Solid ice and snow can turn directly into gas (sublimation). The opposite can also take place when water vapor becomes solid (deposition). 

A large gap between treeline and the water level in California's Shasta Lake on August 25, 2014, is one sign of the state's devastating drought. Shasta Reservoir is at 30 percent of its total capacity and 47 percent of its historical average capacity.
Groundwater: California's big unknown
In the end, to manage water effectively, we need to know all the components of the water budget.

Water influences the intensity of climate variability and change. It is the key part of extreme events such as drought and floods. Its abundance and timely delivery are critical for meeting the needs of society and ecosystems.

Humans use water for drinking, industrial applications, irrigating agriculture, hydropower, waste disposal, and recreation. It is important that water sources are protected both for human uses and ecosystem health. In many areas, water supplies are being depleted because of population growth, pollution, and development. These stresses have been made worse by climate variations and changes that affect the hydrologic cycle.

Predicted changes in the annual number of days of extreme rainfall (defined as rainfall totals in excess of the historic 98th percentile) across the United States by 2041-2070 as compared to 1971-2000 if greenhouse gases continue to increase at a high rate (A2 scenario).
Heavy downpours more intense, frequent in a warmer world
According to the 2009 National Climate Assessment, heavy downpours have increased in frequency and intensity during the last 50 years...

Climate change is affecting where, when, and how much water is available. Extreme weather events such as droughts and heavy precipitation, which are expected to increase as climate changes, can impact water resources. A lack of adequate water supplies, flooding, or degraded water quality impacts civilization — now and throughout history. These challenges can affect the economy, energy production and use, human health, transportation, agriculture, national security, natural ecosystems, and recreation.

Infographic: The science behind atmospheric rivers
What are atmospheric rivers?
Atmospheric rivers are relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport most of the water vapor outside of the tropics.


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. 


Updated February 2019