The water cycle on Earth

The water cycle on Earth

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)

Water, society, and ecology

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.

Water and climate change

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.