The water cycle

Earth's water is constantly on the move, but the total amount stays roughly the same. About 97 percent is salt water in the oceans. Of the fresh water, most is locked in ice sheets and glaciers or stored underground as groundwater, leaving only a small fraction in lakes, rivers, soil and the atmosphere.

The water cycle, or hydrological cycle, describes how water moves between these stores, powered by energy from the sun and pulled downhill by gravity.

Sunlight heats water in oceans, lakes and soil, turning it into water vapour. Oceans supply the great majority of the water that evaporates into the atmosphere. Evaporation leaves salt behind, which is why rain is fresh even though it largely comes from the sea.

Plants also release water vapour through tiny pores in their leaves, a process called transpiration. Together, evaporation and transpiration are called evapotranspiration. Over large forests such as the Amazon, transpiration returns huge amounts of water to the air.

As moist air rises, it expands and cools. Cooler air can hold less water vapour, so the vapour condenses onto tiny particles of dust, salt or smoke, forming the droplets and ice crystals that make clouds.

When droplets or crystals grow heavy enough, they fall as precipitation: rain, snow, sleet or hail. Air rises for several reasons: it is heated from below, forced up mountains, or pushed up at weather fronts where warm and cold air masses meet.

• Runoff flows over the surface into streams, rivers and eventually the sea.
• Infiltration lets water soak into the soil, where plants can use it.
• Percolation carries water deeper to refill aquifers, underground layers of rock that store groundwater.
• Snow and ice can store water for months in mountain snowpacks or for thousands of years in ice sheets.
• A water molecule may spend around nine days in the atmosphere but thousands of years in the deep ocean or an ice sheet.

Humans reshape the water cycle. Dams store river water, irrigation moves enormous volumes onto fields, and pumping groundwater faster than it is replenished lowers water tables. Aquifers such as the Ogallala in the United States and those beneath northern India have been depleted significantly.

Cities also change the cycle. Paved surfaces stop water soaking in, increasing runoff and flood risk. Many cities now use permeable paving, green roofs and restored wetlands to slow and store rainwater.

Warmer air can hold more water vapour, about 7 percent more for each degree Celsius of warming. This tends to intensify the water cycle: heavy downpours become heavier in many regions, while higher evaporation dries soils elsewhere, worsening droughts.

Shrinking glaciers and earlier snowmelt threaten the water supply of regions that depend on meltwater during dry seasons. Understanding and managing the water cycle is therefore central to adapting to climate change.