The Global Water Cycle

The global water cycle, also called the hydrologic cycle, is the continuous movement of water between the oceans, the atmosphere, the land and living things through evaporation, condensation, precipitation and runoff. In brief: the Sun heats surface water, which evaporates, rises, condenses into clouds, falls as rain or snow and returns to the sea through rivers and groundwater; no new water is created, and the same water is recycled.

The water cycle distributes fresh water, shapes landscapes and drives weather. Its processes are the same physical changes studied in chemistry: evaporation, condensation and freezing. This article describes each stage, explains where Earth’s water is stored, includes a table of reservoirs, works through a simple water budget example and lists the effects of human activity.

What are the stages of the water cycle?

1. Evaporation: liquid water at the surfaces of oceans, lakes and rivers turns to vapor.
2. Transpiration: plants take up water through their roots and release vapor from their leaves. Evaporation and transpiration together are called evapotranspiration.
3. Condensation: rising air cools, and water vapor condenses on tiny particles to form cloud droplets or ice crystals.
4. Precipitation: droplets and crystals grow and fall as rain, snow, sleet or hail.
5. Runoff and infiltration: some water flows over the surface into streams and rivers, and some soaks into the ground to become groundwater.
6. Storage: water collects in oceans, lakes, glaciers and aquifers until it moves on again.

The cycle has no starting or ending point, and water can take many different routes. A raindrop may flow into a river and reach the ocean within days, or freeze in a glacier and stay for thousands of years.

What drives the water cycle?

Solar energy provides the heat for evaporation, and gravity pulls precipitation and runoff downhill. During evaporation, water absorbs about 2,260 J per gram. This energy is released again when the vapor condenses in clouds, which warms the air and powers storms. In this way the water cycle also moves energy from warm regions to cooler regions.

Where is Earth’s water stored?

The amounts below are approximate values widely cited from estimates of the U.S. Geological Survey. Different sources vary slightly.

The shares are rounded. Only about 2.5% of the world’s water is fresh, and most of that is locked in glaciers, ice caps and groundwater. Lakes and rivers hold a tiny fraction of the total, yet they supply most of the water people use.

How long does water stay in each reservoir?

Worked example: a water budget

A small lake basin receives 900 mm of precipitation per year. Of this, 540 mm evaporates or is transpired by plants, and the rest leaves as runoff or groundwater flow.

1. Balance. Precipitation = evapotranspiration + runoff, assuming that storage does not change.
2. Runoff. 900 − 540 = 360 mm per year.
3. Fraction. 540 ÷ 900 = 0.60, so 60% returns to the atmosphere.
4. Volume for a basin of 50 km². 360 mm = 0.36 m. The area is 50 × 1,000,000 = 50,000,000 m². The runoff volume is 0.36 × 50,000,000 = 18,000,000 m³, or 18 million cubic meters per year.

If a drought reduces precipitation to 700 mm while evapotranspiration stays at 540 mm, runoff falls to 160 mm, a drop of more than 55%. Small changes in precipitation can have large effects on rivers.

How do humans affect the water cycle?

• Land use: paved surfaces increase runoff and reduce infiltration, which can raise flood risks.
• Water extraction: withdrawing groundwater faster than it refills lowers aquifers.
• Dams and irrigation: change flow patterns and increase evaporation.
• Climate change: warmer air holds more water vapor, which tends to intensify heavy rainfall and drought in different regions.
• Pollution: contaminants travel through the cycle and affect water quality.

How did scientists understand the water cycle?

Ancient thinkers such as Aristotle wrote about evaporation and rain, but many believed that river water came from underground caverns or from the sea. In the 17th century, Pierre Perrault and Edme Mariotte measured rainfall and river flow in France and showed that rain is sufficient to feed rivers. The modern concept of the hydrologic cycle was developed from such measurements, and satellites now monitor precipitation and water storage globally.

Which mistakes are common?

Mistake 1: Thinking that water is used up

Water is not destroyed. It is recycled, though it may become polluted or unavailable where it is needed.

Mistake 2: Saying that clouds are made of water vapor

Clouds are made of liquid droplets or ice crystals. Water vapor is invisible.

Mistake 3: Believing that most fresh water is in rivers and lakes

Most fresh water is in glaciers, ice caps and groundwater.

Mistake 4: Leaving out plants

Transpiration moves a large quantity of water from soil to the atmosphere, especially over forests.