9.2.4 The water cycle The water cycle or hydrologic cycle is how water moves around Earth, powered by the sun. This cycle is always changing water’s form — like vapour, liquid or ice — through the processes of evaporation, condensation and freezing Experts say up to 70 per cent of Earth’s fresh water is trapped in ice in places like the Arctic and Antarctica. This water can’t be used right away, so it’s called a potential resource . Water in vapour, saltwater or wastewater is also a potential resource. The water cycle in FIGURE4 shows how water connects different places. Water flows through the environment in various forms and is essential for life. People often live near rivers and lakes to have a steady water supply, which is also crucial for farming.
FIGURE4 The operation of the water cycle
The sun’s energy drives water cycle processes.
Condensation When water vapour cools, it condenses to form clouds and fog.
Precipitation Water droplets increase in size and weight and fall to Earth’s surface.
Vapour Water vapour moves into the atmosphere and cools as it rises.
Freezing
Transpiration
Plants transpire water from the soil into the atmosphere as water vapour.
Evaporation Water evaporates from soil and water surfaces to form water vapour or gas.
Groundwater Water passes through soil and rock.
9.2.5 How long does water stay in one place? Water can stay in one place for a short time or a very long time. It stays in the atmosphere for about nine days before falling as precipitation . In soil, it lasts one to two months. Water stays in snow and rivers for two to six months, but much longer in lakes, glaciers, oceans and groundwater . In the Antarctic ice sheets, some ice is 800 000 years old, but most is around 20 000 years old. Scientists study ice core samples taken from large ice sheets over three kilometres thick in Greenland and Antarctica to learn how the climate has changed over time.
TOPIC9 Water in the world 249
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