The water cycle

Water is life: every living thing needs it, and next to air it is the most important element for life. Because the earth has so much water on its surface, it is called the 'Blue Planet'.

The hydrological (water) cycle is the circulation of water through the hydrosphere in liquid, solid and gaseous forms, and its continuous exchange between the oceans, atmosphere, land surface, subsurface and living things. It has worked for billions of years, and all life depends on it.

• Water is a cyclic resource: it can be used and reused.
• Most of the planet's water is in the oceans: the book prints 91 per cent, but the usual figure is about 97 per cent. The rest is fresh water in glaciers and ice caps, groundwater, lakes, soil moisture, the atmosphere, streams and living things.
• Water that falls on land partly evaporates, partly runs off the surface, partly infiltrates the ground, and partly becomes glacier ice.

Components and processes:

• Water stored in oceans: evaporation, evapotranspiration (from soil and plants together), sublimation (ice straight to vapour).
• Water in the atmosphere: condensation and precipitation.
• Water in ice and snow: snowmelt runoff to streams.
• Surface water: surface runoff, stream flow, fresh-water storage, infiltration.
• Groundwater storage: groundwater discharge and springs.

The crisis. The amount of renewable water on earth is constant, but demand is rising fast, causing water shortages in many places and seasons. Pollution of rivers makes it worse.

From the video: the teacher corrects the figure: about 97.5 per cent of the earth's water is salty, so only a small share is fresh water people can use.

Example (new): a rooftop rainwater tank in a city does not create new water; it catches some of the rain that would have run off into drains, letting the household use a bit more of the same cycle before it moves on.