condensation
condensation vapour transport evaporation
precipitation
evaporation
transpiration by plants
evaporation precipitation
surface run-off percolation through rocks
river lake land ocean
groundwater flow
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gas) through the process of evaporation. Some water vapour is added by plants through the process of transpiration (the evaporation of water moving through the plant from its leaves). r This warm moist water vapour rises because it is less dense than the surrounding atmosphere. As it rises the air cools and the water vapour condenses (turns back into water droplets) into clouds, which air currents transport around the globe. ∆ Fig. 4.4 Plants intercept some precipitation. r These water droplets then fall as precipitation (rain, snow, fog, mist, dew) onto the land and oceans. Some will be intercepted to provide the water that plants and humans require for their survival, for example, taken up by plant roots or collected in reservoirs. Water will also be captured and stored in freshwater lakes or in ice caps or glaciers. r Most of the precipitation, however, will flow across or run off the surface of the land and into rivers or infiltrate the ground through percolation. Rivers return the water to the oceans. Some of ∆ Fig. 4.5 Ice caps such as the Greenland ice cap will the water that seeps underground through capture and store some precipitation. infiltration will be stored in aquifers (layers of permeable rock that absorb water) while the remainder will eventually seep back into the ocean as groundwater flow.
THE WATER CYCLE
∆ Fig. 4.3 A summary of the water cycle.