In many places, to ensure a constant supply of water, large dams are built to store large amounts of water. These dams are often built in valleys because the water can be trapped by the surrounding hills. The water that is trapped underground is called groundwater. Some of the groundwater seeps into rivers and lakes and may even end up back in the sea. In some places, groundwater is pumped to the surface to be used in irrigation, or treated and used for drinking water.
Water is pumped to towns to be used in houses and factories.
In factories it is used to wash and cool machines. More water is used in factories than in homes. People use water in homes for drinking, cooking and cleaning. Sometimes water has to brought to land that otherwise might be too dry for crops and animals to survive. This is called irrigation and is done by building pipelines and canals to bring the water from the rivers and lakes.