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A catchment is an area of land, usually surrounded by mountains or hills, over which water flows and is collected.

Within a catchment, water runs by gravity to the lowest point. The water is called surface runoff if it stays on the top of the land or groundwater flow if it soaks into the ground. When water reaches the lowest point in a catchment, it eventually flows into a creek, river, lake, lagoon, wetland or the ocean.

Within catchments, dams have been built on rivers to store water and this gives us a more permanent water supply. The rivers leading into the dams form a drinking water supply catchment, which can be different to the river's total catchment.

The water cycle is also known scientifically as the hydrologic cycle and refers to the continuous exchange of water within the hydrosphere, between the atmosphere, soil water, surface water, groundwater, and plants.

In a 100-year period, a water molecule will spend 98 years in the ocean, 20 months as ice, 2 weeks in lakes and rivers, and less than a year in the atmosphere.