Natural and urban water cycle

Water is always cycling around, through and above the Earth: in a natural water cycle existing for billions of years, and a man-made urban water cycle providing cities with safe water, sewerage and drainage.

Natural water cycle

As water moves between the land, ocean, rivers and atmosphere it changes from solid to liquid to gas. This natural water cycle is our planet’s way of recycling water, and is essential for life on Earth.

The stages of the cycle are:

  • evaporation — water molecules get energy from the sun, which breaks the bonds between them and lets them escape into the atmosphere as vapour gas, leaving salt, minerals or metals behind
  • condensation — vapour cools as it rises, changing back into tiny water droplets that join to form clouds
  • precipitation — water falls to earth when too much has condensed for the air to hold, becoming liquid (rain) or solid (snow, sleet or hail) depending on the air temperature
  • infiltration — some water soaks into the ground (groundwater) and slowly seeps into rivers, or is absorbed by plant roots
  • run-off — some water flows across land (surface water) and collects in creeks that flow into larger rivers
  • transpiration — water returns to the air as it evaporates from plants, mainly through their leaves

Urban water cycle

People have changed the natural water cycle, building pipes take water for drinking and remove sewage and stormwater. This is the urban water cycle, and is managed by Melbourne Water in Melbourne.

Water — catchments to tap

Most of Melbourne’s drinking water comes from mountain ash forests high in the Yarra Ranges, which have been closed to the public for over 100 years to keep our water clean.

When it rains, the run-off flows into rivers and onto the reservoirs and water mains that make up our water supply system — eventually making its way to you!

Sewage — toilets to sewage plants

Used water from homes and businesses travels through sewer pipes to two major sewage treatment plants in Melbourne’s east and west — all part of our sewerage system.

These plants remove toxic substances so the sewage can be safely pumped out to sea and back into the natural water cycle, or recycled on farms, parks and golf courses.

Stormwater — drains to our bays

Stormwater drains collect rainwater run-off from roads, roofs and gutters, and send it into nearby rivers and creeks. These eventually carry the water into our bays, where it re-enters the water cycle.

Last updated:
10 October 2017