Teacher background information
What is a catchment?
A catchment is an area of land where water collects when it rains, often bounded by hills. Every inch of land on the Earth forms part of a catchment. Water enters a catchment when rainfall (or snow) lands within the catchment area and makes its way down towards the rivers, lakes, wetlands or dams.
Along the way, some water is absorbed into the ground, some evaporates and some is used by plants. The remaining water that continues to run over the surface of the land is known as surface runoff.
Water usually flows from higher areas to lower areas. A catchment begins at the highest point and the water flows to lower areas with the force of gravity, eventually ending up in a body of water such as a river, a lake, or out to sea.
Why do we need to know about catchments?
Water can move easily from one place to another, even through the tiniest gaps in the soils, so in a catchment anything that happens in one place can eventually affect other areas of the catchment.
Imagine dropping a leaf into a small stream. As the water flows, the leaf will slowly make its way into the larger part of the river, and might even end up at the mouth of the river as it reaches the ocean. Now imagine that leaf is a plastic bag or toxic waste and consider how the environment could be damaged as these pollutants are carried by the water through the catchment.
If we know where the different catchments are in Western Australia, then we know where our streams, creeks and rivers start and where they end up. This means that we can learn to look after the whole area that catches the water.
What are drainage systems?
Water Corporation's drains convey and dispose of stormwater from 76 catchments in the Perth metropolitan area and from many others in the 6 South West drainage districts.
Drainage systems consist of pipes or open channel drains that convey stormwater runoff and groundwater to natural water bodies such as rivers, wetlands or the ocean, or to compensating basins. They often follow the route of natural watercourses that existed before any land development or clearing took place.
Water Corporation manages in excess of 2,500km of drains, diverting water from more than 400 000 hectares of land and preventing the flooding and waterlogging of more than 391,000 properties in the metropolitan area. Local councils manage most of the smaller reticulation drains of Perth's urban drainage network.