We often use the term streamflow (rainfall runoff) when we talk about rainfall and dams. This article will help you understand what streamflow is, how we estimate it, and why it’s so important.

To understand streamflow, you first need to know a bit about catchments. Catchments are areas where water is collected within the natural landscape. The outside edge of the catchment is usually the highest point. Gravity channels water from the high areas to the lower areas in the landscape.

The water eventually makes its way towards a common outlet at the lowest point of the catchment. This could be a creek, stream, river, ocean, lake or other body of water. Some water also seeps below ground, and is stored in the soil for plants to enjoy all year round. Water from catchments can also make its way deeper underground, recharging the aquifers beneath the catchment. The water stored underground in our aquifers is called ‘groundwater’.

In very wet conditions and when it’s not raining, excess water from the catchments can flow across the surface and seep back up out of the ground - that water is what we call ‘streamflow’.

When we mention streamflow, we’re talking about the water that makes its way from catchments into our dams. For us to collect streamflow the ground needs to be soaked and the plants need to have taken what they need. It’s only then that the remaining water runs off the landscape and ends up in our dams.

How do we estimate streamflow?

To estimate streamflow, we use a water balance calculation.

First, we measure the change in the volume of water in the dam over a month. We then subtract any water that was added to the dam within that month. This could be desalinated water we transferred in or rain that fell directly into the dam. We also add any water that was lost due to evaporation and other variables. The value we end up with is the estimated streamflow for that month. 

Why is streamflow so important to understand?

As a result of climate change, rainfall across the South West of WA has been declining since the 1970s. This means that in this part of WA less water soaks into our catchments, with a knock-on effect of less streamflow into our dams.

While streamflow into dams was once Perth’s primary source of drinking water, it now accounts for just 10% of the city’s water supply. Our major sources for Perth are now desalinated water, followed by groundwater.

Perth’s reliance on streamflow has reduced, but the dams themselves still play a very important role in supplying water to the city and in our regional areas. As well as collecting available streamflow, dams are also used to store groundwater and desalinated water for when we need it most.

How can we care for our catchments?

Maintaining healthy catchment areas is important to ensure clean and fresh streamflow.

At Water Corporation, a specialised team diligently care for the health of our catchments. Catchment Managers undertake surveillance, inspections, by-law enforcement, water sampling and give land use advice.

We also rely on the community to do the right thing when visiting catchment areas. Catchment areas are natural parts of our landscape, so the best thing to do when visiting places like Mundaring Weir or Serpentine Dam is to respect the land and follow visitor guidelines, which can be found on our dam directory.

View streamflow and dam data