Yearly streamflow for Perth dams

Students investigate and analyse streamflow data, calculate percentages and use this to predict future water inflow to Perth dams.

Things you will need

  • Computers with internet access for all students
  • Calculators

About this lesson

Year level: 7, 8

Theme: Water Supply

Lesson description

Question: Why is it that when Perth gets a heavy rainfall our dams don’t rise at the same time?

Watch the following video.

Our dam catchments and rainfall

Student Activity

Examine and analyse the streamflow chart and answer the following questions:

  1. In what year did we receive the lowest streamflow? (2010 – 12GL)
  2. When did we last have a streamflow of more than 300 gigalitres? (1996 – 315GL)
  3. In what years did we have a streamflow of more than 600 gigalitres? (1917 – 846GL, 1926 – 742GL, 1945 – 863GL, 1946 – 843GL, 1955 – 775GL, 1963 – 685GL, 1964 – 784GL)
  4. In what year did we have the highest streamflow? How many Gigalitres was this? (1945 – 863GL)
  5. What do you notice about streamflow trends since records began in 1911?

Student work

  • Find the percentage increase/decrease of water inflow from year to year, during World War II  
    (1939 – 465GL, 1940 -59GL, 1941 - 207GL, 1942 – 451GL, 1943 – 219GL, 1944 – 115GL, 1945 – 863GL)
  • Calculation for 1939 to 1940 is as follows: 465 - 59 = 406 ÷ 465 X 100 = 87.3% decrease 
  • Calculate the mean GL for that period (2379GL ÷ 7 = 339GL)
  • Find the percentage increase/decrease of water inflow from year to year, for the years you attended school from Pre-Primary to Year 6. Calculate the mean GL for the period
  • Compare percentage and mean results. What do you notice?
  • Identify a trend. (E.g. The mean has declined by 80% from 339GL to 67GL)
  • Using the information you have collected, calculate what you predict the future water inflow to Perth dams will be in 5 years, 10 years and 20 years

Reflect & summarise


  • What do you think about the data coming from our dams? Are you surprised by the information?
  • What do your predictions about the future of water inflow to our dams suggest in regards to our future water supplies?
  • What could influence future water inflow to our dams?

Extension activities

  • Investigate what Water Corporation is doing to secure our water future
  • Explore if the data coming from Perth dams matches forecasts made in regards to climate change
  • Undertake a similar exercise using the metropolitan total dam storage comparison data and compare results

Key vocabulary

  • Gigalitre (GL): a thousand million litres
  • Streamflow: amount of water flowing into dams in a year (shown in Water Corporation’s graph as GL)
  • Surface water: is water collecting on the ground or in a stream, river, lake, wetland, dam, or ocean
  • Dam: a barrier constructed to contain the flow of water, usually built on a river
  • Climate change: is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years
  • Groundwater: water located beneath the ground surface in soil pore spaces and in the fractures of rock formations
  • Catchment: is an area of land where surface water from rain and melting snow or ice converges to a single point and joins a waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean
  • Inflow: the act or process of flowing in or into

Teacher background

Traditionally Perth has relied heavily on dams as a water source. With our drying climate we can no longer rely on dams or other traditional sources like lower security groundwater. Because of declining rainfall and streamflows, dams traditionally used to supply our Perth customers will be used less as a metropolitan water source and will take on a bigger role as storage reservoirs. Water produced from sources such as deep groundwater and desalination, will be transferred into our dams for storage and distributed out to customers in periods of higher demand. This will help ensure climate independent, year round water provision.

Water Corporation uses dams to move water around, including supplying towns in the Goldfields and Agricultural areas through the Goldfields Pipeline. In recent years more southern dams have been connected. This provides greater flexibility and security to Perth and the South West, as it allows towns access to a wider range of water sources.

Positive rainfall and streamflow trends in some northern areas of our state mean many towns in the Pilbara and Kimberley regions will continue to be supplied by traditional surface water and groundwater sources.

Water Corporation wants to provide a sustainable water supply in the face of strong state population and industry growth, and dwindling surface and shallow groundwater supply. The plan is to do this by becoming climate resilient. This requires a balance of reducing water use, increasing the amount of water we recycle and developing new water sources.

Water Corporations goals for 2030 are:

  • Reduce water use by 15% by working with households, businesses and communities to reduce water use and leaks
  • Increase water recycling in Perth to 30%. Increase the use of recycled water for parks, gardens and in industry
  • Develop up to 100 gigalitres of new water sources. Desalination is an important new source as well as groundwater replenishment and secure groundwater sources
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