Estimating future streamflows

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To make far-seeing management decisions for sustainable water use, you need to predict the impacts of climate change on runoff and streamflow. Research is finding good ways of doing this for Australia.

Climate change is already affecting runoff and hence streamflow in Australia’s catchments — as illustrated by the dramatic decline of inflows to the reservoirs supplying Perth’s water since the mid 70s. But how do we get a meaningful measure of the future impact of changing climate on runoff and streamflow in catchments?

Dr Francis Chiew and his colleagues at CSIRO and eWater CRC say it is important to tackle this question, for three main reasons: rainfall and water availability are predicted to decline over most of southern Australia; water resources there are already fully committed; and demand for water is increasing.

Dr Chiew has found that as a rule of thumb for practical purposes, we can assume a rainfall elasticity of streamflow of 2 to 3 in Australia, meaning that a 10% change in rainfall (whether an increase or decrease) will lead to a 20 to 30% change in streamflow.

This quick method gives water managers, planners, policy-makers and scientists a practical handle on future runoff. Rainfall elasticity of runoff tends to be greater in drier regions and catchments with low runoff coefficients, such as inland Australia, southern Australia and southwest Australia. Similarly, a 10% increase in potential evapotranspiration, another determinant of runoff, will lead to a 5–8% decrease in streamflow.