Background

Climatic Influences on Streamflow

There is now strong evidence that global warming is occurring and will continue in the foreseeable future. This will lead to changes in rainfall pattern and other climate variables, which will in turn impact on water balance in landscapes. CSIRO projections indicate changes in annual rainfall of –15% to +10% in southeast Australia by 2030, with highest decreases estimated for the southern parts of southeast Australia.

Reduced rainfall has essentially the same effect on streamflow as enhanced water use from a vigorously regrowing forest canopy. Both result in less “left-over rainfall” that comprises streamflow. Taken alone, the worse-scenario effect of reduced rainfall caused by climate change translates into a decrease in annual streamflow of up to 35% in catchments in southern Victoria. Dry-weather flows in smaller streams, because of monthly rain variability, will be more severely affected.

Both factors (regrowth and diminished rainfall) can be expected to combine to reduce streamflow most significantly over the next 25 years, with the result that water yield from catchments (and especially dry-weather flows) will be much less that at present. Maintenance of water supplies for in-stream water needs, towns and major cities will therefore require a sea change in the way that government devises contingency plans for future water supplies. This will probably require re-assessment of forest management policies in catchments, revision of water extraction licences from streams, and possibly new (and large) expenditure on water storages. In conjunction with these measures, it is inevitable that future water demand in cities will need to be restricted.

Key reference

• Chiew and McMahon, 2002

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