Expansion of the southern hemisphere Hadley cell linked to reductions in water availability over south-eastern Australia Dr David Post CSIRO Australia

South Eastern Australian Climate Initiative The research involved: –Better understand the factors that drive changes in both climate and streamflow within south-eastern Australia. –Determine how much of the recent “Millennium Drought” across south- eastern Australia was attributable to climate change. –Develop improved long-term climate and streamflow projections for south-eastern Australia out to 2100.

The Millennium drought (1997 – 2009)

Number of ‘very wet’ months per year

The 2010/11 floods

Drivers of the 2010/11 floods

Drought was broken in the warm season

Expansion of tropical influences

Expansion is seen in both reanalyses and radiosonde data

Expansion is correlated with global temperature

Modelling climate impact on water

Projected change in mean annual rainfall Based on all GCMs (col. 1) and the best five GCMs assessed against the measures in col. 2 to 10

Projected changes in rainfall

Projected changes in runoff

Projected changes in future rainfall Dry Median Wet

Projected changes in future runoff Dry Median Wet

What effect does non-stationarity have? Large scale “climate change impact on runoff studies” use rainfall- runoff models in conjunction with future climate projections from GCMs/RCMs. These rainfall-runoff models are typically calibrated to historical conditions. The question therefore is, are there likely to be significant changes in the rainfall-runoff relationship in a warmer, enhanced CO 2, and significantly drier (or wetter) region? And if so, can our rainfall-runoff models capture these changes?

Calibration Simulation Can our models simulate the future? Poorly calibrated models will struggle with a changed rainfall regime

Calibration Simulation Can our models simulate the future? In wetter catchments with a moderate shift in the rainfall regime, well-calibrated models perform adequately 18% decline in rain 48% decline in runoff Simulation overestimation of 3% and 6% Observed runoff of 336 mm Calibration estimates within 2%

Calibration Simulation Can our models simulate the future? In drier catchments, where a moderate shift in the rainfall regime can produce a much larger change in runoff, even well-calibrated models can struggle 16% decline in rain 70% decline in runoff Simulation overestimation of 84% and 41% Observed runoff of 131 mm Calibration estimates within 3%

What’s causing these changes?

Changes in groundwater connectivity

Changes due to enhanced CO 2 VariableWater-limitedEnergy- limited LAI ET Runoff ?

Summary Rainfall and streamflow across southern Australia are likely to be lower in the future, particularly during the cooler months of the year. It is unclear how much of the decline in cool season runoff will be compensated by increased tropical influences across the region in the warmer months of the year. Water resource planning in south-eastern Australia is taking this projected decline in water availability into account in planning future water supply and demand for the region. The current generation of rainfall-runoff models may struggle to predict the hydrologic impact in semi-arid catchments of a change in rainfall greater than around 15-20%.

Questions? Chiew, F. H. S., Prosser, I. P. and Post, D. A. (2011). On climate variability and climate change and impact on water resources, in MODSIM 2011 International Congress on Modelling and Simulation, December 2011, Perth, Australia, pp CSIRO (2012). Climate and water availability in south-eastern Australia: A synthesis of findings from Phase 2 of the South Eastern Australian Climate Initiative (SEACI): 41 pp Petheram, C., Potter, N. J., Vaze, J., Chiew, F. and Zhang, L. (2011). Towards better understanding of changes in rainfall-runoff relationships during the recent drought in south-eastern Australia, in MODSIM 2011 International Congress on Modelling and Simulation, December 2011, Perth, Australia, pp Post, D. A. and Moran, R. J Practical application of climate-induced projected changes in water availability to underpin the water planning process in Victoria, Australia. MODSIM 2011 International Congress on Modelling and Simulation, December 2011, Perth, Australia, Potter, N. J. and Chiew, F. H. S. (2011). An investigation into changes in climate characteristics causing the recent very low runoff in the southern Murray-Darling Basin using rainfall-runoff models. Water Resour. Res. 47(1): W00G10. Potter, N. J., Petheram, C. and Zhang, L. (2011). Sensitivity of streamflow to rainfall and temperature in south-eastern Australia during the Millennium drought, in MODSIM 2011 International Congress on Modelling and Simulation, December 2011, Perth, Australia, pp Vaze, J., Post, D. A., Chiew, F. H. S., Perraud, J. M., Viney, N. R. and Teng, J. (2010). Climate non-stationarity - Validity of calibrated rainfall-runoff models for use in climate change studies. Journal of Hydrology 394(3-4): References