Dr David Post CSIRO Australia

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

2. 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.

3. The Millennium drought (1997 – 2009)

4. Number of ‘very wet’ months per year

5. The 2010/11 floods

6. Drivers of the 2010/11 floods

7. Drought was broken in the warm season

8. Expansion of tropical influences

9. Expansion is seen in both reanalyses and radiosonde data

10. Expansion is correlated with global temperature

11. Modelling climate impact on water

12. 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

13. Projected changes in rainfall

14. Projected changes in runoff

15. Projected changes in future rainfall Dry Median Wet

16. Projected changes in future runoff Dry Median Wet

17. 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 CO2, and significantly drier (or wetter) region? • And if so, can our rainfall-runoff models capture these changes?

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

19. Can our models simulate the future? • 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 In wetter catchments with a moderate shift in the rainfall regime, well-calibrated models perform adequately

20. Can our models simulate the future? • 16% decline in rain • 70% decline in runoff • Simulation overestimation of 84% and 41% • Observed runoff of 131 mm • Calibration estimates within 3% Simulation Calibration 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

21. What’s causing these changes?

22. Changes in groundwater connectivity

23. Changes due to enhanced CO2 ?

24. 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%.

25. References 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, 12-16 December 2011, Perth, Australia, pp 3553-3559. 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 http://www.seaci.org/ 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, 12-16 December 2011, Perth, Australia, pp 3622-3628. Post, D. A. and Moran, R. J. 2011. 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, 12-16 December 2011, Perth, Australia, 3629-3635. http://www.mssanz.org.au/modsim2011/I6/post.pdf 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, 12-16 December 2011, Perth, Australia, pp 3636-3642. 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): 447-457.