Climate change and runoff in south-western Australia R.P. Silberstein a,⇑ , S.K. Aryal a , J. Durrant b , M. Pearcey b , M. Braccia b , S.P. Charles a , L. Boniecka b , G.A. Hodgson a , M.A. Bari c , N.R. Viney d , D.J. McFarlane a a CSIRO Floreat Laboratories, Private Bag 5, PO Wembley, Western Australia 6913, Australia b Department of Water, 168 St. Georges Terrace, Perth, Western Australia 6000, Australia c Bureau of Meteorology, PO Box 1370, West Perth, Western Australia 6872, Australia d CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia article info Article history: Available online 17 February 2012 Keywords: Water resources Hydrological modelling Rainfall Stream flow Model summary This paper presents the results of computer simulations of runoff from 13 major fresh and brackish river basins in south-western Australia (SWA) under climate projections obtained from 15 GCMs with three future global warming scenarios equivalent to global temperature rises of 0.7 °C, 1.0 °C and 1.3 °C by 2030. The objective was to apply an efficient methodology, consistent across a large region, to examine the implications of the best available projections in climate trends for future surface water resources. An ensemble of rainfall-runoff models was calibrated on stream flow data from 1975 to 2007 from 106 gauged catchments distributed throughout the basins of the study area. The sensitivity of runoff to pro- jected changes in mean annual rainfall is examined using the climate ‘elasticity’ concept. Averaged across the study area, all 15 GCMs project declines in rainfall under all global warming scenarios with a median decline of 8% resulting in a median decline in runoff of 25%. Such uniformity in projections from GCMs is unusual. Over SWA the average annual runoff under the 5th wettest and 5th driest of the 45 projections of the 2030 climate declines by 10 and 42%, respectively. Under the 5th driest projection the runoff decline ranges from 53% in the northern region to 40% in the southern region. Strong regional variations in climate sensitivity are found with the proportional decline in runoff greatest in the northern region and the greatest volumetric declines in the wetter basins in the south. Since the mid 1970s stream flows into the major water supply reservoirs in SWA have declined by more than 50% following a 16% rainfall reduc- tion. This has already had major implications for water resources planning and for the preservation of aquatic and riparian ecosystems in the region. Our results indicate that this reduction in runoff is likely to continue if future climate projections eventuate. Ó 2012 Published by Elsevier B.V. 1. Introduction Assessment of the effects of climate change on catchment runoff is crucial for future management of water supplies, however under- taking these assessments on a regional scale with sufficient resolu- tion to be useful to water resources engineers and planners is a challenge. The current suite of global climate models (GCMs) greatly assist the process, but their grid cells are large and interpret- ing the results at the scale of water resources catchments requires downscaling of projections to scales appropriate for catchment het- erogeneity and management. This approach allows the future im- pacts of rainfall and the consequences for water resources and extreme hydrological events to be assessed using suitable catch- ment hydrological models. The impact of climate trends over the latter half of the 20th Cen- tury is well documented in numerous scientific studies reviewed in the Inter-governmental Panel on Climate Change Fourth Assess- ment Report (IPCC AR4, IPCC, 2007a). With a warming global cli- mate there is a general trend for an increasing difference between regions of high and low rainfall. For example, in Iran GCM projec- tions have indicated increasing rainfall in wetter areas and lower rainfall in drier areas and with a greater uncertainty in drier regions (Abbaspour et al., 2009). In Britain drier catchments were found more sensitive to climate change than wetter catchments (Arnell, 1992; Arnell and Reynard, 1996), and catchments where rainfall is most strongly concentrated in winter were projected to be even more so. Countries with a Mediterranean type climate, and partic- ularly those with semi-arid climates, appear to have the greatest impact of climate change on water resources (Ragab and Prud- homme, 2002). D’Agostino et al. (2010) found a 5–10% reduction in rainfall would produce a 16–23% reduction in stream flow in the Candelaro catchment in southern Italy, based on GCM projec- tions with a temperature rise of about 1.5 °C by 2050. In North Alge- ria (Jean-Pierre et al., 2010), Tunisia (Abouabdillah et al., 2010), Brazil (Montenegro and Ragab, 2010) and the USA (Parajuli, 2010) 0022-1694/$ - see front matter Ó 2012 Published by Elsevier B.V. doi:10.1016/j.jhydrol.2012.02.009 ⇑ Corresponding author. Tel.: +61 8 9333 6000. E-mail address: richard.silberstein@csiro.au (R.P. Silberstein). Journal of Hydrology 475 (2012) 441–455 Contents lists available at SciVerse ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol