Analysis of within and between-GCM uncertainties of runoff projections in Mediterranean-like catchments

Streamflow reductions have been reported in mid-latitude Southern Hemisphere (SH) catchments, in particular in the southwest of Western Australia (SWA) and in central Chile (CC), following decreases in precipitation since the mid-1970s. Although projections from Global Climate Models (GCMs) indicate the observed trends are expected to continue during the rest of the 21st century, they are affected by large uncertainties that challenge informed decision making. Quantification and comparison of uncertainties in runoff projections for the period 2050-2080 relative to 1970-2000, driven by an ensemble of a single GCM with perturbed physics (CPDN) and a multi-model ensemble of different GCMs (CMIP5), were used to account for what we term “within-GCM” and “between-GCM” uncertainty in SWA catchments. Between GCM uncertainty of runoff projections was also quantified in CC catchments. Within and between-GCM uncertainties were found to be very similar (∼55 per cent) in SWA catchments. Between-GCM uncertainty for runoff projections in CC catchments is smaller than in SWA. On average, uncertainty of about 51 per cent, under RCP8.5 scenario, was simulated for the period 2050-2080 compared to 1970-2000. For CC catchments a dichotomy was observed in runoff projections under the RCP4.5 scenario, which according to our preliminary analysis might relate to how ozone is specified within different GCMs. We conclude that the number of models sampled by the CMIP5 ensemble, which includes multiple model runs from some GCMs, provides some insight into within-GCM uncertainties. Furthermore, since CMIP5 model runs report values for all regions and are easily accessible, the CMIP5 ensemble is more convenient for regional hydrological assessments than the perturbed physics experiments.