Hydroclimatic projections for the Murray-Darling Basin based on an ensemble derived from Intergovernmental Panel on Climate Change AR4 climate models

We assess hydroclimatic projections for the Murray-Darling Basin (MDB) using an ensemble of 39 Intergovernmental Panel on Climate Change AR4 climate model runs based on the A1B emissions scenario. The raw model output for precipitation, P, was adjusted using a quantile-based bias correction approach. We found that the projected change, Delta P, between two 30 year periods (2070-2099 less 1970-1999) was little affected by bias correction. The range for Delta P among models was large (similar to +/- 150 mm yr(-1)) with all-model run and all-model ensemble averages (4.9 and -8.1 mm yr(-1)) near zero, against a background climatological P of similar to 500 mm yr(-1). We found that the time series of actually observed annual P over the MDB was indistinguishable from that generated by a purely random process. Importantly, nearly all the model runs showed similar behavior. We used these facts to develop a new approach to understanding variability in projections of Delta P. By plotting Delta P versus the variance of the time series, we could easily identify model runs with projections for Delta P that were beyond the bounds expected from purely random variations. For the MDB, we anticipate that a purely random process could lead to differences of +/- 57 mm yr-1 (95% confidence) between successive 30 year periods. This is equivalent to +/- 11% of the climatological P and translates into variations in runoff of around +/- 29%. This sets a baseline for gauging modeled and/or observed changes.