Uncertainties in runoff projections in southwestern Australian catchments using a global climate model with perturbed physics

- Uncertainties quantification in runoff projections using a GCM with perturbed physics.
- Runoff projections for southwest Western Australia indicate drier conditions.
- Current assessments in runoff projections underestimate uncertainties.

SummaryFuture projections of water supply under climate change scenarios are fundamental for efficient water resource planning. However, runoff projections are affected by uncertainties in the modelling process that limit their utility to decision makers. The main source of uncertainty in runoff projections are the Global Climate Models (GCMs) used to produce future climate projections. The impact on projected runoff of this uncertainty has mainly been assessed through comparison of multi-model runs of future climate with little exploration of uncertainties inside the models due to different parameterisations. Here we investigate the uncertainty response of projected runoff due to perturbed physics parameter variations within a GCM using a novel 2500 member ensemble from the HadCM3L model. Our research evaluates the uncertainties in runoff modelling for southwest Western Australia, a Mediterranean climate region which has experienced reductions in precipitation during the last decades. Results for future projections in southwest Western Australian catchments indicate reductions in modelled precipitation between 0% and 40% and increases in temperature that fluctuate between 0.5 °C and 3 °C by 2050-2080 compared to 1970-2000, which lead to reductions in projected runoff of between 10% and 80%. This range of uncertainty for projected runoff is larger than that calculated for previous estimates of within-model uncertainties of runoff. The perturbed physics approach indicates that current water management assessments underestimate uncertainties in runoff projections.