Comparing instrumental, palaeoclimate, and projected rainfall data: implications for water resources management and hydrological modelling

Study Region: The Lockyer Catchment, Queensland, Australia. Study Focus: Future rainfall projections are usually presented as a percentage change from current climate, where current climate is defined using relatively short instrumental records. However, palaeoclimate reconstructions demonstrate that instrumental data does not capture the full range of climate variability that has occurred. Understanding natural climate variability, and how it compares to climate model projections for the future, requires the use of (a) instrumental and palaeoclimate data to quantify the range of historical variability and (b) climate model data to quantify if/how things could change in the future. Considering this, we compare instrumental, palaeoclimate, and projected rainfall statistics for the Lockyer Catchment. New hydrologic insights for the region: We found that, in the Lockyer Catchment, instrumental data insufficiently captures past variability and plausible projections of rainfall decreases in the future. We also found that at mid-21st and late 21st century time periods decreases in annual average rainfall, rainfall variability, and ninetieth percentile rainfall outside the confines of instrumental and palaeoclimate variability are plausible. Also, when considering variability in the palaeoclimate record compounded with projected rainfall trends, much larger decreases in rainfall are plausible than when only considering instrumental and projected rainfall. The implications of these results are discussed in terms of (a) calculating the sustainable yield of water supply catchments and (b) estimating catchment runoff using hydrological models.