Projecting changes in flood event runoff coefficients under climate change

Abstract

Climate change is projected to impact on the magnitude, spatial location, and timing of large precipitation events, with evidence for this already present in the historical record. However, determining the impacts of these precipitation changes on floods is non-trivial as floods are a result of many compounding factors. For example, changes in the timing and spatial distribution of rainfalls influence soil moisture conditions prior to the flood-causing precipitation event, which in turn change the peak and volume of the resultant flood. Despite this, the impact of climate-changed precipitation on runoff and flood volumes has yet to be quantified historically or examined under different scenarios of climate change.It has previously been suggested that in Australia, the impact of increased precipitation on floods have been partially offset by decreases in soil moisture prior to storms. Here, we seek to quantify the change in flood-event runoff coefficients resulting from climate change across Australia, where it is hoped that such information can be used to help inform the projections of floods resulting from climate-change-induced precipitation changes. We use an event runoff coefficient to assess the flood runoff response to precipitation events containing the annual maximum three-day rainfall event. The assessment was applied to 467 unimpaired catchments in Australia ranging across arid, temperate, and tropical climates. Catchments in temperate climates largely show decreasing trends in event runoff coefficients while increasing trends were observed in tropical regions. Statistically significant trend magnitudes of up to 0.13 per decade were found.Runoff outputs from a semi-distributed landscape model were then used to assess projected runoff responses under different scenarios of future climates. The outputs of modelled runoff were first validated and shown to adequately capture the observed spatial and temporal trends in flood event runoff coefficients despite underestimating the trend magnitude. The projections showed that the median proportion of rainfall converted to runoff from large rainfall events will decrease into the future across over 80% of unimpaired catchments in Australia under climate change scenarios of RCP 4.5 and RCP 8.5. Future work will investigate how these reductions in flood volume are best represented by rainfall loss models used widely in operational flood forecasts and design applications.