The impact of differences in rainfall seasonality on antecedent catchment wetness and flood risk

Abstract

The impact of climate change on flood extremes is of major importance globally. It is recognised that increased atmospheric temperatures and specific humidity may cause the intensity and frequency of extreme rainfall events to change. Furthermore, changes in rainfall patterns are likely to affect the wetness of a catchment prior to flood events and in turn the resulting flood risk. The competing interaction of these factors means that their combined impact on flood risk is uncertain. This paper compares and contrasts climate change impacts for different rainfall seasonality on both antecedent catchment wetness (ACW) and flood risk. The paper uses a virtual catchment experimental approach, to investigate changes in two hydro-climatic regimes with different rainfall seasonality. The first regime is dominated by winter rainfall and the second by summer rainfall. Rainfall for both hydro-climatic regimes is derived from statistically downscaled rainfall for the historical climate and for the A2 emissions scenario future climate. The study uses a daily lumped rainfall-runoff model, GR4J, to simulate the ACW and runoff response to the changes in the two regimes. As part of this analysis each regime’s sensitivity to increases in potential evapotranspiration (PET) is assessed. The impact of a decrease in annual total rainfall along with minimal changes in annual maximum rainfall as well as increases in PET on the resulting annual maximum flow and ACW are evaluated. It was determined that flow and ACW decreased under a future climate in both experiments for all PET increases. Also, as ARIs increase the decreases in flow and ACW become smaller. The sensitivity of flow and ACW to changing PET decreased with increasing average recurrence interval (ARI) for the winter-dominant rainfall regime. However, the sensitivity of ACW and flow to changing PET possessed opposing trends for the summer-dominant rainfall regime. Due to these complex relationships between ACW, annual maximum flow and ARI techniques that appropriately account for changes in ACW will be essential to obtaining robust estimates of future flood risk. As a result it was determined that continuous simulation is likely to be required for catchments with similar hydrology to obtain robust estimates of flood risk under a future climate. Further work using more detailed hydrological models, alternative methods of rainfall downscaling and a larger variety of rainfall regimes is required to evaluate the generality of these conclusions.