The impact of a bias-correction approach (delta change) applied directly to hydrological model output when modelling the severity of ice jam flooding under future climate scenarios

Assessing the impact of future climate on the severity of ice jam floods (IJFs) is an essential component of a flood mitigation strategy for many ice jam-prone northern communities. The general circulation model (GCM) outputs are used to derive hydrological conditions under future climate scenarios. Although GCMs are often downscaled to the point of interest, there can still be significant differences between modelled climate scenarios and historically observed climate scenarios. Therefore, the changes between the model simulated baseline and future scenarios are applied to observed baseline values to derive projected future values. In IJF modelling, such projected values are provided as input (boundary conditions) to assess the frequency and severity of future IJF events. Different methods can be used to calculate the climate change signal (difference between model-simulated future vs model-simulated historical conditions), and depending upon the method employed, results can vary. In this study, we evaluate the impact of using different delta change methods (i.e., absolute vs relative) to directly bias-correct the hydrological model output on the assessment of frequency and severity of IJFs under future climate. The method was tested in the Athabasca River at Fort McMurray in western Canada, an ice jam-prone location, to assess the IJF probabilities and intensities in the 2041–2070 period. Our results indicate that there is a notable difference in the projected frequency and severity of IJFs between absolute and relative delta change approaches, suggesting the methods should be carefully selected and results cautiously interpreted.