A holistic, multi-scale dynamic downscaling framework for climate impact assessments and challenges of addressing finer-scale watershed dynamics

•Present a state-of-the-art holistic, multi-scale approach.•Address climate change impact on flow regime at the “scales of human decisions”.•Future climate projections are downscaled to a “point” scale using a weather generator.•Climate natural variability triggers hydrologically-mediated non-uniqueness.•Climate uncertainty masked by uncertainties arising due to non-linearities in smaller-scale processes. We present a state-of-the-art holistic, multi-scale dynamic downscaling approach suited to address climate change impacts on hydrologic metrics and hydraulic regime of surface flow at the “scale of human decisions” in ungauged basins. The framework rests on stochastic and physical downscaling techniques that permit one-way crossing 106–100m scales, with a specific emphasis on ‘nesting’ hydraulic assessments within a coarser-scale hydrologic model. Future climate projections for the location of Manchester watershed (MI) are obtained from an ensemble of General Circulation Models of the 3rd phase of the Coupled Model Intercomparison Project database and downscaled to a “point” scale using a weather generator. To represent the natural variability of historic and future climates, we generated continuous time series of 300years for the locations of 3 meteorological stations located in the vicinity of the ungauged basin. To make such a multi-scale approach computationally feasible, we identified the months of May and August as the periods of specific interest based on ecohydrologic considerations. Analyses of historic and future simulation results for the identified periods show that the same median rainfall obtained by accounting for climate natural variability triggers hydrologically-mediated non-uniqueness in flow variables resolved at the hydraulic scale. An emerging challenge is that uncertainty initiated at the hydrologic scale is not necessarily preserved at smaller-scale flow variables, because of non-linearity of underlying physical processes, which ultimately can mask climate uncertainty. We stress the necessity of augmenting climate-level uncertainties of emission scenario, multi-model, and natural variability with uncertainties arising due to non-linearities in smaller-scale processes.