The Hydrological Challenges of Scale Dependency

English Abstract:

From the earliest beginnings of Hydrology, the scale dependence of catchment response has been explicitly recognised, for example in regional analyses of flood frequency and unit hydrograph response. And when continuous simulation conceptual rainfall-runoff models evolved to the point where the need for parsimony in parameterisation was recognised, regionalisation for ungauged catchments led to explicit quantification of parameter scale-dependence. However, for physics-based models, there has been a failure to account for the scale-dependence of parameters, despite clear evidence that appropriate representation of basic physical properties within models is dependent on the scale of model temporal and spatial discretization. There has perhaps been a naïve belief that physically-based properties are by definition scale-independent. Failure to progress understanding of these issues poses an urgent set of challenges. For example atmospheric models are now routinely applied at km scale over continents, but despite recent discussion of hyper-resolution modelling, the implications for associated hydrological discretization and parameterization are unclear. This paper discusses these issues and a framework for analysis. Methods include numerical experimentation to determine emergent properties, empirical analysis of alternative discretization scales and methodologies, and the analysis of large-scale model performance. Current examples from Western Canada are presented.