Process controls on regional flood frequency: Coefficient of variation and basin scale

The coefficient of variation (CV) of maximum annual floods is examined to understand the effects of process controls and catchment size. A derived flood frequency model is used to interpret data from 489 catchments in Austria. At the core of process controls appears to be the interaction of catchment response time and storm duration, but the magnitude is not large, and often this interaction is hidden by other processes. The dependence of rainfall intensity and duration is clearly very important and reduces CV significantly. Increasing channel travel times with catchment scale tend to translate into decreasing CVs with area for small catchments while they tend to translate into increasing CVs with area for larger catchments. Nonlinear runoff processes, including threshold effects, is the main mechanism for increasing CV. They give rise to complex patterns in the relationship between CV and area. Base flow has been used as a surrogate for a number of processes, such as seasonality of streamflow. It always decreases CV and, in particular, leads to a significant decrease of CV with area. Both the observed tendency of CV to decrease with area and the scatter in the data are the result of a complex interplay of a number of processes which allows various alternative interpretations. Depending on which processes dominate under a particular hydrologic regime, different patterns arise. It appears that the explanations of the relationship between CV and catchment scale suggested in the literature are too simplistic. The case is made for using the concept of hydrologic regimes and process studies of the type presented here to help delineate homogeneous regions for regional flood frequency analyses in a physically consistent way.