The Influence of Spatial Variability of Width Functions on Regional Peak Flow Regressions

The authors investigated the relation between The Width Function (WF) and the regional variability of peak flows. They explored 34 Width Function Descriptors (WFDs), in addition to drainage area, as potential candidates for explaining the regional peak flow variability. First, using hydrologic simulations of uniform rainfall events with variable rainfall duration and constant rainfall intensity for 147 watersheds across the state of Iowa, they demonstrated that WFDs are capable of explaining spatial variability of peak flows for individual rainfall-runoff events under restricted physical conditions. This theoretical exercise indicates that the inclusion of WFDs should drastically improve regional peak flow estimation equations with a reduction of the Root Mean Square Error (RMSE) by more than half in comparison with a regression model based on drainage area only. The authors followed the simulation with an analysis of estimated peak flow quantiles from 94 peak flow gauges in Iowa to determine if the WFDs have a similar explanatory power. The correlations between WFDs and peak flow quantiles are not as high as those found for simulated events, which indicates that results from event scale simulations do not translate directly to annual peak flow quantiles. The spatial variability of peak flow quantiles is influenced by other physical and statistical processes that are also variable in space. These results are consistent with recent work on event-based scaling of peak flows that shows that the spatial variability of flood mechanism is larger than the one expected from geomorphology alone.