Investigating the influence of topography and landscape organization on scaling of small catchment storm runoff generation

Inter-catchment comparison of hydrologic behaviour is challenging in part because of the spatial variability in topography and landscape organization. At the hillslope scale, the influence of surface and subsurface topography on runoff generation is well documented. Both surface and subsurface topography can influence flow on hillslopes delivering water to the stream. At the catchment scale, studies have defined landscape organization as how the stream collects upslope area, in some instances, distinguishing hillslope from riparian or valley-bottom area. Few studies have combined detailed inter-comparison of multiple catchment storm response (e.g. new/old water contributions) with a detailed characterization of topography and landscape organization. In this study, we examine storm runoff generation from 8 small, forested catchments in an attempt to evaluate the influence of catchment topography and landscape organization on scaling of storm response. The term scaling is used here to refer to how storm response changes with catchment area. The 8 nested catchments exhibit significant and varying relief and catchment area. A 1 m x 1 m airborne laser altimetry LIDAR (light detecting and ranging) digital elevation model (DEM) is used to calculate mean slope and topographic wetness index, total catchment area, and distributions of sub-catchment area as collected by the stream channel for each of the 8 nested catchments. Storm runoff generation is quantified by hydrometric, isotopic and hydrochemical methods. Catchment mean residence time (MRT), another measure of hydrologic function by which catchments can be compared, is estimated by baseflow recessional analysis and hydrochemistry. Our analysis shows the three largest and most downstream catchments distinguish themselves with larger MRT and larger valley bottom areas. These three catchments dominate the scaling of storm response, showing significantly larger amounts of new water delivery to the stream channel and suggesting a significant change in dominant runoff mechanisms related to topography and landscape organization.