This study assessed how logging-access roads may have contributed to observed historical increases in peak discharges associated with small and large logged basins in the western Cascades of Oregon. The study was conducted on the Lookout Creek (62km��) and the upper Blue River (118km��) basins. Potential road effects on hydrology were examined using a combination of field surveys and spatial modeling with a geographic information system (GIS). Road networks were similar in both basins with respect to hillslope position, orientation, and stream crossings, but roads in Blue River were constructed one or two decades later than roads in Lookout Creek. A total of 20% (62 km) of the road length was sampled to assess routing of surface flow, using 31 2-km transects stratified by decade of construction and hillslope position. Along each transect, ditches and culvert outlets were examined and this information used to predict the probable routing of water to (1) existing stream channels, (2) newly eroded gullies downslope of culvert outlets, or (3) subsurface flow. Nearly 60% of the surveyed road length appeared to route water directly to stream channels or into gullies. Over time, the length of road connected to stream crossings has decreased, while the length of road discharging runoff that reinfiltrates to subsurface flow has increased, as roads have progressed up hillslopes and onto ridges in Lookout Creek and Blue River. The relatively constant proportion of the road network draining to gullies over time suggests that roads have the potential to become integrated into stream networks, even when constructed on unchannelled hillslope positions. An extended stream network, assumed to exist under storm conditions, was simulated for the basins using a digital elevation model. Although gullies and ditches differ from natural channels, extrapolation of field surveys using the GIS suggested that roads might extend the stream network by as much as 40% during storm events. It is hypothesized that such an effect could decrease the time of concentration of stormflow and contribute to higher peak discharges observed after clearcutting and road construction in these basins. Differences in the magnitude of road effects on peak flow generation may occur among road systems according to hillslope position of roads, road age, soil saturation, geologic substrate, and climate. These differences may explain the range of observed results from paired-basin studies examining road effects on hydrologic response. / Graduation date: 1994

Rainfall interception is a primary control over the moisture input to a forested ecosystem through the partitioning of precipitation into throughfall, stemflow, and an evaporated component (i.e. the interception loss). Rainfall interception is a spatially and temporally varying process at multiple scales, but heterogeneity in interception processes are poorly understood and poorly described in the literature. We need to know how net precipitation varies in ecosystems because natural systems are driven by non-linear ecohydrological processes where mean values cannot capture localized effects or the cumulative consequences associated with an extremely heterogeneous input. In this thesis, we present two studies that investigate the heterogeneity of interception loss and throughfall in a forested catchment in the western Cascades range of Oregon. In one study, we examined the spatio-temporal patterns among point measurements of throughfall depth and isotopic composition to determine the cause of isotopic differences between throughfall and rainfall. Our results indicated that the residual moisture retained on the canopy from previous events plays a major role in determining the isotopic composition of the next event's throughfall. Differences between the isotopic composition of throughfall samples could indicate further partitioning of throughfall into various flow-paths from the canopy. The second project examined the question of how vegetation variability and terrain complexity drive interception loss heterogeneity at the whole-catchment scale. We applied a simple interception model to a watershed gridded at a 50 m resolution to investigate the relative importance of topographic and vegetative controls over the spatial variability of interception loss. We found that storm characteristics are crucial regarding the impact of spatial heterogeneities in vegetation and evaporation rates. In the Pacific Northwest climate, interception loss is not highly variable for the majority of the year because the annual precipitation is dominated by large storms with low interception losses. However, the net precipitation input to a watershed becomes extremely heterogeneous in the summer due to high interception loss variability. Summer interception loss could be an important control over the spatial variability of the availability of moisture, coinciding with when vegetation is most water-limited. / Graduation date: 2013