Simulating the impact of road construction and forest harvesting on hydrologic response

This paper incorporates a conceptual model of the effect of roads and forest harvesting on hillslope soil moisture and runoff production into a hydroecological modelling system and discusses model results for a range of scenarios for a small catchment in the Western Oregon Cascades, USA. The model is used to explore the implications of road cut depth and road drainage patterns on seasonal hydrologic responses including runoff production, soil moisture and ecological processes such as evapotranspiration. By examining hydrologic response within a seasonal and hillslope context, we illustrate the complex role played by roads in terms of both the spatial and temporal persistence of the effects of an increase in local drainage efficiency associated with particular road segments. Model results are compared with observed outflow responses for a paired catchment study using the test case watershed. (catchment area in UK terminology). Results show the potential for an ecologically significant change in soil moisture in the area downslope from the road. These changes are mediated by the drainage patterns associated with roads, specifically whether road culverts serve to concentrate or to diffuse flow. Field verification of these findings presents an avenue for further research. The modelled effects on seasonal outflow response are less significant but do show clear temporal patterns associated with climate pattern, hillslope drainage organization and road construction. Comparison between modelled and observed outflow response suggests that the model does not yet capture all of the processes involved in assessing the effects of forest road construction. Copyright © 2001 John Wiley & Sons, Ltd.