Modeling seasonal hydrologic response to forest harvesting and road construction: the role of drainage organization

This study addresses several challenges in the application of hydro-ecological models as tools for studying the spatially distributed pattern of soil moisture and run off production inthe context of land use change. This research is comprised of two components: a) The development and assessment of a methodology used to model hillslope drainage organization at the small basin scale and b) application of this methodology for assessing forest harvesting and road construction effects on hydrologic response.
This investigation focuses specifically on the Pacific Northwest region where the impact of forest harvesting on hydrologic response is an important and controversial issue. This work presents modifications to the Regional Hydro-Ecological Simulation System (RHESSys)modeling framework and discusses specific issues related to modeling landscape drainage organization in hydro-ecological models. Within RHESSys, tradeoffs between using aspatially explicit routing approach versus a less detailed statistical approach to modeling distributed hydrology are explored. Results from this study illustrate the limitations of using a statistical approach in modeling drainage organization and the sensitivity of both routing methods to calibration and errors in topographic and soil input data. Different strategies for landscape representation are also compared. Results highlight the significant role played by fine scale topographic variability in areas adjacent to streams, and the importance of these local areas in controlling watershed hydrologic response.
Application of the RHESSys modeling tool is further developed by incorporating a conceptual model of forest harvesting and road construction effects on hydrology. Comparing simulations with observed streamflow for a study catchment in the Pacific Northwest provides an illustration of forest harvesting and road construction effects on hydrologic response. The model illustrates how the effects of these disturbances are dependent upon road construction characteristics and the spatial distribution of vegetation and its water use. Road construction characteristics include both culvert drainage topology and the position of the road relative to hillslope drainage organization. Effects on downslope soil moisture and the spatial and temporal dynamics of seasonal runoff production illustrate the potential ecological significance of different disturbance scenarios.