Physics-based simulations of hydrologic-response and cumulative watershed effects

This study quantitatively addresses, via simulation, the impacts of forest management practices on near-surface hydrologic response at the catchment and watershed scales. The simulations were conducted with the Integrated Hydrology Model (InHM) for the North Fork of Caspar Creek Experimental Watershed, located near Fort Bragg, California. InHM is a comprehensive physics-based hydrologic-response model. The North Fork watershed (including 11 tributary catchments) is the site of an ongoing study monitoring the impacts of forest practices.; InHM was parameterized and calibrated using existing data and new field measurements of soil-hydraulic properties. The simulations were conducted for three wet seasons: before logging, after logging, and after a period of regrowth. The increases in throughfall and decreases in potential evapotranspiration related to timber harvesting had significant impacts on the simulated hydrologic response for the Caspar Creek catchments. The simulated increases in discharge depths and peak discharges were considerably higher after partial and full clearcut harvesting.; Hypothetical-reality cumulative watershed effects (CWEs) simulations were carried out to examine potential impacts of alternative timber harvest levels and methods relative to these that occurred in the North Fork watershed. The results from these simulations show that the increases in the simulated discharge after clearcutting were significant for the catchment and watershed scales and that relatively small changes in soil-hydraulic properties produced substantial changes in the hydrologic response.; The simulations in this study clearly illustrate that timber harvesting can alter the streamflow generation mechanisms and patterns within a given catchment. The effort reported here sets a firm foundation for investigating complex processes leading to CWEs, such as flooding, slope-stability, sediment transport, and their impacts on salmon habitat.