| The impact of natural and anthropogenic disturbances on catchment hydrological and biogeochemical dynamics are difficult or impossible to capture through experimentation or observation alone. Process-based simulation models can address this need by providing a framework for synthesizing and analyzing data describing catchment responses to climate, harvest, fire, and other disturbances. When properly constrained, models allow a self-consistent representation and analysis of process-level interactions within catchments, as well as the ability to isolate and make inferences about the contribution of specific processes to observed responses. Models can also extend a data set by allowing behavior of unmeasured system components to be inferred. However, existing models are either too simple to capture important process-level hydrological and biogeochemical controls on ecosystem responses to disturbance, or are too computationally expensive to simulate the local dynamics over large watershed areas, or require a high level of expertise to implement.;To this end, a spatially distributed, physically based, eco-hydrological model (VELMA: Visualizing Ecosystems for Land Management Assessments) that is both computationally efficient and relatively easy to implement was collaboratively developed. The model simulates changes in soil water infiltration and redistribution, evapotranspiration, surface and subsurface runoff, carbon and nitrogen cycling in plants and soils, and the transport of dissolved forms of carbon and nitrogen from the terrestrial landscape to streams.;The first part of the study focuses on exploring catchment hydrological responses to forest harvest amount and spatial pattern. VELMA was applied to a small Pacific Northwest Long Term Ecological Research catchment to elucidate how hillslope and catchment scale processes control stream discharge. Results show that the streamflow response is strongly sensitive to harvest distance from the stream channel. Specifically, a 20% clearcut area in the uplands near the catchment divide resulted in an average annual streamflow increase of 53mm, whereas a 20% clearcut near the stream channel resulted in an average annual streamflow increase of 92mm.;The second part of the study focuses on exploring the impact of fire and harvest on carbon and nitrogen dynamics. VELMA was applied to a small Pacific Northwest Long Term Ecological Research catchment (WS10), where two significant disturbance events have shaped the life history of vegetation growth: The first was a stand-replacing fire in circa 1525 A.D. The second was a clearcut harvest in 1975. VELMA was used to reconstruct, analyze and draw insights into the response of Pacific Northwest catchments and specifically Douglas-fir dominated catchments to natural and anthropogenic disturbances.;The third part of the study focuses on exploring catchment biogeochemical responses to forest harvest amount and spatial pattern. VELMA was applied to the same small Pacific Northwest Long Term Ecological Research (LTER) catchment (WS10), to elucidate how hillslope and catchment-scale processes control soil carbon and nitrogen dynamics in response to clearcut. VELMA was previously calibrated and validated at capturing post-fire and post-harvest hydrological and biogeochemical dynamics in WS10. One hundred scenarios, where harvest amounts ranged from 2% to 100%, irrespective of location were conducted.;In the final part of the study, VELMA was used to simulate the impact of future climate change on catchment hydrology and carbon and nitrogen dynamics. VELMA was applied to an intensely studied watershed in the Pacific Northwest: the H.J. Andrews 64 km2 Experimental Forest. The goal was to provide process level insight into the impact of climate change on ecosystem processes at high spatial resolution relevant to formulating management decision. This analysis provide decision makers and resource managers with critical information on the potential extend of impact climate change will have on forest carbon and nitrogen dynamics, site productivity and water quality and quantity. (Abstract shortened by UMI.)... |
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