Spatiotemporal Modeling of the Impacts of Forest Harvesting, Climate Change and Topography on Stream Nitrates in a Forested Watershed

Abstract

This dissertation is an empirical modeling investigation of the impact of forest harvesting, climate change and topography on stream nitrate fluxes in the Turkey Lakes Watershed (TLW), Ontario, Canada. Data used for this study include topography (DEM), climate (mean monthly temperature and total monthly precipitation), wet nitrogen deposition (total monthly nitrate-N and ammonium-N), nitrate water samples and streamflow in 13 headwater catchments within the TLW. First, a paired-watershed approach was used to examine the impact of forest harvesting intensity on stream water nitrate fluxes by developing transfer function noise (TFN) models that related monthly stream water nitrate fluxes of three treatment catchments to those of one control catchment. Second, TFN models were also developed to relate monthly stream nitrate fluxes in 13 catchments to the temperature, precipitation and wet nitrogen deposition to examine the spatially varying responses of stream nitrate fluxes to changes in climate and bulk deposition. Third, geographically weighted regression (GWR) was introduced to model the spatial and temporal relationships between topography and stream nitrate fluxes in 13 headwater catchments. The results showed that there existed a new phenomenon of clustered wave-up and wave-down of the stream nitrate increases caused by clearcut and selectioncut at the monthly scale, respectively. This phenomenon was never reported by previous studies because it was not possible to be identified with ordinary least squares (OLS) regression at an annual scale. There also existed significant responses of stream nitrate fluxes to wet nitrogen deposition in all catchments at the monthly scale over a long-term record between 1982 and 2003. These responses were previously thought to be lower and masked by the impact of climate variations. There further existed significant spatial and seasonal variability of the relationships between topography and stream nitrate fluxes across space and over time. This variability was largely ignored in previous studies with possibly misleading interpretation on the empirical relations.