EFFECTS OF FOREST MANAGEMENT AND ENVIRONMENTAL VARIABLES ON FOREST HYDROLOGY IN SMALL PACIFIC NORTHWEST WATERSHEDS
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Alteration of the tree canopy cover can substantially influence the hydrologic processes in forest watersheds. Human interventions, via forest harvest or natural disturbances, such as wildfires, are common in the US forests. Differences in topography and climate across varied terrains can result in different effects of tree canopy alteration on the forest ecosystem. This dissertation examines the effects of forest management and environmental variables on forest hydrology in small US Pacific Northwest watersheds. Specific objectives were:1) To investigate relationships between the exposure of mineral soil after wildfire and topographic variables;2) To investigate the effect of all-wave incoming solar radiation derived from LiDAR data on snow processes; and,3) To assess changes in snow accumulation and melt as a result of changes in canopy cover, and compare a hemispheric photograph method and LiDAR for quantifying canopy cover changes.To achieve the first objective, we assessed the spatial distribution of fire effects along hillslopes using Geographic Information System (GIS), and derived a regressional relationship to estimate post-fire exposed mineral soil from key topographic variables, namely, aspect, solar radiation, and profile curvature. Aspect and profile curvature were the leading variables in the regression model for predicting exposed mineral soil.In the second study, we estimated and used values of short- and longwave radiation in Generalized Additive Models (GAMs) to explain variations in snow water equivalent (SWE) and snowmelt over a period of three years with distinct climatic conditions. Elevation, latitude, longitude, and time variables explained most of the variation in both SWE and snowmelt models. Yet the additional LiDAR-derived radiation variables were statistically significant, and including these variables in the original GAMs improved the models, especially those with the time variable.In the third study, we conducted a Before-After/Control-Impact (BACI) analysis to examine how the reduction in canopy cover due to treatments influenced the snow accumulation and melt. The results showed that the treatment effect in the study watersheds that received a thinning operation was significant on SWE and it was not on snowmelt. Field canopy estimates were significant in the models applied to the thinned watersheds while LiDAR estimates, although correlated to the field estimates, were not significant in the statistical models.