Forest management linkages to snow, wildfire, and hydrology in the Oregon Cascades

Forests in the Oregon Cascades play a critical role in mountain ecosystems. Complex linkages between forest land use, climate, snow hydrology, wildfire, and water resources require a mixed approach to forest management. Management techniques include forest thinning and wildfire suppression strategies across watershed elevation zones. Burned area in the Cascades has seen a steady rise with projections showing increases of 200-900% by end of century. Concurrently, the maritime snowpacks of the Cascades are declining with a greater proportion of precipitation falling as rain, affecting snowpack and reservoir water storage. Reduced snowpacks affect water for the forest thereby increasing tree mortality and fire risk. Changing snowpacks in forested watersheds also affect downstream reservoir inflows and hydropower generation.

The McKenzie River Basin (MRB; Oregon) supplies water and hydropower to the city of Eugene, Oregon and is a key tributary of the Willamette River. Like most of the Cascades, the MRB is vulnerable to wildfire and decreasing snowpack. We used the Willamette INFEWS Model, an integrated social-ecological systems watershed model, to explore complex interactions of forest structure, snow, fire, and water resources. The Model uses a series of submodules to describe mountain snowpack evolution, forest state and transition, wildfire, and watershed runoff/streamflow, reservoir management, and hydropower generation. To test snowpack and fire sensitivities we adjusted two parameters; wildfire suppression rate and forest harvest rate.

Preliminary results indicate differences in wildfire activity depending on spatial forest treatments. Differences in maximum snow water equivalent (SWE) and snow disappearance date (SDD) varied with treatment and elevation. We investigate the role of SWE and SDD on fire frequency, burned area, and subsequent upland water yields. We also investigate how changes in the upland water regime affect reservoir inflows and hydropower generation.

Spatially distributed combined models are useful tools to investigating the tradeoffs in forest management practices. This research will help managers develop anticipatory capacity and carefully informed approaches to forest management.