Management of public and private forest lands in the Upper Columbia River basin is necessary to ensure the sustainability of natural ecosystems and enhance protection and recovery of fish and wildlife populations. By 2030, summertime surface water demand is expected to significantly exceed supply in most years in many Upper Columbia tributaries; in some years, a portion of these tributaries will exceed supply even outside the summer months. Forest restoration (i.e., timber harvest, prescribed burning, thinning) reduces canopy cover and, subsequently, has been shown to increase snow accumulation and total runoff volume. Targeted forest restoration actions have the potential to help increase late season flows, while possibly reducing peak flows during the fall and winter to better meet salmonid habitat requirements.
The impact of a particular forest management activity or disturbance on watershed hydrologic processes is highly variable, depending on local climate, soils, vegetation, topography, dominant basin orientation/aspect, and the spatial pattern of disturbance. Hydrologic models can be an important tool for estimating how changes in forest management practices can affect moisture states such as canopy interception storage, snow water equivalent, and soil moisture, and fluxes such as evapotranspiration, sublimation, and streamflow. They also provide a method of evaluating the likely effects of alternative forest management practices in isolation; that is, without layering in the additional complexity of variation in other basin properties and meteorological forcings.
Under this project, funded by the Upper Columbia Salmon Recovery Board (UCSRB) through a grant from the Washington State Department of Ecology’s Office of the Columbia River, researchers at Pacific Northwest National Laboratory (PNNL) evaluated the ability of a distributed hydrologic model to accurately predict hydrologic properties and changes associated with a range of precisely defined forest restoration scenarios in snow-dominated watersheds within the Wenatchee, Entiat, Methow, and Okanogan subbasins under current and future climate conditions. As part of this effort, PNNL researchers evaluated whether restoration treatments of the type and magnitude anticipated for management on the federal forest have the potential to measurably increase flows in UCSRB priority watersheds.
Results derived from this effort:
- demonstrate the ability of the Distributed Hydrology Soil Vegetation Model (DHSVM) to accurately predict hydrologic properties and changes for select forested subbasins in the north-central Washington State.
- demonstrate the ability to prescribe realistic forest restoration scenarios in high spatial detail (90 m) within DHSVM to represent a range of management actions, including mechanical thinning and prescribed burns.
- suggest forest restoration will increase peak snow water equivalent and annual water yield (with reduction in overstory fractional coverage) under both current and future climate conditions, consistent with published paired watershed studies.
- suggest that the impact of forest restoration on the timing of snowmelt and streamflow varies from year to year and is highly dependent on local meteorological conditions (including solar radiation, downward longwave radiation, air temperature, humidity, and wind speed) and particular forest restoration scenarios (i.e., amount of overstory removed).
- suggest that under future climate conditions there will be a general increase in late fall and winter flows for all restoration scenarios and a decrease in flow during the summer and early fall relative to current climate conditions. However, forest restoration tends to decrease fall and winter flows and generally increases summer flows compared to a future climate condition no restoration baseline.