Trees, snow, and flooding: An investigation of forest canopy effects on snow accumulation and melt at the plot and watershed scales in the Pacific Northwest

Several major Pacific Northwest floods over the last decade have been attributed in part to the effects of logging. This concern has heightened the need for better scientific understanding of forest canopy effects on snow accumulation and melt. During a three year field investigation in the Umpqua National Forest, OR (elevation 1200 m), weighing lysimeters and cut-tree experiments were used to observe snow interception and its fate while providing continuous observations of below-canopy snowpack evolution. Approximately 60 percent of annual snowfall was intercepted by the canopy (maximum of 40 mm water equivalent). Approximately 72 percent and 28 percent of the intercepted snow was removed as meltwater drip and large snow masses, respectively. Apparent sublimation rates from the intercepted snow were less than 1 mm per day.; Snow interception and ancillary observations, including standard micrometeorology, were used to develop and test an energy balance model of snow as affected by the forest canopy. Although initial implemented and tested on the Umpqua data, the model was designed for implementation in spatially distributed hydrology models for application at larger scales. Intercepted snow load was modeled as a function of Leaf Area Index. Removal of intercepted snow by sublimation, melt-water drip and mass release was explicitly modeled. A fractional canopy coverage parameter was used to scale from the plot to the stand scale.; The snow model was incorporated into a spatially distributed hydrology model to explore the effect of forest canopy removal on floods in maritime climates like the Pacific Northwest. A sensitivity analysis of the 10 largest floods that occurred between 1988 and 1996 on the Snohomish River, which drains the western slopes of the Cascade Mountains in Washington State, was conducted. Relative to the historic canopy cover, predicted peak streamflow was increased by an average of 3 percent with the current vegetation cover. Larger increases (>10 percent) were predicted for headwater basins, in which forest harvest has been concentrated, and events with a large snowmelt contribution. The contribution of increased snow accumulation and increased snow melt rates resulting from forest harvest was determined for each flood as a function of elevation.