Abstract ID: 3.11478 | Accepted as Talk | Requested as: Talk | TBA | TBA

In western North America many watersheds that span the rain-snow transition zone are undergoing rapid changes in both land cover and rain-snow regime driven by forest operations and climate warming, respectively. A critical question in this region are how flow regimes are projected to change from these two drivers, both independently and in combination. The Distributed Hydrology Soil Vegetation Model (DHSVM) was used to address this question for projected rates of land cover and climate change across a range of scales for a snow-dominated watershed that is projected to shift from a snow to a rain-dominated hydroclimatic regime. The model was parameterized and tested with detailed data collected within the Mica Creek Experimental Watershed, a 28 km2 basin in the interior Pacific Northwest, USA. Streamflows were simulated for harvest rotations of 40 and 80 years to span the ranges of harvest intensities on private and public lands. At the smallest (~1 km2) scale, annual and low flows followed a typical trend of an initial increase followed by a decrease below baseline and gradual return to preharvest conditions. Peak flows remained elevated until canopy closure occurred after approximately 15 years followed by a return to baseline. At the largest (28 km2) scale, all flows were slightly elevated due to the active management regime that produced a landscape mosaic of stand ages. Projected flow changes based on 10 GCM outputs for RCP 4.5 and 8.5 emissions scenarios indicated slight increases in annual yields and peak flows, but relatively large declines in warm season low flows. Flow timing for an extreme case of 100% canopy removal resulted in a 12-day advance in the half-mass date, whereas for the most extreme emissions scenario the half mass date advanced by 56 and 69 days for mid- and late-century conditions, respectively. Canopy reduction due to harvest may therefore positively or negatively interfere with climate-driven flow changes depending on both watershed scale and species age diversity. Changes in flow timing are negligible relative to much stronger climate-driven changes resulting from a shift from a snow to rain-dominated hydrological regime.