Hydrologic landscape classification evaluates streamflow vulnerability to climate change in Oregon, USA

Classification can allow for evaluations of the hydrologic functions of landscapes and their responses to stressors. Here we demonstrate the use of a hydrologic landscape (HL) approach to evaluate vulnerability to potential future climate change at statewide and basin scales in the state of Oregon. The HL classification has five components: climate, seasonality, aquifer permeability, terrain, and soil permeability. We evaluate changes when the 1971-2000 HL climate indices are recalculated using 2041-2070 simulation results from the ECHAM (European Centre HAMburg) and PCM (Parallel Climate Model) climate models with the A2, A1b, and B1 emission scenarios. Changes in climate class were modest (4-18%) statewide. However, there were major changes in seasonality class for five of the six realizations (excluding PCM_B1): Oregon shifts from being 13% snow-dominated to 4-6% snow-dominated under these five realizations, representing a 56-68% reduction in snowmelt-dominated area. At the basin scale, simulated changes for the Siletz Basin, in Oregon's Coast Range, include a small switch from very wet to wet climate, with no change in seasonality. However, there is a modest increase in fall and winter water due to increased precipitation. For the Sandy Basin, on the western slope of the Cascades, HL climate class does not change, but there are major changes in seasonality, especially for areas with low aquifer permeability, which experiences a 100% loss of spring seasonality. This would reduce summer baseflow, but effects could potentially be mitigated by streamflow buffering effects provided by groundwater in the high aquifer permeability portions of the upper Sandy. The Middle Fork John Day Basin (MFJD), in northeastern Oregon, is snowmelt-dominated. The basin experiences a net loss of wet and moist climate area, along with an increase in dry climate area. The MFJD also experiences major shifts from spring to winter seasonality, representing a 20-60% reduction in snowmelt-dominated area. Altered seasonality and/or magnitude of seasonal streamflows could potentially affect survival, growth and reproduction of salmonids in these watersheds, with the greatest effects projected for the MFJD. A major strength of the HL approach is that results can be applied to similarly classified, ungaged basins. Information resulting from such evaluations can help inform management responses to climate change at regional and basin scales without requiring detailed modeling efforts.