Tuesday, October 12, 2004
3:30 to 5:00
Geologic and topographic controls on streamflow response to climate change in the Western U.S
The snowmelt-dominated Cascade and Sierra Mountains of the western U.S. provide critical water supply for agriculture, municipalities, ecosystems, and over 45 million people. Recent analyses show that this region is particularly sensitive to current and projected climate warming trends, resulting in reduced snow accumulation, earlier spring melt, and leading to a reduction in summer streamflow. While these broad regional scale characterizations identify climatic gradients as the first-order controls on spatial variability in changing streamflow regimes, the potential for other landscape controls, notably geology and vegetation, to mediate this response has received less attention.
Previous research has revealed strong contrasts in spatial patterns of summer streamflow in the Cascade Mountains of Oregon between the geologically distinct High and Western Cascade regions. A key control on streamflow response between these two regions is the partitioning of water input between a fast-draining shallow subsurface flow network (Western Cascades) versus a slow-draining deeper groundwater system (High Cascades). These differences result from extremely high contrasts in rock permeability and porosity and drainage density between landscapes dominated by young versus old volcanic rocks.
We consider how geologically-based differences in groundwater storage capacity can significantly alter streamflow response to climatic warming. In particular, we expect that for the young volcanic terrains comprising the High Cascade Range of Oregon and Northern California, ground water storage is of sufficient magnitude to buffer potential changes in snowpack volume, hence summer streamflow, due to changing climate. Older volcanic and granitic landscapes in the Oregon Western Cascades and California Sierras, in contrast, will be much more sensitive to diminished snowpacks and summer streamflow changes. Even within the Sierras, local variations in bedrock geology and associated differences in volume and seasonal fluxes of subsurface water will likely result in significant spatial variability in sensitivity to climate forcing. Taken together, these results imply that current models linking climate and streamflow changes need to account for differences in groundwater storage as a first-order control.
Gordon Grant is a Research Hydrologist with the U.S. Forest Service at
the Pacific Northwest Research Station in Corvallis, Oregon, and also Professor
(Courtesy) in the Departments of Geosciences and Forest Engineering at Oregon
State University. He received a Ph.D. in fluvial geomorphology from Johns
Hopkins University in 1986. Since then, his research has focused on the
response of rivers to changes in stream flow and sediment transport due
to land use impacts, dams and dam removal, and climate change. This work
has included extended collaborations with research groups in Japan, China, and Italy . Currently he is Deputy Editor for geomorphology for Water