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What Will Oregon’s Future Streamflow Regimes Look Like? Integrating Snowpack and Groundwater Dynamics

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  • Spatial patterns of summer streamflow in the Cascade Mountains of Oregon vary dramatically between the geologically distinct High and Western Cascade regions. A key control 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 large contrasts in rock permeability, porosity, and drainage density between landscapes dominated by the older Western Cascade versus younger High Cascade volcanic rocks. How do these geologically-based differences in groundwater storage capacity affect streamflow response to projected climatic warming? We initially expected that for the High Cascades of Oregon and Northern California, large groundwater storage will lead to groundwater recharge independent of precipitation type (rain or snow), thereby buffering low flows against potential changes in snowpack volume due to warming climate. We also expected that low groundwater storage in the older volcanic and granitic landscapes of Oregon and California will result in greater sensitivity to diminished snowpacks and summer streamflow changes. By coupling simple theory with hydrologic modeling, we found that interpreting low flow response to warming involves a convolution of both the snowpack and groundwater dynamics. Using this approach, the High Cascades displays much greater low flow sensitivity to climate change than the Western Cascades. Because the High Cascades discharge groundwater throughout the summer season, both timing of recession and annual fluctuations in total precipitation are reflected in changes in late summer streamflow. The Western Cascades in contrast, displays much less late season sensitivity to changing climate; streamflow is always very low in late summer regardless of winter recharge. We extend these results across the entire western Cordillera and consider implications for water supply in the future. These results imply that current models linking climate and streamflow changes need to account for differences in groundwater storage as a first-order control.
  • Presented at The Oregon Water Conference, May 24-25, 2011, Corvallis, OR.
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