Thermal insulation versus capacitance: a comparison of shading and hyporheic exchange on daily and annual stream temperature patterns
Fogg, Sarah Kathleen
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Channel shading and hyporheic exchange both effect daily and annual stream temperature cycles. In streams with thermal regimes that are too warm to support native biota, increasing shading or hyporheic exchange have largely been thought of as interchangeable management strategies because they influence summertime stream channel temperatures in similar ways. But, shading and hyporheic exchange operate via different mechanisms and influence stream temperature differently at differently times of the year. To understand daily and seasonal differences of shading and hyporheic exchange on stream channel temperature we used a process-based heat-budget model of channel temperature. Our model incorporates stream channel-atmospheric energy exchanges and a novel channel-subsurface heat exchange model that more appropriately represents the effects of hyporheic residence time distributions. We used our model to conduct an in silico experiment where we vary shading and hyporheic exchange on the same stream reach. In summer, the cooling and damping of channel temperatures associated with an expansive, coarse-grained hyporheic zone were similar to shading effects. However, the differences between shading and hyporheic exchange effects were most pronounced in the winter when channel warming associated with hyporheic exchange was substantially greater than warming associated with shade. By interpreting the changes in heat fluxes between shading a stream and adding hyporheic exchange, we find that shading acts as a thermal insulator and hyporheic exchange acts as a thermal capacitor. Our results show that shading and hyporheic exchange can have similar and differently effects on stream channel temperatures depending on what part of the year the effects are investigated, which has important management and modeling implications. Geography and geomorphic context of a stream are important considerations when choosing shading or hyporheic exchange for thermal restoration. In bedrock-confined streams that historically had closed canopies, shading has the greatest potential to reduce summer temperatures while restoration of hyporheic restoration is impractical in these systems. In contrast, in large, coarse-grained alluvial stream reaches, where riparian vegetation is historically sparse, restoring hyporheic influences has the greatest potential for reducing summer temperatures while increasing streamside shading is likely unattainable.