Poster: Thermal fluxes and impacts in small urban headwater catchments: a question of special and temporal scales.
 
Belt, K. T., Pouyat, R. V., Heisler, G., Taylorson, A., Stubbs, J. and B. Smith

 
Water and material fluxes from urban patches to small streams are modulated by extensive “engineered” drainage networks. Small urban headwater catchments are different in character and function from their larger receiving streams and need to be studied as unique functional hydrologic units if impacts on their biota, as well as downstream linkages are to be understood. As part of the BES LTER project, over twenty small catchments from the Gunpowder River, Herring Run and Gwynns Falls watersheds were selected for continuous temperature monitoring to represent a gradient of catchments with various amounts of forest and impervious surface cover (ISC) in the catchment. Sites generally had catchment-wide riparian canopy cover and, many were part of a nested design that allows upstream small catchments (100 ha) to be compared directly with their “small stream” receiving waters (ca 500 ha and larger). We discuss preliminary results from summer monitoring; data analysis and monitoring continue. Suburban sites with greater ISC generally had higher summer temperatures, although some catchments with extensive forest cover resembled 100% forested catchments. Suburban catchments with most of their channel drainage contained within storm drain pipes showed subdued diurnal variation, with low night time temperatures but with very large spikes in summer runoff events. These spikes were also seen in the urban piped streams but the “baseline” temperatures stood well above all the other monitoring sites. There was a pronounced upstream-downstream effect with the nested small headwater catchments experiencing frequent, large temperature spikes related to runoff events that were much larger than at the downstream station. Interestingly these runoff mediated temperature elevations lasted longer than the storm runoff hydrographs. This suggests that for small headwater catchments urban landscapes not only produce a heat island effect on ambient stream temperatures, but also introduce thermal disturbance regimes that are not trivial to thermal stream budgets and may be of important biological significance to biota that utilize these unique aquatic ecosystems.