Threshold Processes and Stream Temperature Distribution in a Small New England Headwater Stream Catchment.
Rising air temperature and decreasing stream flow trends are predicted to result in corresponding increases in stream temperatures. As a result, the future of ectothermic stream fishes, which rely on seasonal and spatial distributions of stream temperature for growth and survival, could be in jeopar...
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| Format: | Others |
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ScholarWorks@UMass Amherst
2020
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| Online Access: | https://scholarworks.umass.edu/masters_theses_2/879 https://scholarworks.umass.edu/cgi/viewcontent.cgi?article=1937&context=masters_theses_2 |
| Summary: | Rising air temperature and decreasing stream flow trends are predicted to result in corresponding increases in stream temperatures. As a result, the future of ectothermic stream fishes, which rely on seasonal and spatial distributions of stream temperature for growth and survival, could be in jeopardy. Fortunately, contradicting stream temperature trends in forested headwater catchments suggest that non-climatic variables, such as baseflow indices and catchment geologic structure, may have an important confounding influence on the future of stream temperature. Most significantly, the annual stability of groundwater temperature has long been recognized as an important buffer between air and stream temperature. The hyporheic zone has been the subject of an emerging focus of new research over the past decade and breakthroughs in our understanding in groundwater and surface water interactions have given rise to new conceptual frameworks in hydrology. Once such framework is the concept of threshold discharge in which inputs to streams are activated once critical metrics are met in antecedent moisture, rainfall intensity, and rising water tables in the riparian areas and topographical depressions. This study investigates the extent to which threshold connectivity between soil saturation, groundwater, precipitation, and streamflow exist in a shallow bedrock/till-mantled headwater catchment, and how threshold characteristics may influence the fluid flux and temperature variability of the stream. This study uses stable water isotopes, fiber-optic distributed temperature sensing, and hydrometric analyses to observe the fluid and heat flux within the catchment. The results indicate that threshold discharge is an inherent property of the saturated soil conductivity curve which drives macropore and fractured-bedrock discharge into the stream. The sequential activation of flow in fractured storage reservoirs and soil can be observed in the isotopic, temperature, and conductivity measurements of the stream. Fiber-optic distributed temperature sensing (FO-DTS) produced the locations of both discrete seeps in areas with bedrock outcrops and thin overburden, and diffuse seeps where thicker alluvial deposits occurred. The net effect of threshold discharge events on seep locations was a reduction of the instream temperature variability and decrease in the persistence of thermal refugia, or seeps buffered from the main stream channel temperature. |
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