Graduate Thesis Or Dissertation

Assessing the Thermal Sensitivity and Stormflow Response of Headwater Stream Temperatures: A Seasonal and Event-scale Exploration in Northern California, USA

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  • Mountainous headwater streams make up ~80 % of stream length globally and are strongly connected with catchment hillslopes and riparian areas, which can influence water quantity, quality, and availability for downstream uses. Accordingly, effective management of headwater streams and riparian zones to maintain desired ecosystem services downstream is critical, particularly in the face of warming climates. Often, a central focus is the protection of stream temperature, which is a critical water quality parameter that influences thermally sensitive species such as salmonid fishes. However, headwater stream temperatures vary depending on characteristics of the stream, catchment, and region. For instance, watershed lithology may control the thermal properties of streamflow during summer baseflow conditions. In addition, there have been few attempts to assess what meteorological or topographic factors control stream temperature change during storm events in forested watersheds and how this response changes seasonally. The goal of this thesis was to improve our understanding of headwater streams by using distributed stream and air temperature monitoring to assess regional differences in summer longitudinal stream temperatures and to determine the factors that influence stream temperature change during storm events in Northern California. In chapter 2, I collected stream and air temperature data along eight headwater streams in two regions (three in the Cascade Range and five in the Coast Range of Northern California) with distinct lithology, climate, and riparian vegetation. My objectives were to compare stream thermal regimes and thermal sensitivity—slope of the linear regression relationship between daily stream and air temperature—within and between streams in both study regions. Mean daily stream temperatures were ~4.7 °C warmer in the Coast Range but were less variable (SD = 0.7 °C) compared to the Cascade Range (SD = 2.3 °C). Median thermal sensitivity was 0.33 °C °C-1 in the Coast Range and 0.23 °C °C-1 in the Cascade Range. I posit that the volcanic lithology underlying the Cascade streams likely supported discrete groundwater discharge locations, which dampened thermal sensitivity. At locations of apparent groundwater discharge in these streams, median stream temperatures rapidly decreased by 2.0 °C, 3.6 °C, and 7.0 °C relative to adjacent locations, approximately 70–90 meters upstream. In contrast, thin friable soils in the Coast Range likely contributed baseflow from shallow subsurface sources, which was more sensitive to air temperature fluctuations and generally warmed downstream (up to 2.1 °C km-1). Overall, my study revealed distinct longitudinal thermal regimes in streams draining contrasting lithology, suggesting that streams in these different regions may respond differentially to forest disturbances or climate change. In chapter 3, my objective was to use hysteresis metrics to assess the relationship between stream temperature and stormflow across ten forested headwater catchments in the Northern California Coast Range during the 2020 water year. I quantified the magnitude and variability of stream temperature hysteresis during rain events and determined whether catchment topographic metrics could explain the stream temperature response to precipitation inputs. I hypothesized that the direction of hysteresis would vary across seasons due to changes in meteorological conditions and that the stream locations most hydrologically connected to the hillslope (as predicted by topographic indices) would exhibit the greatest hysteresis due to lateral throughflow inputs. My results indicate that the stream temperature response to stormflow is seasonally variable and generally exhibits clockwise hysteresis during the summer and spring when air temperatures are warmer than stream temperatures and anti-clockwise hysteresis during the fall and winter when air temperatures are cooler than stream temperatures. In addition, the stream temperature response to stormflow was the most variable across these 10 catchments during the late summer and early fall (SD = 0.18 and 0.10, respectively), when catchment-scale wetness conditions and streamflow were low. As the wet season progressed, stream temperature behavior across these 10 catchments became more similar, and remained coupled through the late spring (SD = 0.04). The magnitude and direction of stream temperature hysteresis was well correlated with the gradient between stream and air temperatures at the start of the event (ρ = 0.49), and air temperature change during the storm hydrograph rising limb (ρ = -0.49), indicating the potential role of regional meteorological conditions on stream temperature change. Contrary to my expectations, none of the derived topographic metrics describing the preponderance of saturated areas and lateral inputs to streamflow were correlated with stream temperature change during events, potentially because subsurface topography and seasonally variable catchment wetness conditions could not be properly characterized with static metrics describing surface topography. Overall, these results indicate seasonally variable stream temperature behavior during storm events that become regionally synchronous as catchment moisture conditions increase during the wet season. Collectively, these results indicate that headwater stream thermal regimes are spatiotemporally variable across regions and in response to precipitation inputs, and riparian management should reflect this variable behavior. Using these results to inform headwater stream management can potentially be accomplished by developing regionally-specific riparian buffers that provide additional protection at locations most sensitive to atmospheric energy exchange or along areas with discrete groundwater discharge.
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  • Pending Publication
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  • 2021-08-08 to 2022-09-09



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