356-5 From Site-Based Thermal Surges to Network-Scale Vulnerability in a Karst Watershed

Abstract:

Groundwater commonly stabilizes stream temperatures, but in urbanized karst watersheds this stability may increase the thermal contrast between cool baseflow and heated stormwater runoff. We analyzed daily stream temperature and discharge records from 2016 to 2024 and sub-daily temperature observations from 2020 to 2024 across 14 sites in the Spring Creek watershed of central Pennsylvania. Groundwater-influenced reaches exhibited dampened seasonal variability, whereas reaches dominated by shallower flow paths more closely tracked atmospheric forcing. Thermal surges were concentrated in urbanized tributaries, with maximum surge magnitudes exceeding 10°C. Surge magnitude increased with developed land cover, impervious cover, and hydrologic flashiness, supporting a contrast-amplification mechanism in which groundwater buffering can enhance storm-driven warming by maintaining cooler pre-storm conditions. We are extending these site-based results across the stream network by modeling reach-scale thermal surge vulnerability from baseline thermal conditions, developed land cover, spring influence, and hydrologic size while accounting for flow-connected spatial dependence. This framework is intended to identify vulnerable reaches where groundwater-supported cold-water habitat and urban runoff converge, supporting targeted monitoring and stormwater management.

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