Abstract
Understanding stream-groundwater connectivity in response to climate variability is critical for water resource management and ecosystem resilience. Groundwater influx moderates stream temperature and is important for cold-water adapted species such as salmonids. As climate change alters precipitation, temperature, and snowpack conditions, groundwater could help sustain thermal refugia in streams. However, monitoring groundwater connectivity remains challenging, particularly in ungauged, snow dominated, headwater streams, limiting our understanding of stream-groundwater connectivity responses to climatic variability of which the increase of meteorological drought is of particular concern across many regions. In this study we explore the use of diel air–water temperature amplitude ratios to assess groundwater connectivity in central Idaho headwater streams from 2017 to 2024, with the objective to evaluate the influence of the 2021 extreme meteorological drought. We analyzed stream and air temperature patterns via amplitude ratios, and applied a Wilcoxon signed rank test to compare years across 12 stream sites. We applied a linear mixed effects model to estimate the effects of snowmelt timing, rate, runoff, and spring SPEI on amplitude ratios. Results indicate that groundwater connectivity increased in some streams during the 2021 drought, while others remained stable or declined. Additionally, some streams exhibited stronger groundwater connectivity and reduced atmospheric coupling during low snow years, while others maintained consistent thermal dynamics. Streams with greater groundwater connectivity also had lower maximum summer temperatures. These patterns suggest that streams with greater groundwater connectivity may buffer thermal extremes, like the 2021 drought, that are becoming increasingly more common. Further research is needed to explore how hillslope hydrologic dynamics, such as how transpiration and topography mediate snowmelt partitioning, shape stream thermal regimes and long-term habitat suitability.