Shallow vs Deep Groundwater Discharge Influences the Thermal Stability of Streams: A Continental-Scale Analysis

Abstract:

Groundwater discharge to streams is essential for maintaining the thermal stability and water quality of groundwater dependent ecosystems throughout stream networks. However, the relative aquifer depth of the contributing groundwater can affect the vulnerability of these dependent ecosystems to landscape and climate change. Here we use paired multi-year air-water temperature signals to categorize 1786 varied size stream sites across the continental United States. These categories parse streams with strong shallow or deeper groundwater connectivity from those with reduced groundwater connectivity and sites with thermal regimes likely influenced by major upstream dam operation. Nationally, 40% of stream sites were identified as having a clear groundwater influence, of which approximately 50% are dominated by shallow groundwater. Of those shallow groundwater sites with a longer-term temperature records (14-30 years) we observed a long-term warming at a similar proportion to sites with reduced groundwater contributions. Sites with a deep groundwater signature tended to have thermal regimes that were more stable or even show cooling. When compared to major physical watershed characteristics, streams with thermal regimes closely coupled to air temperature were associated with low watershed slope and high watershed impervious coverage, indicating groundwater disconnection in human-modified landscapes. These results demonstrate that the vulnerability of groundwater-influenced streams to warming depends on the relative flow path depth of the source groundwater requiring unique management strategies.