Groundwater Temperature Processes and Patterns: Implications for Stream Thermal Regimes

ABSTRACT Groundwater temperature dynamics and spatial patterns influence aquifer biogeochemistry and the quality and ecosystem dynamics of receiving surface waters. Groundwater‐dominated streams are characterized by modulated diel and seasonal thermal regimes and can exhibit pronounced spatial thermal variability near focused groundwater discharge. Temporally modulated thermal regimes impose less thermal stress on aquatic biota than in thermally dynamic streams, and spatial thermal variability creates riverscapes with thermal refuges that sustain poikilotherms during extreme events. Thus, groundwater inflows can (1) lower ambient summer stream temperatures, with changes up to 8°C in past studies, and (2) create anomalous cold‐water plumes, with field studies showing plume temperature differences exceeding 20°C. Because groundwater temperatures are relatively insensitive to seasonal meteorological changes, they are often incorrectly assumed to be thermally resilient to long‐term changes in land cover or climate. However, global evidence reveals that shallow aquifers are warming up to several degrees Celsius in response to urbanization, deforestation, and climate change, which can influence the heat fluxes associated with groundwater discharge to surface waters and resultant stream temperatures. Properly accounting for the changing thermal impacts of groundwater discharge in process‐based stream temperature models is only possible through a rigorous understanding of subsurface thermal processes and how those processes alter spatiotemporal patterns of groundwater temperature. This review advances process‐based understanding of groundwater temperature and its thermal influence on streams and evaluates approaches for representing groundwater discharge impacts in stream temperature models. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water and Environmental Change Water and Life > Stresses and Pressures on Ecosystems