Role of Groundwater in Sustaining Streamflow and Temperature Stability in a Semiarid High-Mountain Watershed (SE Spain)

Abstract

High-mountain streams in semiarid regions are particularly vulnerable to climate change projections due to reduced snow cover and increasing air temperatures. In these environments, groundwater may play a critical role in sustaining streamflow and regulating stream thermal regimes. This study analyzes discharge, air temperature, and water temperature records collected along an altitudinal transect in the Alhorí River watershed (Sierra Nevada, SE Spain) during the hydrological years 2020–2025. The basin is fully deglaciated and characterized by glacial–periglacial deposits and weathered schists that promote snowmelt infiltration and groundwater storage. Results show a progressive downstream increase in discharge, with gains of up to 164% in the upper reaches and an overall increase of approximately 484% from the spring source to the gauging station, indicating substantial groundwater contributions along the river. Strong thermal decoupling between air and water temperatures was observed. Spring water temperature remained nearly constant (~4 °C), with a thermal lag of about 49 days relative to air temperature and an annual amplitude (~0.3 °C) much smaller than that of air temperature (~10 °C). Stream temperature decreased with altitude at a lower rate than air temperature (−0.33 versus −0.54 °C per 100 m), demonstrating persistent groundwater thermal buffering. These findings provide evidence of the role of shallow groundwater storage in sustaining streamflow and moderating thermal variability in Mediterranean mountain catchments, with potential relevance under future hydroclimatic changes.