Monitoring Heat Transfer and Recharge Mechanisms in a Karst System Using Fiber Optic Distributed Temperature Sensing

Abstract Karst aquifers play a crucial role in global water resources. Characterizing their thermal dynamics is essential for improving our understanding of their functioning. However, monitoring karst systems is challenging due to their strong heterogeneity, anisotropy, and limited accessibility. To address these challenges, we deployed an 800‐m‐long fiber‐optic cable within the unsaturated zone of a karst system in the Jura Mountains (eastern France). The objective was to investigate the spatial and temporal variability of cavity air and underground river water temperatures over a 6‐month monitoring period. The results highlight a gradual increase in cavity air temperature (CAT) and cavity river temperature (CRT) over the study period, consistent with the diffusion of surface air temperature (SAT). Superimposed on this long‐term influence, short‐term temperature fluctuations linked to precipitation events reveal the role of advective heat transfer associated with rapid infiltration and mixing in the main conduit. Temperature and electrical conductivity contrasts between the underground river and a lateral tributary indicate that multiple reservoirs contribute to aquifer recharge.