Flash flood events recorded by air temperature changes in caves: A case study in Covadura Cave (SE Spain)

•The effects of a flash flood event on the microclimate of a cave were studied.•The cavity air temperature increased by between 0.9 and 4.1°C due to water intake.•Secondary water accesses, different to the main cave entrance, were detected.•Air temperature in caves enables to identify subterranean flash flood events. On 28th September 2012, more than 150mm rain fell in just two hours in some points of southeastern Spain, triggering intense flash floods that resulted in the death of ten people and widespread material damage. In the gypsum karst of Sorbas, rainfall intensity reached 33mm/h. Air temperature monitoring in different levels of Covadura Cave, down to 85m depth, enabled the effect of this extreme episode on the cave microclimate to be evaluated in real time. The cave air temperature increased by between 0.9 and 4.1°C as a result of water flow into the cavity and intense mixing of air masses, in addition to the displacement of deeper air masses toward shallower levels produced by fast recharge of the surrounding karst aquifer. The lag between peak rainfall intensity and the highest cave air temperature was 5–6h, indicating the response time of the karst to this rainfall event. No trends with depth were observed, suggesting that water not only flowed in through the main cave entrance but also through secondary accesses and fractures. Furthermore, the size of the cave passages and the intensity of air turbulence generated by waterfalls in the cave played an important role in producing these temperature differences. Even though the rainfall event lasted 10h, cave air temperature did not return to pre-flash flood values until more than 20days later. This indicates that, while waterflow through the cave might stop a few hours after the rainfall event, cave air temperature can be affected over a longer period. This can be explained by slow groundwater level decreasing of the surrounding karst aquifer and latent heat liberation produced by moisture condensation on the cave walls. Our results show how continuous monitoring of air temperature in caves can be a useful tool for evaluating the short-term effects of flash floods in subterranean karst systems.