Wildfire impacts on groundwater recharge in mountain catchments

Wildfires significantly alter surface catchments' hydrology, with harsh impacts on water-related ecosystem ser­ vices. Climate change is and will further increase wildfire frequency and intensity, making the assessment of wildfires' hydrological impacts a very hot topic. Wildfires' impacts on surface hydrology have been extensively analysed. The combustion-disruption of the vegetation cover and structural and chemical modification of the soils are the recognised drivers of wildfires' impacts. However, how the surface hydrologic changes impact the groundwater dynamics still lacks consensus. We contribute to this knowledge gap by investigating how wildfires' changes to surface hydrology impacted groundwater in mountain catchments. Our study catchments are located in a Mediterranean mountainous area in Central Italy, and they were hit by a 1200 ha arson wildfire in 2018. The hydrologic impacts of the wildfire were assessed through the paired-catchment method. Surface and groundwater hydrological changes were assessed by combining stream discharge data and chemical and water stable isotope data. A robust hydrogeological conceptualization was crucial to frame the hydrological data with respect to the wildfire impact. We found for the burnt catchments: i) increased stormflow; ii) higher rock-water interactions signal in the baseflow; iii) higher evaporative signal in the baseflow, which were associate with a reduction in the ground­ water recharge. The increased stormflow in burnt catchments led to higher surface runoff and thus less water infiltrating locally, which in turn reduced groundwater recharge and renewal. The impact of wildfire on groundwater is still underexploited. Combining different hydrologic, geochemical and particularly water stable isotope data may provide useful information on this hot topic.