Geology controls hydrological regime and spatio-temporal origin of surface and subsurface water in two adjacent mountain catchments in Central Italy

Understanding hydrological flow pathways and spatio-temporal origin of surface and subsurface water in catchments with highly fractured geology is particularly challenging. In this work, we relied on the integration of hydrometric measurements with stable oxygen and hydrogen isotope data in two adjacent catchments in the Sibillini Mountains National Park, Central Italy, to better understand the drivers of the catchment hydrological response and the spatio-temporal origin of stream and spring waters. The Ussita catchment is 44 km2 and its highest elevation is 2204 m a.s.l. The Nera catchment is 110 km2 and its highest peak is 2233 m a.s.l. The two rivers merge at the Visso Village, at 615 m a.s.l. The area is characterised by heavily fissured and fractured calcareous rocks that foster the occurrence of several springs, some of them of karst origin. Both catchments host a dense hydrometerological network. The experimental apparatus is completed with one piezometer, soil moisture probes at five locations, lysimeters at two depths and four throughfall plots under beeches and oaks. Monthly samples for isotopic analysis are being collected since fall 2020 from precipitation at three different elevations and four locations, the streams at different sections, and four springs in the Nera catchment only. Preliminary results show a distinct hydrological behaviour in the annual streamflow regimes: the Ussita stream slightly reacts only to the largest storms and during intense snowmelt periods, whereas the Nera stream has a very damped response during all the year, revealing a clear buffer effect of the large subsurface reservoir, facilitated by the highly fractured nature of the geological setting. As expected, there is an elevation and seasonal effect in the isotopic composition of precipitation, although the seasonal effect is partly masked by the exceptionally high temperatures occurred in fall 2021. However, the time series of isotope data in stream water show a damped signal and very low seasonal variability in both streams, matching the observed low variability of streamflow. Only the Ussita catchment shows some more enriched outliers likely reflecting runoff response during large storm events. Interestingly, stream and spring samples from both catchments lie along but also above and below the Local Meteoric Water Line, suggesting that the sampled spring and stream water was either originated i) from precipitation fell, infiltrated, and stored well before the collection of the precipitation samples, and released; ii) and/or from areas outside the topographic catchments, and therefore not adequately characterized by the isotopic signal of sampled precipitation. The isotopic composition of the streams and springs is statistically the same, revealing that spring groundwater is the main component of stream runoff. Moreover, the isotope signature of both springs and streams is much closer to that of winter precipitation rather than summer precipitation indicating a major role of winter precipitation in recharging the catchments, consistently with the precipitation seasonal regime. On-going work is assessing the spatial difference in the isotopic composition and quantifying the temporal origin of stream and spring water of the two catchments.