Seasonal hydrological response in nested forested catchments at different spatial scales: insights from Re della Pietra (Tuscan Apennines, Italy)
Authors: Macchioli Grande Marcos , Verdone Matteo , Borga Marco , Cocozza Claudia , Dani Andrea , Fabiani Ginevra , Gourdol Laurent , Klaus Julian , Manca di Villahermosa Francesca , Massari Christian , Pfister Laurent , Preti Federico , Tailliez Cyrille , Trucchi Paolo , Zorzi Ilaria , Zuecco Giulia , Penna Daniele 
Published: March 2022

&lt;p&gt;Runoff generation in mountain forested catchments results from a complex interplay among numerous factors, such as topography and geology, soil properties, meteorological forcing, and vegetation dynamics. Although seasonal patterns are mostly observed in Mediterranean forested catchments, the variability of hydrological processes at multiple scales in these environments remains uncertain.&lt;/p&gt;&lt;p&gt;The Re della Pietra is an experimental catchment of 2 km&lt;sup&gt;2&lt;/sup&gt; located in the Tuscan Apennines, Central Italy. Elevation ranges from 1280 to 650 m asl, mean annual rainfall is 1180 mm and the Mediterranean climate is characterized by a wet (November-May) and a dry period (June-October). Forest is composed mainly of beech trees, with few oak trees and conifers. The instruments installed in the catchment are: a weather station, plus two rain gauges; four stream gauges at different spatial scales (from a 0.3 km&lt;sup&gt;2&lt;/sup&gt; headwater sub-catchment up to the catchment outlet), along with electrical conductivity loggers; six soil moisture probes, located in three sites across the hillslope, at two depths (15 and 35 cm); three groundwater wells equipped with piezometric level loggers; finally, nine sapflow sensors and 12 multiparametric tree probes installed in beech trees on a hillslope.&lt;/p&gt;&lt;p&gt;Preliminary results from 2021 showed that runoff response to rainfall is fast at all scales, leading to consistent streamflow patterns. However, peakflow time-lags between the headwater sub-catchment and the outlet were variable. Streamflow at all scales revealed a marked seasonal variability, with moderate peaks and long recessions during the wet period, and high peaks with flashy response in the dry period. Soil moisture showed a strong seasonal behaviour as well, with a coupled response and very similar values at both depths in the wet season, and a decoupling in the dry season, with lower values at the shallower layer. Conversely, groundwater table rose only during large rainfall events (&gt;10 mm).&lt;/p&gt;&lt;p&gt;The proportion of event and pre-event water contribution to stream runoff was calculated based on electrical conductivity values at the different scales. During the wet period, event water was higher in the headwaters, then becoming similar at all scales during the dry period. By the end of summer until the beginning of the wet season, event water was greater at the outlet and back again to higher values in the headwaters in the next wet season. The relation between event water and soil moisture was non-linear. Diel fluctuation patterns in streamflow, with a consistent time lag between hillslope sapflow, stem radial variations, and streamflow were observed at all scales pointing to a strong influence of evapotranspiration on runoff generation. Interestingly, the outlet showed stronger fluctuations than the headwaters during a very hot period in August although hillslope sapflow decreased to limit water shortage-related physiological damages. This suggests the likely active role in stream water uptake by riparian vegetation.&lt;/p&gt;&lt;p&gt;Overall, these results provide new and useful insights on hydrological processes and the role of forests in runoff generation at different spatial scales in Mediterranean mountain catchments.&lt;/p&gt;