Geographical, spatial, and temporal water sources in a Mediterranean forested catchment.

Forested catchments play a key role in storing and releasing fresh water. Climate changes affect global hydrological and ecosystem processes with effects also observed at small scales. In this context, investigating spatial and temporal water origins in small forested catchments is fundamental to understand and better predict the behavior of hydrological processes. However, very little is known about both the spatial and temporal origin of water across different ecohydrological compartments in Mediterranean forested catchments.

In this study, we collected hydrometeorological and isotopic data in the Re della Pietra experimental catchment (2 km²) located in the Tuscan Appennines (Central Italy) to understand the origin of stream water at different spatial and temporal scales and the sources of tree-water uptake. Starting in April 2019, we collected water samples for isotope analysis (d¹⁸O, d²H) from precipitation, throughfall, springs, and the stream at different sections (4 locations from upstream to downstream). In addition, we sampled soil at different depths (0-20cm, 20-40cm, 40-60cm) and several monitored beech trees. Hydro-meteorological parameters are monitored in the Lecciona subcatchment (0.3 km²).

Results based on the HYSPLIT model revealed that the Northern Lower Atlantic dominates the water vapor of precipitation in both wet and dry periods. In contrast, water vapor from the Arctic Ocean was observed only in wet periods, while in dry ones, there was a small contribution of Mediterranean water vapor. Furthermore, there were significant spatial and temporal variations of isotopes (δ¹⁸O and δ²H) and electrical conductivity among water in various ecohydrological compartments. Both the main stream and the tributary were mainly recharged by spring water and only secondarily by precipitation and soil water with significant seasonal variations. Spring water decreased in wet periods but increased in dry periods, and precipitation and soil compartments showed opposite behaviours. Trees mainly used soil water from shallow layers(0-20 cm: 51.1% ± 13.1%, 20-40 cm: 37.1% ± 15.6%, 40-60 cm: 7.5% ±6.3%) in wet periods, while in dry periods, tree water uptake came from deep soil layers(0-20 cm: 13.41% ± 12.7%, 20-40 cm: 55.6% ± 26.1%, 40-60 cm: 8.35% ± 3.6%). The dominant negative values of the Seasonal Origin Index in all ecohydrological compartments except shallow soil layers revealed that winter precipitation was used even in midsummer by the trees and that both surface and subsurface water reflect larger contributions from winter sources. These results imply the resilient behaviour of this catchment to cope against summer droughts and provide a preliminary theoretical basis for managing forest and water resources in Mediterranean catchments.