Utilizzo idrico e dinamiche ecoidrologiche in meleti alpini: un approccio tramite traccianti ambientali/Water use and ecohydrological dynamics in Alpine apple orchards: an environmental tracer approach

Understanding eco-hydrological processes and the functional relationships between the vegetation and other components of the hydrologic cycle is essential in order to efciently manage water resources. Tracer-based investigations of water exchanges between soil and trees in natural forested catchments have received much attention. However, interactions between tree water use and the hydrological cycle in agricultural environments is still poorly understood. In this work, we use stable isotopes of water as tracers to improve our understanding of the functional interrelations between surface runoff, recharging groundwater and water taken up by apple trees in two orchards in Val Venosta (BZ). The aim of this work is to understand i) at which soil depth apple trees take up water, ii) the relationship between groundwater, soil water and stream water, iii) if and how the position of the two sites influences the isotopic composition of water fluxes. Our results show two water pools with distinct isotopic signature: river water, groundwater and irrigation water show values relatively depleted and consistent with precipitation, whereas soil water and sap show a clear evaporation signal that decreases with increasing soil depth. Sap isotopic data are inconsistent with groundwater data but reflect soil water data in the frst 40 cm, suggesting that apple trees absorb a mixture of rainfall and irrigation water and not from groundwater, which is typically varying between 40 cm and 140 cm depth. There is a marked intra-feld spatial variability in the isotopic composition of soil water, and signifcant differences between the two felds, with the feld closest to the river showing signifcantly more depleted values compared to the feld farther from the river. This difference, which is reflected by sap isotopic composition in summer, is possibly related to the different amount of incident solar radiation between the two felds or to differing depths to groundwater. These results emphasize the heterogeneity of the variability of isotopic composition of water in the different hydrological compartments, as well as the complexity of ecohydrological relationships in agricultural sites. Our study shows the need to continue with monitoring activities and feld experiments aimed at the distinction between rainfall and irrigation water as source of water for apple trees.