Stable isotope-based understanding of water fluxes in the critical zone at the forest plot scale under Mediterranean climate

How, why, and what water flows through the soil-plant continuum are quite complex questions that are not yet well understood quantitatively. Soil and plant-induced heterogeneity, soil evaporation, and root water uptake are some of the main controlling factors of water flow dynamics in the soil-plant continuum. Coupling these processes is thus of quite importance to advance our understanding of subsurface mixing and soil-plant interaction and, especially, water sources used by trees. In this study, we combine hydrological and stable water isotopes (²H and ¹⁸O) field data in an integrated flow and transport model to investigate which water sources are used up by trees under different wetness conditions.

We conducted a field experiment on two sets of three Scots pine trees (Pinus sylvestris) in a forested plot within the Vallcebre research catchments (NE Spain). The experiment was carried out from May to September 2022. We monitored throughfall, sap flow, and dial stem diameter variation, as well as soil water potential and soil water content (in vertical profiles down to 70cm) at high temporal (5min) resolution. Furthermore, we sampled weekly water from the different water pools (throughfall, soil water (bulk and mobile), groundwater, and xylem water (twigs)) for isotopic analysis. The analysis of these data helped in clarifying the interaction between the different water pools and the effect of soil water potential and soil water content dynamics on the isotopic signals in the soil-plant continuum.

To further analyze the field data, we developed a numerical model using R-SWMS to simulate the flow in the vadose zone by solving Richards equation coupled with root water uptake, soil evaporation, and isotopic fractionation. To achieve this, we created a 3-D heterogeneous soil matrix that contains a root system. Field data (soil water retention and conductivity curves, initial water content, environmental conditions) from this and previous studies conducted in the catchment were used as the input data. The root system and its hydraulic properties were determined from theoretical values from literature. The isotopic fractionation during evaporation was modelled using the Craig-Gordon model. The model was used to estimate root water uptake distribution, soil water potential, soil water content, and isotopic composition distribution.