Does high resolution in situ xylem and atmospheric vapor isotope data help improve modeled estimates of ecohydrological partitioning?

Abstract

Ecohydrological partitioning of rainfall into different sources of evaporated and transpired water is crucial to quantify water balance impacts from land cover change. However, resolving ecohydrological partitioning into component fluxes can be ambiguous and uncertain, even where detailed, small-scale measurements are available. To constrain ecohydrological fluxes at the scale of an individual tree in an urban setting, we combined hydrometeorological, sap flow, soil water and high-resolution in situ plant xylem and atmospheric vapor stable isotope measurements over the growing season from April to October 2022. These data were integrated with parsimonious tracer-aided conceptual modeling. The data helped isolate temporal patterns of shifting preferential fractionation in xylem and atmospheric vapor from δ¹⁸O to δ²H mainly depending on air temperature and relative humidity. Modeling high-resolution in situ isotope data revealed the dominant local influence of interception, soil evaporation and transpired water sources on atmospheric vapor particularly during dry periods, whereas wet periods were driven by more variable non-local moisture sources. Additionally, modeling tree water storage did not explain the highly variable and more depleted xylem isotope data compared to enriched and fractionated soil water. Despite volumetrically constrained (within transpiration measurement uncertainty bounds) ecohydrological partitioning, the atmospheric vapor isotope data showed that fine-scale variations of interception and soil evaporation vapor sources can have nuanced impacts on the atmospheric vapor mixture. The comparison of a more complex conceptualization of modeled soil storages (three soil storages) with a minimalist two-storage model indicated the notoriously difficult isotopic discrimination of root water uptake depths. Nonetheless, the combination of soil moisture, transpiration and high-resolution in situ isotope measurements with modeling helped enhance our understanding of plot-scale vegetation-mediated urban hydrological processes.