Isotopic signatures and soil water partitioning in a humid temperate forest catchment: Implications for the 'two-water-worlds' hypothesis.

[Display omitted] • Ten years of stable water isotope data collected at a forest site were analyzed. • A numerical model was used to simulate soil water and isotope dynamics. • The model successfully located the root uptake of pre-event water in the soil profile. • Isotopic differences became apparent when flux-type weighting was taken into account. • Mechanisms for the existence of isotopically distinct subsurface waters were explored. The availability of long-term data series of natural stable isotopes of water (18O and 2H) observed in a small mountainous headwater catchment provides an opportunity to study soil water partitioning and runoff generation processes. In this study, the analysis of the isotopic composition of rainwater, soil water, subsurface stormflow, groundwater, and streamflow was carried out over a 10-year period. A one-dimensional vertical dual-continuum model was used to analyze the preferential flow of water and the transport of 18O in a hillslope. The model, previously validated against observed hillslope discharge and isotope transport data dynamics, provided a framework for evaluating seasonal and episodal runoff regimes. For episodal simulations, the partitioning of soil water into pre-event and event water was done using virtual tracer concentrations instead of the observed 18O contents – to maximize the contrast between pre-event and event water signatures. Simulated seasonal isotopic variations of 18O in stormflow and soil water compared well with the observed 18O contents. When evaluated as long-term median values, the observed isotopic composition did not suggest significantly different water in the different discharge components. However, when the flux-type weighting of isotopic signatures in hillslope stormflow, deep percolation, and root water uptake was applied, isotopically distinct waters became evident. For the selected episodes, approximately 62 % of stormflow and water transpired by plants was associated with pre-event water. The results suggested that isotopic differences between water associated with different discharge mechanisms prevail despite intense pre-event and event water mixing above the soil–bedrock interface. The modeling approach helped to explore the potential mechanisms behind the existence of isotopically distinct subsurface waters. [ABSTRACT FROM AUTHOR]