Abstract Quantifying how rainwater infiltration modifies the stable isotope composition of soil pore water is essential for tracing water movement through the critical zone. We monitored intact soil columns under natural rainfall at high temporal resolution, jointly measuring seepage flux and the hydrogen and oxygen isotope composition of drainage water. The data reveal a robust, threshold-like pattern: faster seepage is consistently associated with more isotopically depleted drainage, behavior that is not readily explained by evaporation or simple mixing alone. We interpret this pattern as evidence for an interfacial hydrodynamic isotope fractionation process, in which transport dynamics and soil–water interfacial interactions partition water isotopologues without phase change. This framework refines how isotopes are used to infer soil water mixing and has implications for predicting recharge and ecosystem water use under a changing climate.
Zhao et al. (Fri,) studied this question.