Deriving Subsurface Stormflow Mechanisms with Isotopes and ERT

Abstract. Subsurface stormflow (SSF) is a dominant process in the generation of runoff in many catchments. However, the mechanisms by which new event water triggers the activation and release of stored pre-event water are not well understood. To identify the flow paths and mechanisms governing SSF generation, we conducted a controlled sprinkling experiment using deuterium- labeled water alongside time-lapse electrical resistivity tomography (ERT), groundwater monitoring and soil core sampling at two trenched hillslopes in the Black Forest, Germany. Water was applied for three hours, with the supply switching to deuterated water halfway through the experiment. Both hillslopes responded rapidly, with event runoff coefficients of approximately 0.6, and tracer breakthrough occurred at the trenches within 7–18 minutes of application, even at depths greater than 70 cm. However, post-irrigation soil cores detected the tracer only in the uppermost 20 cm of the soil matrix, where approximately 45 % of the labeled water remained stored. Time-lapse ERT revealed that the wetting front was largely confined to the upper 0.2–1.5 m; meanwhile, the deeper, near-saturated zone exhibited negligible resistivity change despite sustained lateral discharge as measured at the trenches.

Together, these observations suggest the presence of a dual-domain flow system, whereby a small proportion of event water bypasses the matrix via vertically and laterally connected preferential pathways. Meanwhile, the majority of SSF consists of pre-event water, which is mobilized by pressure-driven (piston) displacement within the saturated zone. Subsequent natural rainfall remobilized the stored tracer, raising the cumulative recovery rate to approximately 30 % within one month, thereby confirming the soil matrix to be both a sink and a delayed source of event water.