Spatial and Temporal Variability in Event Water in Near-Surface Flow Pathways in a Humid Steep Headwater Catchment

In steep humid catchments with low permeability soils, lateral flow at, or near, the soil surface is an important runoff generation mechanism. However, so far, few studies have investigated near-surface runoff for vegetated hillslopes in temperate climates. To better understand the relative importance of rainfall (event water) to near-surface flow in pre-Alpine headwater catchments underlain by gleysols, we installed 14 small (1 m wide) runoff plots across the Studibach catchment and collected samples of overland flow and lateral subsurface flow in the topsoil (i.e., topsoil interflow), soil water, groundwater and rainwater during the snow-free seasons of 2021 and 2022. The samples were analysed for the stable isotopes of hydrogen and oxygen and used in a mixing model to infer the importance of soil water (i.e., pre-event water) and rainfall (i.e., event water) for overland flow and topsoil interflow. The spatial and temporal variability of the event water fractions was high. Event water was the dominant source (> 50%) for nearly half of the overland flow samples and almost 40% of all topsoil interflow samples. The best predictors for the event water fractions in overland flow and topsoil interflow were, according to a random forest model, the duration of the rainfall event, the rainfall intensity, and the antecedent wetness conditions. The event water fractions were not clearly related to site characteristics. These findings show that overland flow and topsoil interflow consist of a mixture of rainwater and soil water and that the relative contribution of rainwater depends on the complex interplay between antecedent soil moisture conditions, rainfall event characteristics and site characteristics. These results have large implications for the interpretation of the event water fractions in streamflow and related inferences about the dominant runoff generation mechanisms.