The separation of flow pathways in a sandstone catchment of the Northern Black Forest using DOC and a nested Approach

In order to identify runoff generation processes, investigations of the spatial and temporal dynamics of dissolved organic carbon (DOC) export and oxygen isotope ratio (δ 18O) in precipitation, soil water and runoff were carried out in the catchment Duerreychbach (northern Black Forest, Germany). For a headwater DOC measurements during selected precipitation-runoff events were used to distinguish the dominating runoff processes. The DOC-rich water originated from the upper soil horizons and formed a rapid runoff component. Runoff was found to be dominated by lateral subsurface flow through the upper soil horizons. The combination of DOC and δ 18O data in a mixing diagram allowed to quantify the fraction of event water. In opposition to δ 18O, DOC concentrations always showed a distinct contrast between soil water and precipitation water. Determination of a representative mean pre-event value for both DOC and δ 18O was difficult, but the possible range of soil DOC concentrations was rather limited and fell between 68 and 80 mg l −1. Using a DOC mass balance approach, a hydrograph separation for the Duerreychbach catchment succeeded for low antecedent soil moisture or low to medium precipitation sums. Increasing precipitation sums or wetter antecedent soil moisture increased the connectivity of other areas to the main channel, which then resulted in a more complex pattern of runoff contributing areas.