LAUSR

Abstract. Understanding components of the total streamflow is important to assess the ecological functioning of rivers. Binary or two-component separation of streamflow into a quick- and slow (often referred to as baseflow) component, and the associated and often used baseflow index (BFI), have been criticised for their arbitrary choice of separation parameters. These methods also merge different delayed components in one baseflow component. As streamflow generation during dry weather often results from drainage of multiple sources, we propose a novel delayed flow index (DFI) considering the dynamics of multiple delayed contributions to streamflow. The DFI is based on characteristic delay curves where the identification of breakpoint estimates helps to avoid rather arbitrary separation parameters and allows distinguishing four types of delayed streamflow contributions. The methodology is illustrated using streamflow records from a set of 60 headwater catchments in Germany and Switzerland covering a pronounced elevational gradient of roughly 3000 m. We found that the quickflow signal often diminishes earlier than given by BFI-analyses with only two flow components, and further distinguished a variety of flow contributions with delays shorter than 60 days controlling the seasonal streamflow dynamics. For streamflow contributions with delays longer than 60 days, the method was used to assess catchments’ water sustainability during dry spells. Colwells’s Predictability, a measure of streamflow periodicity and sustainability, was applied to attribute the identified delay patterns to streamflow dynamics and catchment storages. The smallest storages were consistently found for catchments between approx. 800 and 1800 m a.s.l. Above an elevation of 1800 m the DFI suggests that seasonal snowpack provides the primary contribution, whereas below 800 m groundwater resources are the major streamflow contributions. Our analysis also indicates that subsurface storage in high alpine catchments may be large and is overall not smaller than in lowland catchments. The DFI can be used to assess the range of sources forming catchments’ storages and judge their long-term sustainability of streamflow. Combining the DFI with a low flow stability index, allows us to better understand controls of seasonal low flow regimes across different regimes.