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 a slow (often referred to as baseflow) component are often based on arbitrary choices of separation parameters and also merge different delayed components into one baseflow component and one baseflow index (BFI). As streamflow generation during dry weather often results from drainage of multiple sources, we propose to extend the BFI by a delayed-flow index (DFI) considering the dynamics of multiple delayed contributions to streamflow. The DFI is based on characteristic delay curves (CDCs) where the identification of breakpoint (BP) estimates helps to avoid rather subjective separation parameters and allows for distinguishing four types of delayed streamflow contributions. The methodology is demonstrated using streamflow records from a set of 60 mesoscale catchments in Germany and Switzerland covering a pronounced elevation gradient of roughly 3000 m. We found that the quickflow signal often diminishes earlier than assumed by two-component BFI analyses and distinguished a variety of additional flow contributions with delays shorter than 60 d. For streamflow contributions with delays longer than 60 d, we show that the method can be used to assess catchments' water sustainability during dry spells. Colwell's predictability (P_T), a measure of streamflow periodicity and sustainability, was applied to attribute the identified delay patterns to dynamic catchment storage. The smallest dynamic 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 most likely the major streamflow contributions. Our analysis also indicates that dynamic storage in high alpine catchments might be large and is overall not smaller than in lowland catchments. We conclude that the DFI can be used to assess the range of sources forming catchments' storages and to judge the long-term sustainability of streamflow.

[1] Due to temperature differences of groundwater and streamwater, localized groundwater inflows into small streams can directly be detected with ground‐based thermographic systems in summer or winter. Infrared radiation temperatures of surface water were used to determine mixing length and to calculate the relative fraction of groundwater inflow to downstream discharge. These fractions were comparable to groundwater inflow fractions derived from electrical conductivity, kinetic water temperatures and discharge measurements. This approach advances the immediate detection and quantification of localized groundwater inflow for hydrology, geology and ecology.

Salt tracers (sodium bromide/sodium chloride) and two different fluorescent tracers, uranine (UR) and sulforhodamine-B (SRB), were injected as a pulse into six different surface flow wetlands (SFWs). Salt tracers documented wetland hydraulics. The fluorescent tracers were used as a reference to mimic photolytic decay (UR) and sorption (SRB) of contaminants as illustrated by a comparison to a real herbicide (Isoproturon), which was used as a model for mobile pesticides. Tracer breakthrough curves were used to document residence time distributions, hydraulic efficiencies, peak attenuation and retention capacities of completely different wetland systems. A 530 m² forest buffer zone showed considerable peak attenuation but limited retention capabilities despite its large area. Approximately 80% of SRB was permanently retained in a re-structured 325 m² flood detention pond. These two non-steady SFWs indicated long-term tracer washout. The remaining four SFWs displayed constant outflow rates and steady-state flow conditions. Due to photolytic decay in a 330 m² row of three wetlands, UR was almost entirely degraded, but the SRB breakthrough suggested relatively low sorption. A 65 m² shallow flow-through wetland yielded negligible photolytic decay but showed considerable sorption losses. Finally two types of vegetated ditches were analysed. In one case, vegetation was removed from a 413 m long ditch immediately prior to tracer injection. A 30% loss by sorption to sediment and plant remnants occurred at the very beginning of the tracer breakthrough. Inside a second ditch, 80 m long and densely vegetated by Phragmites australis, sorption was even higher and yielded eightfold higher specific SRB retention rates. Although the present findings are only valid for low flow conditions, they indicate that a shallow water depth seems to be a key variable which may increase sorption of tracers and therefore contaminants. Large wetlands with deep water bodies may attenuate concentrations efficiently, but unit load reduction was found to be more significant in shallow systems even at much higher flow velocities.