Mobilization of particulate matter in intermittent and forested headwater streams

Headwater streams extend and retract both seasonally and in response to individual rainfall events. Stream network extension is typically accompanied by an increase in stream water velocity and water depth, which may overcome mobilization thresholds of particulate matter that may have accumulated in previously dried-out streams. Although this process is commonly conjectured, data documenting the pacing and mechanisms leading to the transfer of particulate matter from terrestrial to aquatic environments remains scarce. An improved mechanistic understanding of these processes in forested headwater streams is particularly needed because they are reportedly highly sensitive to the changes in timing, magnitude and duration of precipitation expected under a changing climate. In Luxembourg, the health of forest ecosystems has also declined severely over the past two decades. Together, these changes might ultimately affect flow persistence, alter the transport and transformation of water, energy, dissolved and suspended materials, and impact organisms throughout the river network. The potentially considerable consequences of these changes on our water resources, aquatic ecosystems and bio-geochemical cycles remain largely unknown. In this study, we investigate the relationship between catchment storage, water flow paths, stream network extension and particulate matter mobilization. During rainfall events, water might flow overland in previously dry streams if a shallow, perched, transient water table builds up and generates runoff, or if a deeper water table rises to the upper transmissive soil horizons. The former mechanism is more likely to occur when antecedent catchment storage is low, whereas the latter is expected when storage is high. Despite it has never been demonstrated with observations, these two processes leading to overland flow might be associated to different sediment mobilization mechanisms. To test these hypothesis, we designed a field study to gather unprecedented datasets on (i) stream network dynamics (i.e., network extension/retraction and intermittency) documented using time-lapse cameras, (ii) suspended sediment fluxes measured at the catchment outlet, and (iii) catchment storage estimated from an extensive, high-resolution hydrometric time series collected in the Weierbach Experimental Catchment (WEC; 0.45 km²; north-western Luxembourg). Our results show that stream extension during rainfall events drives particulate matter mobilization during single peak hydrographs in the WEC, when water rapidly reaches the stream network during precipitation pulses and catchment storage is low. In contrast, double peak hydrographs occur when catchment storage is high, resulting in limited stream network extension and low particulate matter mobilization. Building on these newly gained datasets, we aim to develop a novel conceptual framework linking particulate matter mobilization to its subsequent controlling factors, including rainfall characteristics, catchment storage, regolith structure, land cover and topography.