Changes in the Flowing Drainage Network and Stream Chemistry During Rainfall Events for Two Pre-alpine Catchments

Many headwater catchments contain non-perennial streams that flow only during wet conditions or in response to rainfall events. The onset and cessation of flow result in a dynamic stream network that periodically expands and contracts. The onset of flow can flush sediment and nutrients from previously dry streambeds and enhance the rates of carbon and nitrogen mineralization. The expansion of the flowing drainage network also increases hydrologic connectivity between hillslopes and streams because it decreases travel distances to the stream. However, datasets on the dynamics of the flowing drainage network and short-term changes in stream chemistry during rainfall events are rare. This limits our interpretation of hydrological processes and of changes in stream chemistry during events.Here, we present hourly measurements of solute concentrations and stable isotopes from precipitation and streamflow at the outlets of two 5 ha catchments in the Swiss pre-Alpine region during seven rainfall-runoff events in the snow-free season of 2021. Samples were also collected from soil water and groundwater across the catchments. We combine these data with 10 min information on the flowing drainage network length to infer the dominant runoff-generating mechanisms for the two experimental catchments.Despite their proximity and similar size, soil, and bedrock characteristics, the flowing drainage network dynamics were very different for the two catchments. In the flatter catchment (average slope of 15 degrees), the stream network was more dynamic and expanded rapidly, up to 10-fold, while in the steeper catchment (average slope of 24 degrees), it remained relatively stable (only a 2-fold change). The event water contributions were higher for the flatter catchment. The dilution of calcium at the time of the rapid expansion of the network and the increase in discharge suggested that the contribution of rainfall falling directly on the stream channels is important, especially for the smaller events during dry conditions. During wet conditions, event water must have been delivered from areas outside the channels. In the flatter catchment with the more dynamic stream network, a "flush" of nitrate was detectable, possibly due to the transport of material from previously dry stream segments. In the catchment characterized by a more stable flowing drainage network, such a flush was not observed, and nitrate concentrations decreased, suggesting larger contributions from riparian groundwater with reducing conditions during rainfall events. Our experimental study not only highlights the large differences in stream network dynamics and stream chemical responses for neighboring catchments but also shows the value of fine-scale observations of both the channel network dynamics and stream chemistry to understand runoff-generation mechanisms.