Changes in flowing drainage network and stream chemistry during rainfall events for two pre-Alpine catchments

Abstract. Many headwater catchments embed non-perennial streams that flow only during wet conditions or in response to rainfall events. The onset and cessation of flow results 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 carbon processing rates. 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 flowing drainage network dynamics during rainfall events and short-term changes in stream chemistry are rare. This limits our interpretation of hydrological processes and changes in stream chemistry during events. Here, we present joint hourly measurements of solute concentrations and stable isotopes from precipitation and streamflow at the outlets of two 5-ha catchments in the Swiss pre-Alps during seven rainfall-runoff events in the snow-free season of 2021. Relevant samples were also collected from soil- and groundwater across the catchments before and after rainfall events. In addition, 10-min frequency information was collected on the flowing drainage network length. We used these synoptic measurements to infer the dominant runoff-generating mechanisms for the two experimental catchments. Despite their proximity and similar size, soil and bedrock features, the flowing drainage network dynamics proved very different for the two catchments. In the flatter catchment (average slope: 15°), the stream network was more dynamic and expanded rapidly, up to 10-fold, while in the steeper catchment (average slope: 24°), it remained relatively stable (only a 2-fold change). The event water contributions were also higher for the flatter catchment. The dilution of calcium at the time of the rapid expansion of the network and 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. In wet conditions, unchanneled areas must have contributed event water as well. In the flatter catchment endowed with the more dynamic stream network, a “first flush” of nitrate was detectable, possibly attributed to the transport of material from previously dry stream segments. In the catchment characterized by a more stable flowing drainage network, such flush was not observed and nitrate concentrations decreased, suggesting enhanced contributions from riparian groundwater with reducing conditions during rainfall events. Our experimental study not only highlights the large differences observable in stream network dynamics and stream chemical responses for neighboring, nearly equal-size catchments but also shows the value of fine-scale observations on both the channel network dynamics and stream chemistry to fully understand runoff generation mechanisms.