Short-Term Dynamics of the Flowing Stream Drainage Density

Many headwater streams are non-perennial. The flowing stream network dynamically expands and contracts during and after rainfall events, resulting in temporal changes in the flowing stream drainage density (DD). This dynamic behavior has consequences for solute transport and the organisms that live in the streams. Therefore, it is important to understand the hydrological processes responsible for these changes in DD to better predict the impacts of climate change on riverine ecosystems. However, until now, our knowledge of event-scale DD dynamics is limited because experimental data remain sparse.

We monitored DD in two 5-ha catchments in the Swiss Alpine foothills from June to October 2021. We installed a dense wireless sensor network to monitor the water levels in the streams and groundwater, soil moisture, and precipitation. In addition, we did multiple mapping surveys during different hydrological conditions and developed a simple model to calculate DD from these measurements at a 10-min resolution. We used these data to explore how short-term changes in DD relate to water storage in the catchments.

Our surveys showed that during the wet 2021 summer, DD varied considerably both in space and time, ranging from 2.7 to 32.2 and 7.8 to 14.6 km/km² for the flatter and steeper catchment, respectively. The model provided reliable estimates of DD variations at 10-min resolution for both catchments (accuracies >0.94). In the flatter catchment, the relations between DD and either discharge or groundwater became steeper when DD was larger than 20 km/km².DD increased rapidly with wetter conditions when the groundwater levels rose to 20 cm from the surface and streamflow was initiated in multiple shallow-incised channels. From analyzing multiple consecutive rainfall events, we found that the discharge-DD relationship was counterclockwise when conditions were dry. This is likely caused by the streamflow coming from nearby the outlet where the topographic wetness index is high. Surface flow in the upstream tributaries emerges only once the maximum subsurface transport capacity is exceeded, causing a rapid increase in DD. After the rainfall ends, discharge recedes quickly, whereas DD remains high due to ongoing groundwater seepage at the channel heads. For events with wetter conditions, there was no hysteresis, likely because the maximum subsurface transport capacity is exceeded faster throughout the catchment. Such threshold behavior and hysteresis were also not observed for the steep catchment, where multiple groundwater springs were flowing throughout the study period, resulting in much smaller DD variations.