This study aimed to investigate the seasonal variability of runoff generation processes, the sources of stream water and the controls on the contribution of event water to streamflow for a small forested catchment in the Italian pre-Alps. Hydrometric, isotopic and electrical conductivity data collected between August 2012 and August 2013 revealed a marked seasonal variability in runoff responses. Noticeable differences in runoff coefficients and hydrological dynamics between summer and fall/spring rainfall events were related to antecedent moisture conditions and event size. Two-and three-component hydrograph separation and end-member mixing analysis showed an increase in event water contributions to streamflow with event size and average rainfall intensity. Event water fractions were larger during dry conditions in summer, suggesting that stormflow generation in summer consisted predominantly of direct channel precipitation and some saturated overland flow from the riparian zone. On the contrary, groundwater and hillslope soil water contributions dominated the streamflow response during wet conditions in fall. Seasonal differences were also noted between event water fractions computed based on isotopic and electrical conductivity data, likely due to the dilution effect during the wetter months.

Abstract. Flowing stream networks dynamically extend and retract, both seasonally and in response to precipitation events. These network dynamics can dramatically alter the drainage density and thus the length of subsurface flow pathways to flowing streams. We mapped flowing stream networks in a small Swiss headwater catchment during different wetness conditions and estimated their effects on the distribution of travel times to the catchment outlet. For each point in the catchment, we determined the subsurface transport distance to the flowing stream based on the surface topography and determined the surface transport distance along the flowing stream to the outlet. We combined the distributions of these travel distances with assumed surface and subsurface flow velocities to estimate the distribution of travel times to the outlet. These calculations show that the extension and retraction of the stream network can substantially change the mean travel time and the shape of the travel time distribution. During wet conditions with a fully extended flowing stream network, the travel time distribution was strongly skewed to short travel times, but as the network retracted during dry conditions, the distribution of the travel times became more uniform. Stream network dynamics are widely ignored in catchment models, but our results show that they need to be taken into account when modeling solute transport and interpreting travel time distributions.

While temporary streams account for more than half of the global discharge, high spatiotemporal resolution data on the three main hydrological states (dry streambed, standing water, and flowing water) of temporary stream remains sparse. This study presents a low-cost, multi-sensor system to monitor the hydrological state of temporary streams in mountainous headwaters. The monitoring system consists of an Arduino microcontroller board combined with an SD-card data logger shield, and four sensors: an electrical resistance (ER) sensor, temperature sensor, float switch sensor, and flow sensor. The monitoring system was tested in a small mountainous headwater catchment, where it was installed on multiple locations in the stream network, during two field seasons (2016 and 2017). Time-lapse cameras were installed at all monitoring system locations to evaluate the sensor performance. The field tests showed that the monitoring system was power efficient (running for nine months on four AA batteries at a five-minute logging interval) and able to reliably log data (<1% failed data logs). Of the sensors, the ER sensor (99.9% correct state data and 90.9% correctly timed state changes) and flow sensor (99.9% correct state data and 90.5% correctly timed state changes) performed best (2017 performance results). A setup of the monitoring system with these sensors can provide long-term, high spatiotemporal resolution data on the hydrological state of temporary streams, which will help to improve our understanding of the hydrological functioning of these important systems.

Floodplain deposition rates have increased markedly under influence of human impact throughout the late Holocene in many western and central European catchments. Consequently the geomorphology and ecology of many floodplains changed. In this study we discuss this human impact and its influence on the floodplain geoecology during the middle and late Holocene for the headwaters of the Dijle catchment, located in the Belgian loess belt. The floodplain geoecology and the regional vegetation was reconstructed from sedimentological and palynological analyses. An age-depth model for the studied sequences was obtained using 17 radiocarbon dates. Statistical analyses of the pollen data (cluster analysis and canonical correspondence analysis) were used to detect changes in the pollen record. Our data show that until c. 2500 cal. BP, human impact was nearly absent or localized with no discernible influence on the floodplain geoecology. The floodplain was in a stable phase and consisted of a marshy environment where organic material could accumulate, which is interpreted as the natural state of the floodplain. From c. 2500 cal. BP onwards, human impact gradually increased. However, only when human impact in the catchment crossed a threshold around 500 cal. BP, the floodplain geoecology changed with clearing of the Alder carr forest, the establishment of a single channel river and the dominance of minerogenic overbank sedimentation. Spatial variability in the coupling between increasing human impact and changes in floodplain geoecology can be attributed to differences in hillslope-floodplain connectivity and local differences in human impact.

We used a dual stable isotope approach ( 2 H and 18 O) and electrical conductivity data in combination with hydrometric measurements to trace water fluxes in the soil, stream and trees in a small forested watershed in the Italian Pre-Alps. The aim was to understand the main water sources for plant transpiration and runoff generation. The data were collected between events and during rainfall events in order to assess the sources of tree uptake under different conditions. Sampling and analysis are still continuing but preliminary results show that the isotopic composition of tree water was similar to soil water and rain water but statistically different from streamflow and groundwater. This suggests that trees used predominantly soil water, rather than groundwater, during the study period. No marked difference was found between the isotopic composition of the sap of trees in the riparian zone and the sap of trees on the hillslope. However, during dry conditions sap in riparian trees slowly became more similar to deep soil water and groundwater, indicating a possible switch in water uptake from shallow to deeper soil water. In contrast to the quick response of the stream, the composition of tree water did not change immediately after a rainfall event, even though the composition of shallow soil water had changed, which may be caused by storage of water in the tree and the low vapor pressure deficit that suppressed transpiration after the rainfall event. Future work will be carried out to monitor the seasonal variability in water uptake and its response to rainfall events in more detail.