Abstract
Abstract. Expansion of the hydrologically connected area during rainfall events causes previously disconnected areas to contribute to streamflow. If these newly contributing areas have a different hydrochemical composition compared to the previously connected contributing areas, this may cause a change in stream water chemistry that cannot be explained by simple mixing of rainfall and baseflow. Changes in stormflow composition are, therefore, sometimes used to identify when transiently connected areas (or water sources) contribute to stormflow. We identified the dominant sources of streamflow for a steep 20 ha pre-Alpine headwater catchment in Switzerland and investigated the temporal changes in connectivity for four rainfall events based on stream water concentrations and groundwater level data. First, we compared the isotopic and chemical composition of stormflow at the catchment outlet to the composition of rainfall, groundwater and soil water. Three-component end-member mixing analyses indicated that groundwater dominated stormflow during all events, and that soil water fractions were minimal for three of the four events. However, the large variability in soil and groundwater composition compared to the temporal changes in stormflow composition inhibited the determination of the contributions from the different groundwater sources. Second, we estimated the concentrations of different solutes in stormflow based on the mixing fractions derived from two-component hydrograph separation using a conservative tracer (δ2H) and the measured concentrations of the solutes in baseflow and rainfall. The estimated concentrations differed from the measured stormflow concentrations for many solutes and samples. The deviations increased gradually with increasing streamflow for some solutes (e.g. iron and copper), suggesting increased contributions from riparian and hillslope groundwater with higher concentrations of these solutes and thus increased hydrological connectivity. The findings of this study show that solute concentrations partly reflect the gradual changes in hydrologic connectivity, and that it is important to quantify the variability in the composition of different source areas.
Highlights
During dry periods only a small part of a catchment is connected to the stream, but the connected area can expand dramatically during rainfall or snowmelt events (Stieglitz et al, 2003; Bracken and Croke, 2007; Jencso and McGlynn, 2011; van Meerveld et al, 2015)
The change in stream water composition during the four events presented in this study was not large enough to distinguish contributions from the different groundwater sources, it is evident that pre-event water dominated streamflow
The results of this study show that the spatial variability in soil water and groundwater compositions across the small, pre-Alpine headwater study catchment was large
Summary
During dry periods only a small part of a catchment is connected to the stream, but the connected area can expand dramatically during rainfall or snowmelt events (Stieglitz et al, 2003; Bracken and Croke, 2007; Jencso and McGlynn, 2011; van Meerveld et al, 2015). Knowledge of which areas are connected and contribute to streamflow is important because it helps us to shape our conceptual understanding of how catchments function. Ladouche et al (2001) showed for the 0.8 km Strengbach catchment in France that the upper layers of saturated areas contributed up to 30 % of the discharge during the initial stages of a rainfall event, even though these areas occupied only 2 % of the catchment area. During the final stage of the event, upslope and downslope areas contributed to flow. Oswald et al (2011), showed for a 0.8 km catchment in northwestern Ontario, Canada, that a large part of the catchment area was hydrologically disconnected from the stream during most events, and that there was a threshold catchment storage at which a larger area contributed to streamflow.
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