Do stream water solute concentrations reflect when connectivity occurs in a small, pre-Alpine headwater catchment?

Abstract

<jats:p>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. </jats:p>

Vertical Connectivity Regulates Water Transit Time and Chemical Weathering at the Hillslope Scale

“…Solutes that are enriched in shallow soils, such as DOC, almost always show flushing CQ patterns (Zarnetske et al., 2018). Metals that form complexes with DOC often exhibit flushing patterns (Herndon et al., 2015; Kiewiet et al., 2020; McIntosh et al., 2017).…”

Missing connectivity during summer drought controls DOC mobilization and export in a small, forested catchment

“…As it controls runoff response during events, it has a direct impact on solute export (Kiewiet et al, 2020;Detty and McGuire, 2010). Therefore, it influences nutrient dynamics across different temporal and spatial scales (Covino, 2017).…”

Hydrological trends and the evolution of catchment research in the Alptal valley, central Switzerland

“…Stream chemistry during baseflow conditions does not differ much among the catchments in the Alptal (Fischer et al, 2015) but can vary over short distances due to inputs from chemically distinct groundwater sources (Kiewiet et al, 2019; van Meerveld et al, 2018). The high temporal variability in stream water quality suggests large variations in runoff flow pathways and mixing, as well as dynamically changing connectivity of runoff‐generating areas (Kiewiet et al, 2020; Rinderer et al, 2019) during rainfall events.…”

“…Stream chemistry during baseflow conditions does not differ much among the catchments in the Alptal (Fischer et al, 2015) but can vary over short distances due to inputs from chemically distinct groundwater sources (Kiewiet et al, 2019;van Meerveld et al, 2018). The high temporal variability in stream water quality suggests large variations in runoff flow pathways and mixing, T A B L E 1 Statistical significance of the trends in average air temperature, total precipitation, maximum snow water equivalent (max SWE), duration of snow cover, and average and median runoff, and the direction of the trends (positive: +, negative: -) as well as dynamically changing connectivity of runoff-generating areas (Kiewiet et al, 2020;Rinderer et al, 2019) during rainfall events.…”