Spatial variability in specific discharge and streamwater chemistry during low flows: Results from snapshot sampling campaigns in eleven Swiss catchments

Abstract

Catchments consist of distinct landforms that affect the storage and release of subsurface water. Certain landforms may be the main contributors to streamflow during extended dry periods, and these may vary for different catchments in a given region. We present a unique dataset from snapshot field campaigns during low‐flow conditions in 11 catchments across Switzerland to illustrate this. The catchments differed in size (10 to 110 km2), varied from predominantly agricultural lowlands to Alpine areas, and covered a range of physical characteristics. During each snapshot campaign, we jointly measured streamflow and collected water samples for the analysis of major ions and stable water isotopes. For every sampling location (basin), we determined several landscape characteristics from national geo‐datasets, including drainage area, elevation, slope, flowpath length, dominant land use, and geological and geomorphological characteristics, such as the lithology and fraction of quaternary deposits. The results demonstrate very large spatial variability in specific low‐flow discharge and water chemistry: Neighboring sampling locations could differ significantly in their specific discharge, isotopic composition, and ion concentrations, indicating that different sources contribute to streamflow during extended dry periods. However, none of the landscape characteristics that we analysed could explain the spatial variability in specific discharge or streamwater chemistry in multiple catchments. This suggests that local features determine the spatial differences in discharge and water chemistry during low‐flow conditions and that this variability cannot be assessed a priori from available geodata and statistical relations to landscape characteristics. The results furthermore suggest that measurements at the catchment outlet during low‐flow conditions do not reflect the heterogeneity of the different source areas in the catchment that contribute to streamflow.

An Increase in Specific Discharge With Catchment Area Implies That Bedrock Infiltration Feeds Large Rather Than Small Mountain Headwater Streams

“…Previous studies that demonstrated decreasing specific discharge with catchment area (Figure 1c, Table 5) implied that more water is stored in bedrock fractures and quaternary deposits in smaller catchments compared to larger catchments, and thus smaller catchments contribute disproportionately to low flows. They also indicated that the infiltration of streamflow into the streambed in larger catchments can result in the observed decreasing pattern (Floriancic et al, 2019; Tetzlaff & Soulsby, 2008). A study in one catchment with a constant baseflow‐specific discharge relationship reported that the average contribution of bedrock flow was similar for both small and large streams, with contributions of bedrock groundwater ranging between 42% to 62% for third‐ to sixth‐order streams under baseflow conditions (Uchida & Asano, 2010).…”

“…The second pattern is that specific discharge increases with catchment area (Figure 1b; e.g., Fujimoto et al, 2016; Shaman et al, 2004), presumably because of the contribution of groundwater flow paths, as demonstrated theoretically by Tóth (1963). The third pattern is a decrease in specific discharge with catchment area during low‐flow conditions (Figure 1c; Floriancic et al, 2019; Tetzlaff & Soulsby, 2008). Many of the studies that have demonstrated independent, increasing, or decreasing trends in specific discharge with increased catchment size have also found highly variable specific discharge in smaller catchments (0.1–1 km 2 ) that show no coherent pattern with catchment area, whereas larger catchments tend to show less variation (e.g., Tetzlaff & Soulsby, 2008; Woods et al, 1995).…”

Spatiotemporal dynamics of water sources in a mountain river basin inferred through δ2H and δ18O of water

“…For each streamflow gaging station, we calculated two streamflow statistics based on the period of record to characterize individual stream hydrology: unit discharge at baseflow and baseflow index. Unit discharge at baseflow, or specific discharge, was calculated by dividing average streamflow for the month of September by the upstream contributing area for each gage location (Floriancic et al, 2019;Tague & Grant, 2004). Baseflow index is the ratio of annual baseflow to total streamflow and it represents the contribution of groundwater to river flow (Beck et al, 2013;Smakhtin, 2001 Outlets of major tributaries and the mainstem sampled biweekly…”

Sources of variability in springwater chemistry in Fool Creek, a high‐elevation catchment of the Rocky Mountains, Colorado, USA

“…The high ANC springs originate within a faulted section of brecciated granodiorite dikes and exposed outcrops that is more fractured than the surrounding gneiss and schist bedrock, and where increased surface area and contact time with bedrock contributes to higher springwater ion concentrations. Elsewhere, unmapped local geologic features rather than broader landscape factors determined variability in groundwater and stream chemistry for 11 Swiss catchments (Floriancic et al, 2019). Similarly, high alkalinity and SC emerged from springs in the Scottish Highlands adjacent to small calcareous outcrops (Scheliga et al, 2017).…”

“…Elsewhere, unmapped local geologic features rather than broader landscape factors determined variability in groundwater and stream chemistry for 11 Swiss catchments (Floriancic et al, 2019).…”