Effects of Spatial Variability in the Groundwater Isotopic Composition on Hydrograph Separation Results for a Pre‐Alpine Headwater Catchment

Abstract

Isotope hydrograph separation is a powerful tool to investigate catchment functioning. In most hydrograph separation studies, a pre‐event baseflow sample is used to represent the pre‐event water, and thus, baseflow is assumed to be a mixture of all the water that is stored in the catchment. However, baseflow may not be representative of all water stored in the catchment because some sources may not contribute to baseflow. This is problematic when the isotopic composition of the sources is highly variable. We quantified the effects of spatial variability in the shallow groundwater isotopic composition on pre‐event water characterization and hydrograph separation results. We compared the composition of groundwater sampled at 38 wells in a 0.2 km2 pre‐alpine catchment with stream water sampled before, during, and after three rainfall events. We estimated the number of groundwater samples needed to characterize the average groundwater composition in the catchment and its spatial variability and compared the results of two‐component hydrograph separations for different ways to characterize the pre‐event water. We found that differences in the calculated pre‐event water fractions and uncertainties were large and depended on which and how many samples were used to characterize the pre‐event water composition. Analyses based on a limited number of groundwater samples likely underestimate the real uncertainty and can give a false impression of accuracy. Our results highlight the importance of representing the variability in the pre‐event water composition when applying hydrograph separation analyses. We therefore recommend sampling pre‐event water at multiple locations or estimating the variability based on literature values.

Optimal Baseflow Separation Through Chemical Mass Balance: Comparing the Usages of Two Tracers, Two Concentration Estimation Methods, and Four Baseflow Filters

“…A possible explanation is that SEC is more sensitive to streamflow change, a more definite “C‐Q” relationship than turbidity does as illustrated by the higher SCC magnitude in Figure 2f. The reason that SEC has a more definite relationship with streamflow than turbidity may be explained by the differences in sources and exporting mechanisms of solutes and particulates (Kiewiet et al., 2020; Liu et al., 2021; Zarnaghsh & Husic, 2021). On the one hand, the dynamic of SEC is determined by the longitudinal and lateral expansion/contraction of active channels which connect/disconnect the different solute reservoirs (Kiewiet et al., 2020; Wymore et al., 2019; Yang et al., 2021).…”

Evaluation of hydrograph separation techniques with uncertain end‐member composition

“…Other studies have also noted spatial (Penna & van Meerveld, 2019) and temporal (Feng et al, 2002; Liu et al, 2004) variability in end‐member composition. Furthermore, the spatial and temporal variability in end‐member composition and its effect on the hydrograph has been observed in catchments <1 km 2 in size (Cayuela et al, 2019; Kiewiet et al, 2020). Studies such as these emphasize the importance of spatially diverse and temporally detailed end‐member data for hydrograph separation.…”

“…Detailed instream sampling during baseflow along with samples from a single spring in the study area were used to characterize concentrations of an old‐water end‐member. Many studies have highlighted the issues with using poorly characterized end‐member concentrations to perform hydrograph separations (Cayuela et al, 2019; Kiewiet et al, 2020; Penna & van Meerveld, 2019), but characterizing end‐member concentrations via detailed spatial and temporal sampling is not always possible, particularly in remote catchments. This highlights the need to develop methods to overcome inevitable end‐member data limitations.…”

Comparison of two isotopic hydrograph separation methods in the Hydrological Open Air Laboratory, Austria

“…In certain cases, this condition does not hold true. For instance, if old water is not completely mixed within the catchment, for example, if soil water with different isotopic composition is not being mixed, other water sources may contribute (e.g., Kiewiet et al, 2020; Tetzlaff et al, 2014; von Freyberg et al, 2017). This condition also does not hold true, if the selection of the new water end member is incorrect, for example, due to not capturing enough samples during intense rainfall events when the temporal variability of rainfall isotope values may be large, leading to an incorrect weighted mean of the samples (McDonnell et al, 1990).…”