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

Abstract

AbstractIsotope 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.