Spatial variability in the isotopic composition of water in small catchments and its effect on hydrograph separation

Abstract

Hydrograph separation is a widely applied technique that uses the stable isotopes of water ( 2 H and 18 O) or other tracers to quantify the contribution of different water sources to streamflow. For its successful application it is critical to adequately characterize these sources (end-members). In most small catchment studies, water samples are collected from end-members at one or a few locations that are assumed to be representative for the entire catchment. We tested this assumption by reviewing 148 papers that used the stable isotopes of water to investigate hydrological processes in catchments up to 10 km 2 . We assessed the typical spatial variability in the isotopic composition of different hydrological compartments when they were sampled at five or more locations across a catchment. The median reported spatial variability was largest for snowmelt and soil water, followed by throughfall and shallow groundwater. To determine how this spatial variability might affect isotope-based hydrograph separation results, we used three-component hydrograph separation for two real rainfall-runoff events and a synthetic rainfall-runoff event and adjusted the isotopic composition of the end-members (throughfall, soil water, and shallow groundwater) by the median observed spatial variability. The estimated maximum contributions of the three components differed by up to 26% from the reference scenario. This suggests that caution is needed when interpreting hydrograph separation results if they are based on samples taken at one or only a few locations. Above all, these results show that the assumption of negligible spatial variability may not be valid for small catchments.

On the Spatio-Temporal Under-Representation of Isotopic Data in Ecohydrological Studies

“…In essence, the world is intrinsically enormously heterogeneous (Pfister and Kirchner, 2017;Penna and van Meerveld, 2019), and heterogeneity is responsible for the spatio-temporal variability of hydrological and ecohydrological states as well as scale-dependent flow and transport properties (Troch et al, 2009). For ecohydrological systems, we distinguish here between structural heterogeneity (physical structure of soils and plants) and process heterogeneity (physical and physiological processes in soils and plants).…”

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

“…Wiekenkamp et al, 2016). Antecedent soil moisture conditions also affect runoff amounts and stream chemistry (Zehe et al, 2010;Uber et al, 2018;Knapp et al, 2020) as well as hillslope-stream connectivity (Penna et al, 2011). Further investigation of the response of soil water, the distribution of soil water chemistry and the interaction between soil water and groundwater during rainfall events is thus important if we want to understand the influence of soil water on hydrologic connectivity and when and where soil water contributes to streamflow.…”

What Ecohydrologic Separation Is and Where We Can Go With It

“…In reality, soil structure is three‐dimensional, and not only do vertical flow rates vary laterally (as conceptualized in Figure 1c), but water also flows laterally at different rates. Both may yield lateral isotopic variations that can be as large as the vertical variations (Goldsmith et al, 2019; Penna & van Meerveld, 2019; Yang et al, 2016), which further complicate identifying depths of root‐water uptake. Theoretically, where preferential flow and heterogeneous transport dominate, depth of uptake may not be inferable from isotope variations unless a strong evaporation signal is present (Figure 1c).…”