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

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Abstract Hydrograph separation is a widely applied technique that uses the stable isotopes of water (2H and 18O) 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 km2. 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. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Methods
World map with all study sites included in this review (red dots). Note that some sites include multiple catchments (e.g., H.J. Andrews in USA, Krycklan in Sweden, Maimai in New Zealand, etc.). The legend refers to the Köppen‐Geiger climate classification (based on Peel, Finlayson, & McMahon, )
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Time series of the calculated contributions of throughfall, soil water, and groundwater to streamflow for the different realizations for scenarios 1a (top) and 1b (bottom), as well as the fraction of the 100 realizations for which the calculations were not possible due to either contributions to streamflow >1.025 or <−0.025. See Figure for the mixing diagrams
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The mixing diagrams with the composition of the end‐members (EC on x‐axis and δ18O on the y‐axis—see Table for the actual values) and the streamflow (blue circles connected by the blue line) for the different scenarios. The error bars represent the standard deviation in the δ18O and EC of the end‐members. The maximum difference between the 10th and 90th percentile of the calculated contributions to streamflow derived from the 100 hydrograph separations with varying end‐member compositions. Note that the contributions to streamflow for the three components (without the errors) are the same for all scenarios. The maximum differences between the 10th and 90th percentile usually occurred at the time of the maximum contribution to streamflow. See Figure for the time series for the relative contributions of the end‐members to streamflow and the calculated variability in these contributions for scenarios 1a and 1b
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Three component hydrograph separation results for the reference (left column) and alternative scenarios for the two example rainfall‐runoff events in the Ressi catchment. For the changes in the composition of the end‐members for the alternative scenarios see Table
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The spatio‐temporal variability (i.e., maximum minus minimum δ18O per record, in ‰) and the maximum spatial variability in the isotopic composition for each record (indicated by different symbols) for which a hydrological compartment was sampled at five or more locations in the same catchment and reported the data. The dashed lines indicate the median values
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(Panel a): Number of records for each hydrological compartment in the database, grouped by number of sampling locations. (Panel b): Box‐plots of the number of sampling locations per record for each hydrological compartment. The boxes indicate the 25th and 75th percentile, the whiskers the 10th and 90th percentile and the horizontal line the median. The pink line marks the mean and the gray dots indicate all outliers. Surface runoff, soil water, shallow groundwater, and deep groundwater are missing one record each because the number of sampling locations was not reported in the paper. Irrigation, stemflow, and ice melt are not included due to the small number of papers in the database that reported data for these compartments
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Main land cover of the 173 catchments included in the database. The class “Other land covers” includes grass + urban; moorland + bare soil; tundra; forest + tundra; urban; wetland + grass
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Number of papers included in the database for each seven‐year period (except the last period, which includes only the last 6 years) for the different climate classes. The class “Others” includes the arid, tropical, and polar classes. Note that two papers reported data from catchments that belong to two different climate classes and were thus both counted twice
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