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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/hess-2019-420
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2019-420
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 15 Aug 2019

Submitted as: research article | 15 Aug 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Seasonal partitioning of precipitation between streamflow and evapotranspiration, inferred from end-member splitting analysis

James W. Kirchner1,2,3 and Scott T. Allen1,4 James W. Kirchner and Scott T. Allen
  • 1Dept. of Environmental Systems Science, ETH Zurich, 8092 Zurich, Switzerland
  • 2Swiss Federal Research Institute WSL, 8903 Birmensdorf, Switzerland
  • 3Dept. of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA
  • 4Dept. of Geology and Geophysics, University of Utah, Salt Lake City, UT, 84112, USA

Abstract. A key attribute of the terrestrial water cycle is the partitioning of precipitation into its two ultimate fates: green water that is evaporated or transpired back to the atmosphere, and blue water that is discharged to stream channels. Measuring this partitioning is difficult, particularly on seasonal timescales. End-member mixing analysis has been widely used to quantify streamflow as a mixture of isotopically distinct sources, but knowing where streamwater comes from is not the same as knowing where precipitation goes, and this latter question is the one we seek to answer. Here we introduce end-member splitting analysis, which uses isotopic tracers and water flux measurements to quantify how isotopically distinct inputs (such as summer vs. winter precipitation) are partitioned into different ultimate outputs (such as evapotranspiration and summer vs. winter streamflow). End-member splitting analysis has modest data requirements, and can potentially be applied in many different catchment settings. We illustrate this data-driven, model-independent approach with publicly available biweekly isotope time series from Hubbard Brook Watershed 3. A marked seasonal shift in isotopic composition allows us to distinguish rainy-season (April–November) and snowy-season (December–March) precipitation, and to trace their respective fates. End-member splitting shows that about one-sixth (18 ± 2 %) of rainy-season precipitation is discharged during the snowy season, but this accounts for over half (60 ± 9 %) of snowy-season streamflow. By contrast, most (55 ± 13 %) snowy-season precipitation becomes streamflow during the rainy season, where it accounts for 38 ± 9 % of rainy-season streamflow. Our analysis thus shows that significant fractions of each season's streamflow originated as the other season's precipitation, implying significant inter-seasonal water storage within the catchment, as both groundwater and snowpack. End-member splitting can also quantify how much of each season's precipitation is eventually evapotranspired. At Watershed 3, we find that only about half (44 ± 8 %) of rainy-season precipitation evapotranspires, but almost all (85 ± 15 %) evapotranspiration originates as rainy-season precipitation, implying that there is relatively little inter-seasonal water storage supplying evapotranspiration. We show how results from this new technique can be combined with young water fractions (calculated from seasonal isotope cycles in precipitation and streamflow) and new water fractions (calculated from correlations between precipitation and streamflow isotope fluctuations) to infer how precipitation is partitioned on multiple time scales. This proof-of-concept study demonstrates that end-member mixing and splitting yield different, but complementary, insights into catchment-scale partitioning of precipitation into blue water and green water. It could thus help in gauging the vulnerability of both water resources and terrestrial ecosystems to changes in seasonal precipitation.

James W. Kirchner and Scott T. Allen
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Status: open (until 10 Oct 2019)
Status: open (until 10 Oct 2019)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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James W. Kirchner and Scott T. Allen
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Short summary
Perhaps the oldest question in hydrology is Where does water go when it rains? Here we present a new way to measure how the terrestrial water cycle partitions precipitation into its two ultimate fates: green water that is evaporated or transpired back to the atmosphere, and blue water that is discharged to stream channels. Our analysis may help in gauging the vulnerability of both water resources and terrestrial ecosystems to changes in rainfall patterns.
Perhaps the oldest question in hydrology is Where does water go when it rains? Here we present a...
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