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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/hess-2020-562
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2020-562
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  05 Nov 2020

05 Nov 2020

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This preprint is currently under review for the journal HESS.

Hydrologic regimes drive nutrient export behavior in human impacted watersheds

Galen Gorski and Margaret A. Zimmer Galen Gorski and Margaret A. Zimmer
  • Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA, 95064, United States

Abstract. Agricultural watersheds are significant contributors to downstream nutrient excess issues. The timing and magnitude of nutrient mobilization in these watersheds are driven by a combination of anthropogenic, hydrologic, and biogeochemical factors that operate across a range of spatial and temporal scales. However, how, when, and where these complex factors drive nutrient mobilization has previously been difficult to capture with low-frequency or spatially limited datasets. To address this knowledge gap, we analyzed daily nitrate concentration (c) and discharge (Q) data for a four-year period (2016–2019) from five nested, agricultural watersheds in the midwestern United States that contribute nutrient loads to the Gulf of Mexico. The watersheds span two distinct landforms shaped by differences in glacial history resulting in natural soil properties that necessitated different drainage infrastructure across the study area. To investigate nutrient export patterns under different hydrologic conditions, we partitioned the hydrograph into stormflow and baseflow periods and examined those periods separately through the analysis of their concentration-discharge (c-Q) relationships on annual, seasonal, and event time scales. Stormflow showed consistent chemostatic patterns across all seasons, while baseflow showed seasonally dynamic c-Q patterns. Baseflow exhibited chemodyanmic conditions in the summer and fall and more chemostatic conditions in the winter and spring, suggesting that water source contributions during baseflow were nonstationary. Baseflow chemodynamic behavior was driven by low-flow, low-NO3 conditions during which in-stream and near-stream biological processing likely moderated in-stream NO3 concentrations. Additionally, inputs from deeper groundwater with longer residence times and lower NO3 concentration likely contributed to low-NO3 conditions in-stream, particularly in the larger watersheds. Stormflow c-Q behavior was consistent across watersheds, but baseflow c-Q behavior was linked to intensity of agriculture and density of built drainage infrastructure, with more drainage infrastructure associated with higher loads and more chemostatic export patterns across the watersheds. This suggests that how humans replumb the subsurface in response to geologic conditions has implications for hydrologic connectivity, homogenization of source areas, and subsequently nutrient export during both baseflow and stormflow. Our analysis also showed that anomalous flow periods greatly influenced overall c-Q patterns, suggesting that the analysis of high-resolution records at multiple scales is critical when interpreting seasonal or annual patterns.

Galen Gorski and Margaret A. Zimmer

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Galen Gorski and Margaret A. Zimmer

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Selected hydrograph events from Raccoon River Watershed Galen Gorski and Margaret Zimmer https://doi.org/10.4211/hs.173cff98da3c4263a110cba8c6d62406

Galen Gorski and Margaret A. Zimmer

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Short summary
Understanding when, where, and how nitrate is exported from watersheds is key to addressing the challenges that excess nutrients pose. We analyzed daily nitrate and streamflow data for five nested, agricultural watersheds that export high levels of nitrate over a four-year time period. Nutrient export patterns varied seasonally during baseflow but were stationary during stormflow. Additionally, anthropogenic and geologic factors drove nutrient export during both baseflow and stormflow.
Understanding when, where, and how nitrate is exported from watersheds is key to addressing the...
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