Hydrologic regimes drive nitrate export behavior in human-impacted watersheds

Gorski, Galen; Zimmer, Margaret A.

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 data sets. To address this knowledge gap, we analyzed daily nitrate concentration (inline-formulac) and discharge (inline-formulaQ) data for a 4-year period (2016–2019) from five nested, agricultural watersheds in the midwestern United States that contribute nutrient loads to the Gulf of Mexico. These records allow us to investigate nutrient mobilization patterns at a temporal and spatial resolution not previously possible. 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 (inline-formulacinline-formulaQ) relationships on annual, seasonal, and event timescales. Stormflow showed consistent chemostatic patterns across all seasons, while baseflow showed seasonally dynamic inline-formulacinline-formulaQ patterns. Baseflow exhibited chemodynamic 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-NOinline-formula M7inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimg76bbd0535ad9c3e987722e2e722d5d00 hess-25-1333-2021-ie00001.svg9pt16pthess-25-1333-2021-ie00001.png conditions during which in-stream and near-stream biological processing likely moderated in-stream NOinline-formula M8inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimg7de8959157e6c258409d4c11688ca166 hess-25-1333-2021-ie00002.svg9pt16pthess-25-1333-2021-ie00002.png concentrations. Additionally, inputs from deeper groundwater with longer residence times and lower-NOinline-formula M9inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimg1933cd4f78557ae19e1c84fa4d0b5473 hess-25-1333-2021-ie00003.svg9pt16pthess-25-1333-2021-ie00003.png concentration likely contributed to low-NOinline-formula M10inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimg78ed0f7e81615226176402cdd6a1afd5 hess-25-1333-2021-ie00004.svg9pt16pthess-25-1333-2021-ie00004.png conditions in stream, particularly in the larger watersheds. Stormflow inline-formulacinline-formulaQ behavior was consistent across watersheds, but baseflow inline-formulacinline-formulaQ behavior was linked to the intensity of agriculture and the density of built drainage infrastructure, with more drainage infrastructure associated with higher loads and more chemostatic export patterns across the watersheds. This suggests that the way 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 inline-formulacinline-formulaQ patterns, suggesting that the analysis of high-resolution records at multiple scales is critical when interpreting seasonal or annual patterns.

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Gorski, Galen / Zimmer, Margaret A.: Hydrologic regimes drive nitrate export behavior in human-impacted watersheds. 2021. Copernicus Publications.

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