Hydrologic regimes drive nitrate export behavior in human-impacted watersheds

• Published in: Hydrology and earth system sciences 2021-03, Vol.25 (3), p.1333-1345
• Main Authors: Gorski, Galen, Zimmer, Margaret A
• Format: Article
• Language: English
• Subjects: Agricultural watersheds; Agriculture; Analysis; Anthropogenic factors; Base flow; Biogeochemistry; Creeks & streams; Discharge; Drainage; Exports; Glacial periods; Glaciers; Groundwater; Human acts; Human behavior; Human influences; Hydrologic regime; Hydrology; Infrastructure; Land use; Landforms; Loads (forces); Low flow; Mineral nutrients; Nitrogen; Nutrient loading; Records; Resolution; Rivers; Seasons; Soil properties; Spatial discrimination; Spatial resolution; Storms; Water quality
• ISSN: 1607-7938; 1027-5606; 1607-7938
• DOI: 10.5194/hess-25-1333-2021

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 (c) and discharge (Q) 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 (c–Q) relationships on annual, seasonal, and event timescales. Stormflow showed consistent chemostatic patterns across all seasons, while baseflow showed seasonally dynamic c–Q 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-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 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 connecti