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.
Landscape Controls on Nutrient Export during Snowmelt and an Extreme Rainfall Runoff Event in Northern Agricultural Watersheds
