Differences in Groundwater Contributions to Streamflow Versus Watershed Nitrate Export Reveal the Importance of Scale in Evaluating Agricultural Conservation Practices.

Abstract:

Diffuse nutrient pollution from agricultural fields can result in the eutrophication of downstream water bodies, highlighting a need for conservation efforts to reduce nutrient loading to adjacent waterways. However, few studies explore how the impacts of field-scale conservation manifest at the watershed scale. For example, in the Shatto Ditch Watershed (SDW), planting cover crops decreased field-scale nitrate loss from subsurface tile drains by 69-90%; yet watershed nitrate export only decreased by 13%. Here, we explored how sources of streamflow and nutrients may influence the conservation “signature” at a field- versus watershed-scale at SDW. We used a water budget approach paired with stable isotope analysis to determine the composition of streamflow across seasons, sampling five times from Nov. 2018 to Sept. 2019. While we hypothesized that watershed-scale patterns in nutrient loss were driven by direct groundwater upwelling, we found that this pathway only accounted for 43% of streamflow on average. We also sampled synoptically in November 2018 to develop a nitrogen mass balance for the watershed; results indicated groundwater upwelling contributed only ~1% of nitrate export at the watershed outlet, while tile drains contributed the remaining ~99%. Specifically, three large tile drains with unknown drainage area and landscape management contributed ~69% of the watershed nitrate export, reducing the impacts of field-scale conservation. Our study demonstrates the importance of cross-scale analysis to accurately evaluate the complex interactions between sources of streamflow and nutrient loss at the watershed scale.