Water transport pathways influence the propagation of field‐scale NO3−‐N reductions to the watershed scale

• Published in: Hydrological processes 2022-02, Vol.36 (2), p.n/a
• Main Authors: Grose, Amelia L., Speir, Shannon L., Thellman, Audrey N., Dee, Martha M., Tank, Jennifer L.
• Format: Article
• Language: English
• Subjects: Agricultural land; Agricultural practices; Agricultural runoff; Conservation; Conservation practices; Cover crops; cross‐scale analysis; Downstream effects; Drainage area; Drains; Eutrophication; Groundwater; Mass balance; Mineral nutrients; Nitrates; nitrification; Nitrogen; nutrient budget; Nutrient loss; Nutrient sources; Nutrients; Ocean circulation; Phosphorus; Propagation; Runoff; Sampling; Stable isotopes; Stream discharge; Stream flow; Tile drains; Upwelling; Water budget; Water resources; water stable isotopes; Water transport; watershed export; Watersheds; Waterways
• ISSN: 0885-6087; 1099-1085
• DOI: 10.1002/hyp.14476

Diffuse nutrient runoff from agricultural fields can result in the eutrophication of downstream water bodies, highlighting a need for conservation efforts to reduce dissolved nitrogen (N) and phosphorus (P) loading to adjacent waterways. However, few studies explore how the impacts of field‐scale conservation manifest at the watershed scale. We explored how sources of streamflow and nutrients may influence the magnitude of the impact of conservation practices at a field versus watershed scale at the Shatto ditch watershed (SDW), where the planting of winter cover crops reduced field‐scale nitrate‐N (NO3−‐N) losses from subsurface tile drains by 69%–90%; yet watershed NO3−‐N export only decreased by 13%. To resolve this discrepancy, we used a water budget approach paired with water stable isotope (18O and 2H) analysis to determine the composition of streamflow across seasons, sampling five times from November 2018 to September 2019. While we hypothesized that watershed‐scale patterns in nutrient export were driven by direct groundwater upwelling, we found that this pathway only accounted for 43% of streamflow on average. We also developed a NO3−‐N mass balance for the watershed during our November 2018 sampling; results indicated groundwater upwelling contributed only ~1% of NO3−‐N export at the watershed outlet, while subsurface tile drains contributed the remaining 99%. Specifically, three large county tile drains (with an unknown drainage area and extent of conservation practice implementation) contributed ~69% of the watershed NO3−‐N export, obscuring the impacts of field‐scale conservation in SDW. Our study highlights the importance of cross‐scale analyses to accurately evaluate the effect of conservation given the interactions between sources of streamflow and nutrient loss at the watershed scale. In an agricultural system with conservation practices implemented to reduce NO3−‐N runoff, groundwater was a significant source of streamflow (average = 43% of total streamflow); however, groundwater contributed ~1% of the total NO3−‐N load. Four large tile drains with unknown drainage areas contributed most of the NO3−‐N export (69%), obscuring the impacts of field‐scale conservation when looking at the watershed‐scale. This study emphasizes the importance of cross‐scale analyses for accurately quantifying the efficacy of conservation.