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Multiple factors affect the downstream attenuation of wetland-mediated nitrate reductions
Citation:
Forgrave, R., G. Evenson, H. Golden, J. Christensen, C. Lane, Q. Wu, AND E. D'Amico. Multiple factors affect the downstream attenuation of wetland-mediated nitrate reductions. AGU 2023 Fall Meeting, San Francisco, CA, December 11 - 15, 2023.
Impact/Purpose:
Minimal research exists assessing the extent to which these expected benefits in nutrient reductions are propagated downstream at various watershed scales to meet nutrient management goals. We ask the question: How far downstream do wetland restoration effects propagate?
Description:
Wetlands can be an effective management tool for mitigating excess nitrate loads from agricultural runoff, motivating federal and state conservations programs to incentivize wetland restoration and construction. However, uncertainties remain about which types of wetland implementation approaches are most effective at reducing nitrate loads, as well as the scale at which these expected nutrient load reduction benefits from wetland additions are measurable in watersheds with high baseline nitrate loads. In this study, we ask: how far downstream are wetland-mediated nitrate reductions realized and what factors influence variations in this nitrate response? We use a modified SWAT model to simulate wetland additions at the hydrologic response unit scale in a 9,397 km2 watershed in Iowa, USA, divided into 108 model subbasins. We simulate subbasin-scale nitrate load reductions compared to baseline data and calculate how far along the downstream path these reductions can be discerned. Results show that (1) subbasin-scale nitrate load reductions depend on wetland area added and wetland catchment area to wetland area ratios, and that (2) the distance downstream nitrate reductions continue to be observable is largely dependent on the proportional decreases in subbasin-scale nitrate. Additionally, model results show that sizable subbasin reductions in non-headwater subbasins are masked by upstream loads, obscuring the effects of wetland additions on reducing whole-watershed nitrate loads. Outcomes of this research can inform wetland incentive programs to ensure that wetland restoration and construction efforts are targeted at the most effective locations for management of loads at a watershed scale.
