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Modeling Downstream Attenuation of Wetland-Mediated Nitrate Reductions
Citation:
Lane, C., B. Forgrave, G. Evenson, H. Golden, J. Christensen, Q. Wu, E. D'Amico, AND J. Prenger. Modeling Downstream Attenuation of Wetland-Mediated Nitrate Reductions. 2024 Soil and Water Conservation Association Annual Conference, Myrtle Beach, SC, July 21 - 24, 2024.
Impact/Purpose:
Our work focuses on simulating and analyzing the downstream effects of NFWs at two scales: the ~0.5 million km2 Upper Mississippi River Basin (UMRB) and the Raccoon River Watershed (~9400 km2), Iowa. In the UMRB we ask: (1) what is the potential for NFWs to reduce nitrate within UMRB subbasins as well as at the UMRB outlet and (2) what NFW and NFW catchment characteristics influence nitrate reductions at these scales? Our more recent focus in the Raccoon River Watershed is to address how far downstream nitrate reductions from restored NFWs will affect water quality. Overall, we find that NFW catchment area-to-NFW area ratios, tile drainage, and distances to watershed outlets – along with other wetland characteristics – are key factors in determining the magnitude of NFW-mediated nitrate reductions, at both river basin (UMRB) and watershed (Raccoon River) scales.
Description:
Connections between agricultural runoff and excess nitrogen in the Upper Mississippi River Basin are well-documented, as is the potential role of constructed wetlands in mitigating this surplus nitrogen. However, limited knowledge exists about the “best” placement of these wetlands for downstream nitrogen reductions within a whole watershed context as well as how far downstream these benefits are realized. In this study, we simulate the cumulative impacts of diverse wetland restoration scenarios on downstream nitrate reductions in different subbasins of the Raccoon River Watershed, Iowa, USA, and spatially trace their relative effects downstream. Our simulated results underscore previous work demonstrating that the total area of wetlands and the wetland-catchment-to-wetland area ratio are both significant factors for determining the nitrate load reduction benefits of wetlands at subbasin scales. However, we found these wetland-mediated nitrate reduction benefits are quickly attenuated downstream throughout the watershed, despite the magnitude of the subbasin-scale nitrate decreases. The relatively rapid attenuation of wetland effects is largely due to downstream nitrate load contributions from untreated subbasins. However, higher subbasin-scale nitrate reductions from wetland-based conservation practices resulted in longer downstream distances prior to attenuation. This study highlights the importance of considering the spatial location of constructed or restored wetlands relative to the area within the watershed where nitrogen reductions are most needed.
