Citation:

Evenson, G., H. Golden, J. Christensen, C. Lane, M. Kalcic, A. Rajib, Q. Wu, T. Mahoney, E. White, AND E. DAmico. Factors affecting wetland-based nitrate reductions at river-basin scales: a model-based analysis. Soil and Water Conservation Society (SWCS) Annual Meeting, Denver, CO, July 31 - August 03, 2022.

Impact/Purpose:

Wetland restoration and construction are potential management solutions for water quality problems attributable to surplus nitrogen. We used a model to project wetland restoration and construction impacts on river basin nitrate yields and loads using varying assumptions regarding the quantity, location, catchment-size, nitrate removal efficiency, and tile-interception capacity of restored and constructed wetlands in the Upper Mississippi River Basin. We found that projections can vary substantially under varying conditions and that these conditions should be managed so that constructed and restored wetland intercept greater quantities of surplus nitrogen. Projections can help program and regional partners guide wetland restoration and construction efforts.

Description:

River-basin scale wetland restoration and construction is as a potential management solution for large-scale water quality problems attributed to surplus nitrogen. Yet the magnitude of change that these management actions will affect upon river-basin scale nitrogen losses is contingent on several factors, including: (1) the aggregate area, (2) location, (3) catchment size, (4) nitrogen-removal efficiency, and (5) tile-interception capacity of the restored and constructed wetlands. We used a Soil and Water Assessment Tool (SWAT) model of the Upper Mississippi River Basin (UMRB) (~440,000 km2) to complete over five hundred simulations depicting a range of possible factor conditions to assess factor impacts on simulated nitrate loads at the UMRB-outlet and local (i.e., hydrologic response unit or HRU) nitrate yields. Our results showed that these factors must be co-managed to increase nitrate loading rates to the restored and constructed wetlands; and that the magnitude of surplus nitrate reduction from wetland restoration and construction will vary widely under different factor conditions – e.g., if ~4,400 km2 (or 1% of UMRB area) of wetland were restored or constructed, mean annual UMRB nitrate loads could decrease by 6 to 67%, with larger load reductions requiring impractical factor conditions. Our analysis provides an improved understanding of how wetland restoration and construction impacts river-basin scale nitrogen dynamics. Our analysis also highlights the implications of uncertain restored and constructed wetland factor conditions.

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