Submitted to: Ecological Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 7, 2012
Publication Date: July 1, 2013
Citation: Tomer, M.D., Crumpton, W.G., Bingner, R.L., Kostel, J.A., James, D.E. 2013. Estimating nitrate load reductions from placing constructed wetlands in a HUC-12 watershed using LiDAR data. Ecological Engineering. 56:69-78. Interpretive Summary: The wide installation of nutrient removal wetlands in tile drained watersheds of the Midwest may represent the best opportunity to reduce nitrate loads in the Mississippi River Basin and mitigate Gulf of Mexico hypoxia with a single conservation practice. However suitable locations to place wetlands must be identified, along with their potential impact on nutrient load reductions at the watershed scale, to implement this approach. In this study, we used results of an aerial laser altimetry survey to develop a detailed topographic map of a Hydrologic Unit Code (HUC-12) (16,000 acre) watershed in Illinois. We applied a conservative set of criteria, modified from a wetlands program in Iowa, to identify eleven sites that could readily be converted to wetlands with minimal loss of productive cropland. These wetlands could intercept and treat tile drainage from 30% of the watershed. A modeling exercise showed that these wetlands could reduce nitrate loads from the watershed by as much as 16%. However load reductions among the wetland locations varied considerably, depending on watershed-to-wetland area ratios, land use in the upslope area, and nitrate loads generated under that land use. These issues will need to be considered by policy makers interested in developing incentive structures that encourage wetlands, including the establishment of nutrient trading schemes.
Technical Abstract: Mitigating Gulf of Mexico hypoxia will require practices to reduce nitrate losses from tile drains throughout the upper Mississippi River basin. Wetlands are a key practice to help accomplish this but the locations of feasible wetland sites will need to be determined on a watershed specific basis. This study’s objective was to demonstrate that LiDAR topographic data can be used to identify the locations of feasible wetland sites in a 6500 ha HUC-12 watershed in northern Illinois, and then estimate the impact on watershed nitrate loads from wetlands hypothetically constructed at those locations. The evaluation resulted in the identification of eleven sites where wetlands could intercept tile drainage from 30% of the watershed. The USDA AnnAGNPS model was used to estimate nitrate loads delivered to those locations and from the watershed during a 30-year period of simulated climate. The model results were consistent with discharge and nitrate load regimes in the region. Nitrate reduction by the wetlands was estimated based on published regression models. The wetlands could reduce average watershed nitrate loads by 11-16%, a significant reduction from treating just 30% of the watershed with a single practice. Results illustrated a wide variation in N-removal performance among wetlands based on contributing-to-wetland area ratios, and the impacts of land use on discharge and N loads to individual wetland locations. These conclusions are relevant for planning watershed conservation efforts and establishment of nutrient trading schemes.