Potential for saturated riparian buffers to treat tile drainage among 32 watersheds representing Iowa landscapes

Authors: Tomer, Mark; PORTER, SARAH - Former ARS Employee; James, David; VAN HORN, JESSICA - Former ARS Employee

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2019
Publication Date: 7/6/2020
Citation: Tomer, M.D., Porter, S.A., James, D.E., Van Horn, J.D. 2020. Potential for saturated riparian buffers to treat tile drainage among 32 watersheds representing Iowa landscapes. Journal of Soil and Water Conservation. 75(4):453-459. https://doi.org/10.2489/jswc.2020.00129.
DOI: https://doi.org/10.2489/jswc.2020.00129

Interpretive Summary: The saturated buffer is a conservation practice that diverts agricultural tile drainage into riparian (streamside) soils to reduce nitrate loads. The cost effectiveness of the saturated buffer practice compares favorably with other practices (bioreactors, wetlands). Conservation planners want to understand the potential role this practice may have for reducing nitrate loads from agricultural watersheds. The Agricultural Conservation Planning Framework (ACPF) provides a method to identify riparian zones where the saturated buffer practice can be installed. Assessing the potential role for saturated buffers, however, requires that suitable sites be associated with tile-drained fields that drain to that site. This study compared the extent of riparian sites suited for saturated buffers in 32 Iowa watersheds, and estimated the extent of tile drained land in each watershed that could be treated with the saturated buffer practice. Most watersheds had suitable saturated buffer sites along 30-70% of streambank lengths, into which tile drainage from 15-40% of the watershed areas could be diverted. Therefore, the SRB has an important potential role for water quality improvement in most but not all tile-drained watersheds in Iowa. These results will be of interest to conservation planners seeking to identify viable options to reduce nitrate loads from Midwestern agricultural watersheds.

Technical Abstract: The saturated riparian buffer (SRB) is a new conservation practice that diverts agricultural tile drainage for subsurface discharge within riparian buffers. The cost efficiency of the SRB practice compares favorably with other practices (bioreactors, wetlands) that require excavation and/or land easements. Conservation planners want to understand the potential role of the SRB practice for reducing nitrate loads from agricultural watersheds. The Agricultural Conservation Planning Framework (ACPF) includes a tool for identifying riparian zones where the SRB practice can be installed, i.e., where there are shallow water table conditions, sufficient soil carbon available to facilitate denitrification, and where risks of unintended consequences (crop inundation and stream bank failure) should be minimal. Watershed assessment of the potential role for SRBs, however, must determine where SRB-suited sites actually receive drainage from tile-drained fields. This study compared the extent of SRB-suited riparian sites among 32 Iowa watersheds, and estimated the proportion of each watershed that was tile drained and located above SRB-suited riparian zones. Results from ACPF tools that identify tile-drained fields and delineate riparian catchments were overlaid and linked to results from the SRB tool to estimate the extent of tile drainage that could be treated by the SRB practice in each watershed. Results showed the extent of sites suited for SRBs did not significantly differ among three Major Land Resource Areas (MLRAs) in Iowa, from which the selected watersheds were sampled. Most watersheds had suitable sites along 30-70% of streambank lengths, where tile drainage from 15-40% of the watershed areas could be diverted, based on estimated extents of tile drainage above suitable sites. Therefore, the SRB has an important potential role for water quality improvement in most but not all tile drained watersheds in Iowa.