Comparing riparian buffer design classification data among watersheds representing Iowa landscapes

Authors: Tomer, Mark; PORTER, SARAH - Former ARS Employee; James, David; VAN HORN, JESSICA - Former ARS Employee; NIEMI, JARAD - Iowa State University

Submitted to: Agrosystems, Geosciences & Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/2021
Publication Date: 4/26/2021
Citation: Tomer, M.D., Porter, S.A., James, D.E., Van Horn, J.D., Niemi, J. 2021. Comparing riparian buffer design classification data among watersheds representing Iowa landscapes. Agrosystems, Geosciences & Environment. 4(2). Article e20159. https://doi.org/10.1002/agg2.20159.
DOI: https://doi.org/10.1002/agg2.20159

Interpretive Summary: Riparian buffers can improve water quality, but large-area evaluations of riparian buffering opportunities are rare. A landscape segmentation tool called riparian catchments, part of the Agricultural Conservation Planning Framework Version 3, was applied to evaluate riparian zones and map riparian runoff-contributing areas (i.e., riparian catchments) for 32 headwater watersheds representing three landscape regions (Major Land Resource Areas or MLRAs.) in Iowa. Riparian zones (each with 820 feet of nominal streambank length) were classified based on elevation above channel and runoff contributing area. Results showed where buffers primarily designed to intercept runoff, treat nitrate in shallow groundwater, and/or protect streambanks could be placed to benefit water quality. Riparian zones with small riparian catchments were common, typically occupying >50% of streambank lengths in landscapes of northern Iowa (MLRA 103) and southeast Iowa (MLRA 108). In these settings, narrow buffers (20-30 feet wide) will provide a buffer-to-upslope area ratio of >0.02 to filter surface runoff, while providing streambank protection. This similarity occurred despite the contrasting landscapes of these two regions because narrow buffers can be effective both along ditches in the flat terrain of MLRA 103, and along streams found below short hillslopes in well-dissected terrain of MLRA 108. However, in east-central Iowa (MLRA 104), headwater streams often had broad, low-lying riparian zones, where wide buffers (>80 ft) could be placed to mitigate nitrate transport in shallow groundwater. The ACPF riparian catchments approach enabled comparison of riparian zones among landscape regions, while providing spatial data to inform riparian planning efforts in small watersheds. This research is of interest to agroecosystem scientists and agricultural conservationists seeking better linkage between large-area and riparian management planning technologies.

Technical Abstract: Riparian buffers can improve water quality, but watershed-scale evaluations of riparian buffering opportunities are rare. A landscape discretization tool called riparian catchments, part of the Agricultural Conservation Planning Framework Version 3, was applied to evaluate functional riparian settings for 32 headwater watersheds representing three Major Land Resource Areas (MLRAs.) in Iowa. Riparian settings of 250-m length were classified based on height-above channel and upslope contributing area to show where to place buffers primarily designed to intercept runoff, treat nitrate in shallow groundwater, and/or protect streambanks. Riparian zones found below small riparian catchments were common, typically occupying >50% of streambank lengths in MLRA 103 (northern Iowa) and MLRA 108 (southeast Iowa). In these settings, narrow (6-10 m wide) buffers provide a buffer-to-contributing-area ratio of >0.02 to filter surface runoff, while providing streambank protection. This similarity occurred despite these two MLRAs having contrasting landscapes. Whereas, the narrow buffers suggested are associated with ditches and flat terrain in MLRA 103, they occur below short slopes along streams that have well dissected the watersheds in MLRA 108. In MLRA 104 of east-central Iowa, headwater alluvial streams often had broad low-lying riparian zones, where wide buffers (>25 m) may be placed to help mitigate nitrate transport in shallow groundwater. The ACPF riparian catchments approach enabled cross-watershed analyses of riparian settings, while providing spatial data to inform watershed-scale riparian planning efforts.