Using spatially rich datasets to assess the influence of channel characteristics on biogeochemical behavior in agricultural watersheds

Authors: Herreid, Allison; Dalzell, Brent; FLYNN, KADE - University Of Minnesota; BAKER, JOHN - Retired ARS Employee

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2025
Publication Date: 2/25/2025
Citation: Herreid, A.M., Dalzell, B.J., Flynn, K., Baker, J. 2025. Using spatially rich datasets to assess the influence of channel characteristics on biogeochemical behavior in agricultural watersheds. Water Resources Research. 61(2). Article e2024WR038265. https://doi.org/10.1029/2024WR038265.
DOI: https://doi.org/10.1029/2024WR038265

Interpretive Summary: Many farmlands have been altered to drain water and increase crop production. These changes often involve straightening natural stream channels, which reduces their ability to use nutrients and carbon. In this study, we collected detailed water quality data from two different areas of an agricultural watershed in south-central Minnesota to see how features like straightened channels, drainage systems, and the presence of lakes or wetlands affect the processing of nitrate (NO3-) and dissolved organic carbon (DOC). The upstream area is mostly farmland with drainage systems that empty directly into the stream, while the downstream area has more lakes and wetlands, creating a more natural stream environment. We used a method called reach leverage to understand whether areas of the watershed were sources of NO3- and DOC, or if they removed them. Our results showed differences in nutrient processing between the two areas. The upstream, straightened region mainly transported or slightly increased NO3- and DOC, while the downstream, wetland-rich region alternated between acting as a source and a sink for NO3- depending on water flow and other factors. These findings highlight the importance of collecting detailed, location-specific data to understand nutrient processing and for developing better land and water management strategies. This approach to stream data interpretation will help scientists and conservationists understand how agricultural landscapes can contribute to water pollution and identify strategies for improving water quality.

Technical Abstract: Many agricultural landscapes have undergone significant modifications to drain farmland and improve crop productivity. Subsurface field drainage, ditching and channelization of streams limit opportunities for biogeochemical processing of carbon and nutrients within the channel network. In this study, we used spatially rich water quality data collected from two contrasting regions of an agricultural watershed in south-central Minnesota, USA to assess how watershed features, such as channelization, tile drainage, and presence of lakes or wetlands, influence biogeochemical processing of nitrate (NO3-) and dissolved organic carbon (DOC). In the channelized upstream region, land use is predominantly agricultural (> 92%) with subsurface tile drainage commonly discharging directly to the stream channel. Further downstream, the channel is more natural with increasing lakes and wetlands, including riparian wetlands. We used the concept of reach leverage to interpret biogeochemical behavior (i.e., source vs. sink) in each region of the watershed. Results indicate variability in biogeochemical behavior between the distinct watershed regions, suggesting that channel characteristics and the presence of lentic waters play a role in regulating biogeochemical processing. The upstream, channelized region acts primarily as a conservative transporter or small source of both NO3- and DOC across sampling dates. In contrast, the lentic-influenced region exhibited shifts between source and sink behavior over time, especially for NO3-, influenced by factors such as hydrologic connectivity and discharge. These findings highlight the value of collecting spatially resolved data to enhance our understanding of biogeochemical processing which may be useful to inform effective management and conservation strategies.