Landscape heterogeneity drives spatial and temporal patterns of nitrate export within agricultural watersheds

Authors: FLYNN, KADE - Soil Health Institute; Dalzell, Brent; Herreid, Allison; Porter, Sarah; BAKER, JOHN - Retired ARS Employee

Submitted to: Biogeochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/2026
Publication Date: N/A
Citation: N/A

Interpretive Summary: Streams and rivers draining agricultural landscapes are often polluted with high levels of nutrients. With traditional fixed-location water quality monitoring approaches, it is not possible to determine which parts of the watershed are more important sources of pollution. We developed a new approach for monitoring water quality by deploying sensors from a small inflatable raft and analyzing data to match inputs from the landscape. Results from this work show that: (1) changes in stream concentrations correspond with nearby land use, and (2) wetlands and lakes can also impact nearby water quality measurements. These results can help to guide planning about where to put water quality conservation practices.

Technical Abstract: For watershed management, it is important to monitor the impact of mitigation practices on water quality. Effective monitoring can foster further adoption of conservation actions by demonstrating success. However, the spatial resolution of most agricultural watershed monitoring is low, and linking field-scale management or the influence of individual watershed features to a single downstream monitoring station is difficult. We investigated how surface flow-path sampling can be used in low-order agricultural streams to gain a better understanding of processes controlling nitrate-nitrogen (NO3--N) export. We characterize spatial patterns in stream NO3--N concentrations and the temporal stability of these patterns in two contrasting agricultural stream reaches. In a watershed region with homogenous land use, NO3--N concentrations are stable through time and spatial dependence persists across large distances. In a heterogeneous region of the watershed which includes wetlands, spatial patterns are variable through time and spatial dependence persists across variable distances during contrasting stream conditions. Our results indicate that surface flow-path sampling is most valuable in landscapes with multiple, changing influences on analytes. In these landscapes, surface flow-path sampling can identify control points and capture how the influence of these control points change through time. In homogenous landscapes, surface flow-path sampling can be most valuable for establishing the location of control points to inform where fixed-site monitoring should occur. By coupling surface flow-path sampling with traditional fixed-site monitoring, researchers and watershed managers can develop a deeper understanding of watershed processes which can help plan for more effective management.