Mapping pollutant source to enhance water quality conservation in agricultural watersheds: Nonpoint no more?

Authors: JAMES, DAVID - Retired ARS Employee; TOMER, MARK - Retired ARS Employee; Porter, Sarah; CRUSE, RICHARD - Iowa State University; ZIMMERMAN, EMILY - The Nature Conservancy

Submitted to: Journal of Soil and Water Conservation
Publication Type: Other
Publication Acceptance Date: 8/22/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Abatement of nonpoint source pollution (NPSP) has been a focus of environmental science since the 1970s, when the US Clean Water Act was passed. Since that time, research has focused on assessment of pollutant transport and conservation effectiveness. Today, high-resolution data are available to a wide extent and could be better used to identify pollutant sources and effective conservation measures. This paper argues for wider use of these data and shows examples of how crop-history, terrain, and soils data can be used to de-mystify NPSP, by using field specific data to identify and prioritize conservation options, considering costs and likely pollution reduction. Based on an approach developed by the Iowa Nutrient Reduction Center, examples are shown for N loss abatement. The potential to extend the approach to sediment and P losses is discussed. This publication will benefit agricultural researchers and conservation professionals on the state of the science and technology regarding nonpoint source pollution.

Technical Abstract: The term “nonpoint-source pollution” (NPSP) has been used to describe land-sourced, precipitation-driven contamination of surface waters by nutrients, sediment, and land-applied wastes and chemicals. Given the importance of agriculture to society and the uncertainties involved in bringing about NPSP reductions, research efforts to understand NPSP transport/delivery processes and mitigation options using soil and water conservation practices began in earnest with the passage of the Clean Water Act (CWA). We argue that advances such as high-resolution Light Detection and Ranging (LiDAR) topographic survey data, terrain analyses, remotely sensed crop cover data, modernized soil surveys, and computerized data access and analyses can be leveraged to map relative sources of agricultural nutrients and sediments at a scale that is meaningful in watershed- and farm-scale conservation efforts. More specifically, the Agricultural Conservation Planning Framework (ACPF) (Tomer et al. 2013, 2015, 2017) provides three types of layered information for identifying and ranking candidate locations for conservation treatment at the scale of practice implementation—the individual field. To address nitrogen (N) losses, land use and by-field crop rotation data can be used to estimate the spatial distribution of fertilizer-N applications, by field and across watersheds. Second, high-resolution (ranging from <1 to 3 m) topographic information can be used to map cumulative overland flow pathways by which water moves across the landscape to streams, rivers, and lakes. Using contributing area and slope distributions, risks of erosion and sediment transport can be mapped at greater resolution than was possible before LiDAR-derived topographic data became available. Third, locations suitable for installation of a range of interceptive conservation practices can be identified using conservation practice placement tools found in the ACPF toolbox, including practices that treat subsurface drainage and others that slow overland flow. Overall, these data and geographic analyses can be used to consistently identify locations meeting threshold criteria (i.e., qualifying as a “hotspot”), where a given conservation practice could be installed to provide measurable benefits. The qualified sites can then be ranked according to the costs and benefits of conservation-practice implementation (Bravard et. al 2022). These techniques can provide ranking/prioritization among sites to help watershed coordinators engage landowners in achieving water quality improvement goals.