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Dr. Chad Penn holds a bottle to collect a water sample beside of a phosphorus trapping structure, with a soybean field in the background.

USDA Legacy Phosphorus Assessment Project

Water quality can be persistently degraded due to historical phosphorus sources, creating a challenge for watershed management. USDA's Legacy Phosphorus Assessment Project leverages watershed research to advance legacy phosphorus understanding and support water quality improvements.

About Legacy Phosphorus

Phosphorus is an essential fertilizer nutrient that is used in agriculture to support crop production. It is a highly reactive, or “sticky,” element that builds up in agricultural soils over time. This accumulated phosphorus, referred to as legacy phosphorus, can contribute to downstream water quality degradation. Examples of legacy phosphorus include phosphorus that has accumulated in soils as a result of historical fertilization practices, phosphorus that now resides in sediment as a result of its transfer from upstream sites, and phosphorus transported through groundwater systems. 

Legacy phosphorus represents a major challenge to watershed management and conservation outcomes. Water quality may be persistently degraded as a result of past fertilization activities, even after SMART nutrient management planning is in place to improve current phosphorus applications within cropland fields. Effectively addressing legacy phosphorus requires targeted conservation systems that avoid, control, and trap losses within and at the edges of fields as well as in ditches and catchments throughout the watershed.

About USDA's Legacy Phosphorus Assessment Project

USDA's Legacy Phosphorus Assessment Project seeks to leverage watershed research led by the Natural Resources Conservation Service (NRCS) and Agricultural Research Service (ARS) in partnership with universities to advance the science, innovation, and assessment of legacy phosphorus mitigation strategies. This project utilizes Conservation Effects Assessment Project (CEAP) watersheds within the Chesapeake Bay, Lake Champlain, Lake Erie, Mississippi River, and Snake River as shown in the map below. The ultimate goal of this effort is to improve recommendations for legacy phosphorus management through assessment of the relative contributions of legacy phosphorus from fields and watersheds.

Participants in the project coordinate their research to characterize legacy phosphorus in diverse settings across the contiguous United States. Established monitoring sites – both at the stream and edge-of-field scales – provide data on the nature of phosphorus export over time. USDA and partners are carrying out watershed modeling to extrapolate findings from field research and to assess cost-effective strategies for mitigating legacy phosphorus.

ARS offers a multimedia publication that details field research activities and watershed monitoring efforts associated with USDA’s Legacy Phosphorus Assessment Project.

Map of USDA legacy phosphorus assessment projects in the contiguous U.S.

Publications

Publications associated with the USDA Legacy Phosphorus Assessment Project will be available on this webpage as they are published.

Journal Article

Blog

Webinar

Phosphorus Modeling Resources

USDA and partners are pursuing watershed modeling to extrapolate findings from field research and to assess effective legacy phosphorus mitigation strategies. To ensure that this modeling is broadly applicable, the USDA Legacy Phosphorus Assessment Project convened a workshop with partners to review how legacy phosphorus may be better represented in watershed models. The below resources, grouped by associated model, are the result of workshop presentations by model developers to describe how leading watershed models represent legacy phosphorus sources and, in some cases, processes, to account for these as part of watershed loads.

A major aspect of the USDA Legacy Phosphorus Assessment Project is to further develop leading USDA water quality models to better account for legacy phosphorus sources and processes. Future versions of models will include enhancements to improve these capabilities. Specifically, efforts are currently underway to advance the first four models below. 

Soil and Water Assessment Tool

The Soil and Water Assessment Tool (SWAT) is a field-to-basin scale hydrologic and water quality model used to predict sediment and nutrient losses from agricultural catchments.

Agricultural Policy Environmental Extender

The Agricultural Policy Environmental Extender (APEX) is a farm scale water quality model used to simulate a wide range of agricultural management practices, cropping systems, and other land uses across whole farms and small watersheds.

Annual Phosphorus Loss Estimator

The Annual Phosphorus Loss Estimator (APLE) is a spreadsheet-based model that runs on an annual time step and estimates field-scale, sediment-bound, and dissolved phosphorus loss in surface runoff for agricultural fields.

Annualized Agricultural Non-Point Source Pollution Model

The Annualized Agricultural Non-Point Source Pollution Model was developed to quantify the effects of agriculture practices on the generation and transport of non-point source pollutants in un-gauged agricultural watersheds.

SPAtially Referenced Regression on Watershed Attributes

The SPAtially Referenced Regression on Watershed Attributes (SPARROW) is a mass-balance model used to estimate sources and environmental factors that affect the long-term supply, transport, and fate of contaminants in streams.

Chesapeake Bay Program Phase 6 Watershed Model

The Chesapeake Bay Program Phase 6 Watershed Model is a participatory creation of the Chesapeake Bay Program Partnership using a process responsive to decisions made by the Chesapeake Bay Program’s Management Board, Modeling Workgroup, and Water Quality Goal Implementation Team.

Additional Resources

Conservation Effects Assessment Project (CEAP)

USDA’s Conservation Effects Assessment Project (CEAP) is a multi-agency effort led by the Natural Resources Conservation Service (NRCS) to quantify the effects of conservation practices across the nation’s working lands.

Learn More

Watershed Assessments

Conservation Effects Assessment Project (CEAP) watershed studies provide in-depth assessments of water quality, water availability, and soil concerns at a finer scale than national or regional assessments.

Learn More

Lisa Duriancik

Watershed Lead, Conservation Effects Assessment Project