Modeling Phosphorus Runoff in the Chesapeake Bay Region to Test the Phosphorus Index


To assess and advance site assessment for P based management. This presentation is part of a symposium addressing the P Index.

What did we do?

The revision of USDA-NRCS’s standard for nutrient management coincided with significant assessment of the performance of Phosphorus (P) Indices in the six states that are tied to the Chesapeake Bay watershed. The 64,000 square mile watershed is the focus of unprecedented activity around nutrient management as a result of a 2011 Total Maximum Daily Load for P, nitrogen (N), and sediment under the Clean Water Act. In addition, the state of Maryland had required updates to it’s original P Index, resulting in broad scrutiny by various interest groups. Within this setting, USDA-NRCS funded a multi-state project to help advance the testing and harmonization of P-based management in the Chesapeake region.

States in the Chesapeake Bay region have had a long history of collaboration over P management. As a result, participants in the project decided to employ a unique approach to evaluating site assessment and P-based management on the basis of physiographic groupings. Expert panels were established for each of the major physiographic provinces in the region: (1) the Allegheny Plateau (NY, PA and WV); (2) the Appalachian Valley and Ridge (PA, MD, VA, WV) and the Appalachian Piedmont (PA, MD, VA); (3) and the Atlantic Coastal Plain (DE, MD, VA). Panels were comprised with representatives from the nutrient management planning community, the conservation community, the action agency community (state and federal), the farming community and researchers.

At the beginning of the project, each physiographic panel was charged with helping to identify the primary P management concerns for a province and the range of conditions that are most appropriate for site assessment. As the project evolves, panels will be expected to provide feedback on results, look for consistencies and differences between state approaches to site assessment, and, for states working to modify they site assessment approaches, offer input on suitable options.

What have we learned?

Modeling P runoff plays a fundamental role in the Chesapeake project. For each physiographic region, watersheds were identified where the SWAT model and the local P Index would be run to compare results. This objective has proven challenging to the SWAT model, as it requires a common scale of spatial inference with the P Index, i.e., the field. At the start of the project, the performance of SWAT was evaluated with regard to its representation of hydrologic and P cycling processes. It was determined that a version of SWAT that better represents variable source area hydrology, TOPO-SWAT, is best suited to the uplands of the Chesapeake Bay region. In addition, it was determined that the original P routines in SWAT are insensitive to the key nutrient management factors considered by the P Index (rate, timing, method and form of P application). Therefore, a new set of P routines was developed to correct the problem.

To-date, SWAT has been applied to watersheds in all of the physiographic regions of the Chesapeake Bay watershed: Factory Brook (NY) and Dressler Run (PA) in the Allegheny Plateau; Spring Creek (PA), Mahantango Creek (PA), Conewago Creek (PA), Upper Potomac (VA) in the Appalachian Mountains (Valley and Ridge as well as Piedmont). On the Atlantic Coastal Plain, where subsurface P losses to artificial drainage dominate, existing models were deemed inadequate. Therefore, an empirical approach to describing P transfers has been used that includes the application of geophysical imagining techniques to describe the flow paths connecting fields with drainage ditches.

Future Plans

Next steps of the project are to begin comparing P Index output with SWAT output for select field conditions under an array of management scenarios. At that point it is expected the physiographic panels will be re-engaged to work through the assessment of P Index performance and generate insight into the potential for regionally consistent approaches to P-based management.


Peter Kleinman, Research Leader, USDA-ARS Pasture Systems and Watershed Management Research Unit

D. Beegle, Z. Easton, A. Collick, Q. Ketterings, D. Fuka, T. Veith, A. Shober, S. Cela, M. Reiter, A. Allen, J. Liu and T. Basden

Additional information

Information on this project can be obtained by contacting the corresponding author.


This project is funded, in part, by a USDA-NRCS Conservation Innovation Grant.

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2015. Title of presentation. Waste to Worth: Spreading Science and Solutions. Seattle, WA. March 31-April 3, 2015. URL of this page. Accessed on: today’s date.