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ARS Home » Northeast Area » University Park, Pennsylvania » Pasture Systems & Watershed Management Research » Research » Research Project #444762

Research Project: Modeling the Contribution of Legacy Phosphorus to Watershed Outcomes in Mahantango Creek

Location: Pasture Systems & Watershed Management Research

Project Number: 8070-13000-015-029-S
Project Type: Non-Assistance Cooperative Agreement

Start Date: Aug 1, 2023
End Date: May 30, 2026

The chronic release of legacy phosphorus (P) from soils and sediments is a water quality problem that poses persistent challenges to watershed P cleanup programs. In many agricultural watersheds across the U.S., past nutrient management activities have built up soil P reserves to levels that greatly exceed crop nutrient requirements. In sloping landscapes of the Upper Chesapeake Bay, for instance, the coincidence of legacy P hotspots with areas of frequent runoff generation results in legacy P losses that subsequently accumulate in downslope soils and in the sediments of ditches, streams, and rivers. Identifying, targeting, and managing legacy P in agricultural soils and sediments is therefore central to improving the outcomes of watershed management programs focused on P reduction. To enhance progress on this front, the Conservation Effects Assessment Project (CEAP) initiated a multi-location project to better understand the role of legacy P in watershed outcomes across the U.S. The Mahantango Creek experimental watershed – a long-term USDA-ARS research site and a CEAP watershed since 2002 – was selected by the national legacy P project team to represent the Upper Chesapeake region of central Pennsylvania. Scientists with USDA-ARS’s Pasture Systems and Watershed Management Research Unit (PSWMRU) have partnered with Penn State’s Department of Agricultural and Biological Engineering to undertake legacy P modeling in the Mahantango Creek watershed that contributes to regional and national components of the legacy P project. This agreement establishes four objectives that PSWMRU and Penn State will pursue in partnership: 1. Use empirical data from the characterization of P hotspots in Mahantango Creek to inform SWAT modelling parameters that best reflect critical source areas (CSAs) of legacy P loss. 2. Apply the updated SWAT+ model to simulate baseline legacy P fate and transport processes in the Mahantango Creek watershed. 3. Use SWAT+ to evaluate the recommended watershed strategy for cost-effective mitigation of legacy P sources developed at the conclusion of the first phase of the project. 4. Work with the legacy P modeling community and SWAT developers to ensure that any improvements to the representation of legacy P processes are shared across the project.

ARS will use SWAT+, the recently revised version of the Soil and Water Assessment Tool (SWAT), to simulate legacy P processes and mitigation options in the Mahantango Creek watershed. At the outset of the study, we will coordinate our modeling efforts with the SWAT developers in Temple, TX to support the application of SWAT+ to the physiographic, hydrologic, and agricultural conditions in Mahantango Creek. In year one, ARS will develop a baseline SWAT+ simulation of legacy P fate and transport processes in Mahantango Creek using the latest modeling routines recommended by modelers and model developers that are part of the legacy P project team. In Mahantango Creek, the SWAT+ model will be informed by long-term data on water quality and agricultural management, as well as the Phase 1 characterization of legacy P CSAs in soils and sediments. In year two, ARS will use the baseline SWAT+ model to simulate a range of legacy P mitigation strategies and quantify their efficacy in reducing legacy P losses from Mahantango Creek. In consultation with the legacy P project team, ARS will select a set of legacy P mitigation strategies that are most appropriate to the agricultural conditions in Mahantango Creek and the broader Upper Chesapeake Bay region. The results from this work will lead to watershed-specific estimates of the potential to reduce legacy P losses in agricultural watersheds that typify the sloping agricultural landscapes of the Upper Chesapeake Bay. These recommendations will be integrated into an NRCS technical note that summarizes opportunities for ameliorating legacy P losses across the legacy P project domain.