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United States Department of Agriculture

Agricultural Research Service

Title: Dynamic spatially-explicit mass-balance modeling for targeted watershed phosphorus management I: Model development

Authors
item Meals, D - ASSOC. IN RURAL DEV., VT
item Cassell, E - ASSOC. IN RURAL DEV., VT
item Hughell, D - ASSOC. IN RURAL DEV., VT
item Wood, L - ASSOC. IN RURAL DEV., VT
item Jokela, William
item Parsons, R - UNIV. OF VERMONT

Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 3, 2008
Publication Date: June 3, 2008
Citation: Meals, D.W., Cassell, E.A., Hughell, D., Wood, L., Jokela, W.E., Parsons, R.L. 2008. Dynamic spatially-explicit mass-balance modeling for targeted watershed phosphorus management I: Model development. Agriculture, Ecosystems and Environment. 127:189-200.

Interpretive Summary: Surface waters are frequently impaired by excessive phosphorus (P) from nonpoint sources, especially in regions of intensive livestock agriculture. Despite concerted efforts to apply new management measures, reductions in runoff losses from nonpoint source P loads have been difficult to achieve. Improved management practices aimed at reducing P losses could be more cost-effective if they were targeted at the specific areas in the watershed that have the highest risk for P runoff losses. These critical sources areas, as they are termed, occur in areas with a combination of a large P source, e.g. soil test P or manure application, and high runoff potential. Our research team developed an approach to identify, analyze, and map these high-risk areas by integrating land use and crop data with spatial watershed characteristics such as streams and soil type. Our computer simulation model shows changes in soil P concentration and P runoff losses over time across the watershed in response to changes in management and P inputs to the watershed. The results of computer simulations are analyzed and displayed spatially through color-coded maps as part of a geographic information system (GIS). This approach allows the spatial distribution of P runoff risk to be tracked through time in response to the long-term balance between P inputs (e.g. manure, fertilizer) and outputs (e.g. milk, crops). Baseline simulations showed that if present-day management continues, both soil test P and P losses will increase dramatically in some parts of a test watershed. Critical P source areas in a watershed will evolve over time and are likely to occur in limited areas that can be identified so they can be treated with improved management practices. Model results can contribute to improved targeting of public resources to those areas at highest risk of nutrient loss, resulting in more cost-effective use of scarce funds and greater improvements in water quality.

Technical Abstract: Surface waters are frequently impaired by excessive phosphorus (P) from nonpoint sources, especially in regions of intensive livestock agriculture. Despite concerted efforts to apply new management measures, reductions in nonpoint source P loads have been difficult to accomplish. Watershed management to reduce P export could be more cost-effective if treatments were targeted to critical source areas at high risk for excessive P export. These critical source areas can be defined as the intersection of P source areas and active runoff contributing areas; such areas vary in space and time due to watershed characteristics and management practices. We developed an approach to identify, analyze, and map high-risk areas for P export by integrating spatial data with land use and agronomic data. We evaluated changes over time and space in soil P concentration and P export in response to changes in inputs and outputs with a dynamic mass-balance simulation model running in grid cells across a watershed. The temporal and spatial relationships that define the risk of P export are captured simultaneously using a raster-based distributed dynamic modeling approach and related to management interventions. Simulated responses to management interventions are analyzed and displayed spatially through a geographic information system (GIS). This approach allows the spatial distribution of P runoff risk to be tracked through time in response to long-term P input/output balance, evolving from either continuation of current practices or from management changes specifically targeted to areas of high P loss risk. Baseline simulations shows that if present-day management continues, both soil test P and P export will increase dramatically in some parts of a test watershed; critical P source areas in a watershed will evolve over time and are likely to occur in limited areas that can be identified and tracked. Model results can contribute to improved targeting of scarce resources by focusing management interventions on those areas at highest risk of nutrient loss. This paper describes the underlying principles of the model, discusses the process of model development, and presents the final modeling system. Application of the model to alternative management scenarios is discussed in a subsequent paper.

Last Modified: 4/24/2014
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