Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/3/2008
Publication Date: 5/27/2008
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 II: Model Application. Agriculture, Ecosystems and Environment. 127:223-233. Interpretive Summary: To be cost-effective, programs to control phosphorus (P) runoff should target the land areas at greatest risk for P loss. We 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. The DISPLA computer 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. We used the model to evaluate these changes in response to management interventions targeted specifically to these high-risk areas, known as critical P source areas. Five scenarios were simulated in a test watershed dominated by dairy agriculture in Vermont's Champlain Valley: (1) baseline, (2) nutrient management applied to corn and hay land and to urban lawns; (3) erosion control applied to silage corn land, (4) conversion of critically eroding cropland to permanent grass, and (5) all management changes combined. The model simulations showed that if present-day conditions continue, soil test P and P export will inevitably increase (by two-fold or more) as P inputs continue to exceed outputs. Targeted nutrient management was the most effective management scenario. It reduced soil test P concentrations by 50 to 90 percent and reduced P export by 64 percent compared to baseline. Application of all management measures combined yielded a 74 percent reduction in P export. These results show the potential of the DISPLA model as a tool for use by watershed managers to evaluate alternative management options and target watershed areas for most cost-effective use of public resources.
Technical Abstract: Cost-effective nonpoint source phosphorus (P) control should target the land areas at greatest risk for P loss. We combined mass-balance modeling and geographic analysis to identify and map high-risk areas for P export by integrating long-term P input/output accounting with spatially variable physiographic, land use, and agronomic factors. The DISPLA model evaluates changes over time and space in soil P concentration and P export in response to management interventions targeted specifically to critical P source areas. Five scenarios were simulated in a test watershed dominated by dairy agriculture in Vermont's Champlain Valley: (1) baseline, (2) nutrient management applied to corn and hay land and to urban lawns; (3) erosion control applied to silage corn land, (4) conversion of critically eroding cropland to permanent grass, and (5) all management changes combined. If present-day conditions continue, soil test P and P export will inevitably increase as P inputs continue to exceed outputs. Soil test P levels on corn land are projected to increase more than four-fold over 80 years if present management continues; estimated P export is expected to more than double over the same period. Increases in soil test P over time in the watershed are not uniform, but varied spatially in response to variability in initial conditions. Targeted nutrient management was effective in reducing soil test P concentrations (50 to 90 percent) and appeared to hold the line on P export for the test watershed over the 80-year simulation. Simulated P export in the test watershed at the end of the nutrient management simulation was reduced by 64 percent compared to baseline. Implementation of erosion control on row cropland had little effect on soil test P and achieved only a transitory reduction in P export. Exclusive reliance on cropland erosion control to manage nonpoint source P is unlikely to succeed over the long term. Conversion of critical row cropland to permanent grass reduced P export by 54 percent, but did not affect soil P levels. Because row cropland converted to grassland retains its soil test P concentration, management of converted grassland to reduce runoff and P export is very important; row cropland with elevated soil test P converted to riparian buffer may still serve as a source of dissolved P to runoff. Application of all management measures combined yielded a 74 percent reduction in P export. Implications to watershed P management are discussed.