Dynamic phosphorus mass balance modeling of large watersheds: long-term implications of management strategies

The principles of mass balance, compartment;nux diagramming, and dynamic si mulation modeling are integrated to create computer models that estimate ph osphorus IF) export from large-scale watersheds over long-term futures. The se Watershed Ecosystem Nutrient Dynamics (WEND) models are applied to a 275 ,000 ha dairy-documented watershed and a 77,000 ha poultry-dominated waters hed in northeastern USA. Model predictions of present-day P export loads ar e consistent with monitoring data and estimates made using P export coeffic ients. For both watersheds P import exceeds P export and P is accumulating in the agricultural soils. Agricultural and urban activities are major cont ributors to P export from both watersheds. Continued urban growth will incr ease P export over time unless wastewater management is substantially enhan ced and/or rates of urban growth are controlled. Agriculture cannot rely so lely on the implementation of increasingly stringent conservation practices to reduce long-term P export but must consider options that promote P inpu t/output balance. The WEND modeling process is a powerful tool to integrate the diversity of activities in watersheds into a holistic framework. Model outputs are suited to assist managers to explore long-term effects of over all watershed management strategies on P export in comparison to environmen tal and economic goals.