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Manure and litter management currently uses some form of phosphorus indicator to determine the correct amount of fertilizer and/or manure to be applied to a field. Therefore, the manure application rate to a field can be variable from year to year, and the maximum allowable application rate is a function of some phosphorus indicator.

The watershed model SWAT and the field-level model APEX allow the user to specify a priori multi-year management by application field; however, interaction of manure management in a dynamic manner with phosphorus indicators is limited. Though APEX has been recently modified to adjust manure and fertilizer application based on soil test P levels, management adjustments do not include accounting for changing application area requirements with changing application rates and a fixed quantity of manure for disposal. The watershed model, SWAT, was altered using algorithms already developed for APEX to allow manure application rates to change dynamically within the simulation as a function of soil test P. In addition, ARCVIEW SWAT (AVSWAT) output and SWAT were modified to adjust the area receiving fertilizer in response to dynamically changing application rates, and the amount of manure requiring land application each year based on simple constraints (such as mandated manure haul-off).

Initial testing of the modified SWAT gave indications that the modifications were performing as expected. This modified SWAT will be used to assess the effect of comprehensive nutrient management plans (CNMPs) in a watershed.