Dynamic simulation modelling (DSM) is a computer modelling technique that promotes description of complex natural systems, such as watersheds and BMP systems. When DSM is accomplished in an object oriented programming framework, modelling is highly interactive. The Field Phosphorus Model (FPM) was created with STELLA(R) II, an object oriented DSM programming environment. FPM is based on sophisticated accounting of all annual inputs and outputs of phosphorus for a farm field and simulates, on a yearly basis, patterns of phosphorus export from farm fields used for manure management on dairy farms. Model inputs include atmospheric deposition, manure application and chemical fertilizer; outputs include surface runoff, crop harvest, and movement into deeper soil layers. Using two BMP scenarios, the implications for long-term phosphorus export levels from the field to surface and ground waters are discussed. Changes in the land treatment or BMP levels on the field results in a lag time between when the BMP is implemented and when the full impact on phosphorus export is seen. The Field Phosphorus Model suggests that the simultaneous reduction of long-term phosphorus output to surface and ground waters is accomplished by implementation of BMPs that limit the input of phosphorus to the field.
