Dissolved phosphorus (P) in runoff from surface-applied fertilizers can be relatively great, but commonly used, field or watershed-scale computer models often do not simulate direct transfer of fertilizer P to runoff. Using data from our own simulated rainfall experiments and published runoff studies, we developed a simple model to predict fertilizer P release during rain and the concentration of dissolved P in runoff. The model operates on a daily time-step and requires input data on the amount of fertilizer P applied, type of soil cover (bare, residue-covered, grassed), and amount of rain and runoff for each storm during the simulation period. The model applies fertilizer to the soil surface, adsorbs fertilizer P to soil before the first rain, releases P from fertilizer for each rain event, and distributes released fertilizer P between runoff and infiltration based on the runoff to rain ratio. Using data from 11 runoff studies, we validated that our model accurately predicts dissolved P in runoff from surface-applied fertilizers. Validation data represented a series of runoff events for a variety of fertilizer types, soil cover types and subsequent fertilizer P adsorption amounts, storm hydrology conditions (i.e., runoff to rain ratio), and plot or field sizes (0.2mpo to 9.6ha). An analysis showed model predictions can be quite sensitive to rainfall and runoff data. However, the simplicity of our model should make it straightforward to incorporate into more complex P transport models, thus improving their ability to reliably predict P loss to the environment for a variety of agricultural land uses.
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