Modeling phosphorus transport in agricultural watersheds: Processes and possibilities

Modeling phosphorus (P) loss from agricultural watersheds is key to quantifying the long term water quality benefits of alternative best management practices. Scientists engaged in this endeavor struggle to represent processes controlling P transport at scales and time frames that are meaningful to farmers, resource managers, and policy makers. To help overcome these challenges, we reviewed salient issues facing scientists that model P transport, providing a conceptual framework from which process-based P transport models might be evaluated. Recent advances in quantifying the release of soil P to overland and subsurface flow show that extraction coefficients relating soil and flow P are variable but can be represented as a function of land cover or erosion. Existing information on best management effects on P export should be linked to watershed models to better represent changes in P transport. The main needs of P transport models are inclusion of flexible coefficients relating soil and overland flow P, fertilizer and manure management and P loss, stream channel effects on edge-of-field P losses prior to water body input, and linkage of watershed and water-body response models. However, it is essential that the most appropriate model be carefully selected, according to a user's needs in terms of available input data, level of predictive accuracy, and scale of simulation being considered.