Movement of phosphorus (P) from farm fields to surface waters can elevate P above critical levels for aquatic plant growth and thus enhance nutrient enrichment and seasonal deficient oxygen, a process called eutrophication. Phosphorus commonly controls vegetative production in freshwater bodies, and hence the potential for eutrophication. The sourcing of P from production fields (including P from soil, manure, and fertilizer) is one area identified as being an important contributor of total P entering surface waters, and hence significantly contributing to water quality concerns. In recent years considerable effort has focused on developing methods to assess risk of P loss from production fields to surface water systems.