Field-to-stream delivery in the P Index

The sum of the field particulate and soluble P losses are multiplied by a field-to-stream total P delivery ratio to account for deposition and infiltration of P as runoff moves to perennial surface water. This total P delivery ratio is derived from modeling with an assumption that flow from the field to surface water is in a channel similar to a grassed waterway with a constant slope.  This total P delivery ratio method is an over-simplification of complex transport processes, but has the advantage of not requiring extensive data inputs that would be a burden to nutrient management planners. The only information required for the total P delivery calculations are selection of below-field slope and distance ranges. These can easily be obtained from the soil maps used in nutrient management planning. Use of the field-to-stream delivery ratio with its current assumptions undoubtedly results in over-estimation of average P delivery for many fields, and in under-estimation for those fields where runoff flow rates are high enough to create down-slope gully erosion, effectively adding to the particulate P delivery from that field.