Within the model, riparian strips only remove P - they have no effect on N. The P runoff is a calibrated risk model that predicts loss to second order streams. A lot of this P is attached to fine particles that do not readily settle out.
Riparian / grass filter strips capture runoff. In New Zealand, P in runoff is generally present in very fine particle sizes, which is the particle size range that riparian strips are least effective at removing. Hence, the impact of riparian strips on reducing P loss in runoff is not always high. On blocks with low rainfall, modelled runoff is low or non-existent and so the riparian strip will not impact on the modelled P-loss.