Phosphorus Movement Through Macroppores: Are Current Models Relevant to a P Index?.

Subsurface tile drainage is present in a large proportion of the agricultural land in North America, particularly in the Great Lakes basin, and in Europe.  These drains have been identified as a major conduit for phosphorus export into surface water, and therefore implicated in the degradation of surface water quality.  While it is probable that the majority of P reaching tile drains has travelled directly from the surface through macropores, none of the existing process models for water and solute movement adequately describe this pathway.  The result is a poor concordance between the model predictions and the actual patterns of spatial and temporal distribution of P movement from the landscape.  This, in turn, leads to reduced effectiveness of P mitigation strategies. The authors will address the assumptions that have gone into a few of the most popular models of tile flow (i.e. DRAINMOD) and macropore flow (i.e. MACRO), discuss how these assumptions can interfere with accurate predictions of P movement from the landscape, and humbly suggest some approaches that might overcome these deficiencies.