A dual porosity model to quantify phosphorus losses from macroporous soils

Diffuse phosphorus losses from arable land contribute significantly to eutrophication in many regions. To enable the most efficient measures for reducing P losses to be identified, it is important to quantify the contributions from different sources and the effects of various scenarios aimed at reducing these losses. To account for transport to tile-drains through macropores, we extended the field-scale simulation model ICECREAM with descriptions based on mixing layer and dual porosity concepts. For losses of particulate phosphorus (PP) to tile-drains, a new description of particle generation and detachment was included. The model was applied on a 6-year field experiment on a structured clay soil in south-west Sweden and simulation results were compared with measured water flow and losses of suspended particles (SP), PP and dissolved reactive phosphorus (DRP) to tile-drains. After calibration of parameters related to macropore flow and transport, and to particle generation and detachment, the model could describe the extremely episodic losses of SP, PP and DRP reasonably accurately. However, some short-term fluctuations were not captured (model efficiency during the validation period −2.1 for PP and 0.43 for DRP). Thus, further development and testing are required before the model can be used with confidence for predictive applications or management purposes. Based on this application, we suggest that direct leaching from non-decomposed plant material and infiltration in partly frozen soil be included in the ICECREAM model. We also suggest that the controls on the sorption distribution coefficient for partitioning between DRP and labile phosphorus (PL) and the effects of using the ‘storage routing’ equation for calculation of soil water content instead of the Richards equation be further investigated.