Extended SWAT model for dissolved reactive phosphorus transport in tile-drained fields and catchments

• We developed extensions DrainP for Soil and Water Assessment Tool version 2012 (SWAT2012) to simulate dissolved reactive phosphorous (DRP) transport from tile drains.
• DrainP calculates DRP leaching with the Langmuir isotherm and DRP transport from tile drains to streams.
• DrainP was tested against observations in two tile-drained fields and a 428 km2 Danish lowland catchment.
• DrainP simulated well DRP yields from tile drains, improves DRP yields and change source allocations at the catchment scale.
• Simulated soil P accumulations in tillage layer fell within the range of measurements.

We developed extensions (DrainP) for the Soil and Water Assessment Tool version 2012 (SWAT2012) to simulate leaching of dissolved reactive phosphorus (DRP) throughout soils with the Langmuir isotherm, and the subsequent transport from tile drains to streams. The DrainP module was evaluated for two small tile-drained fields and the River Odense catchment (486 km2) in Denmark. DrainP not only simulated well monthly DRP yields in the tile-drained fields but also improved the overall catchment DRP yields, and provided new insights into the source allocation of DRP at the catchment scale. The simulated soil total phosphorus (TP) accumulation in the tillage layer fell within the range of measurements.