Modeling Soluble Phosphorus Desorption Kinetics in Tile Drainage

This paper describes a simple model for the desorption and transport of soluble reactive phosphorus (SRP) to subsurface drains. The model assumes first-order kinetically rate-limited desorption in a soil surface mixing layer over a soil profile layer that rests on an underlying, shallow restricting layer. Input data include precipitation, soil hydraulic properties, drain outflow, free water surface fluctuation, sorbed P concentrations for the mixing layer and profile, desorption rate and equilibrium soil-SRP partitioning. Model results are compared to data on flow and SRP concentrations in drain outflow collected during natural rainfall events under field conditions. The concentration time series simulated follow the sharp rise, peak, and gradual recession of the observed field data. Predicted event mass loads resulting from observed and simulated tile discharges differ from the observed load by +8.2% and –9.7%, respectively. Sensitivity analysis indicate that equilibrium assumptions would not provide satisfactory results and that mass transfer limits SRP release to the tile drain.