Modeling soluble phosphorus desorption kinetics in tile drainage

This paper describes a simple model for the desorption and transport of sol uble reactive phosphorus (SRP) to subsurface drains. The model assumes firs t-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 o utflow, free water surface fluctuation, sorbed P concentrations for the mix ing layer and profile, desorption rate and equilibrium soil-SRP partitionin g. Model results are compared to data on flow and SRP concentrations in dra in outflow collected during natural rainfall events under field conditions. The concentration time series simulated follow the sharp rise, peak, and g radual recession of the observed field data. Predicted event mass loads res ulting from observed and simulated tile discharges differ from the observed load by +8.20% and -9.7%, respectively. Sensitivity analysis indicate that equilibrium assumptions would not provide satisfactory results and that ma ss transfer limits SRP release to the tile drain.