Evaluation of the Langmuir model in the Soil and Water Assessment Tool for a high soil phosphorus condition

Phosphorus adsorption by a water treatment residual was tested through Langmuir and linear sorption isotherms and applied in the Soil and Water Assessment Tool (SWAT). This study uses laboratory and greenhouse experimental Phosphorus data to evaluate the performance of a modified version of SWAT for high P concentration simulation development. A combination of vegetative filter strips (VFS) and water treatment residuals (WTR) were used to reduce soluble P runoff concentration. To effectively simulate the concentration measured in the experiments, a Langmuir model was incorporated into SWAT. The effective depth of surface runoff and soil interaction over a small subwatershed (0.07 km2) was based on an experimentally determined WTR rate of 64 Mg ha−1. A continuous flow method for rapid measurement of soil hydraulic properties was used to determine soil water contents and hydraulic conductivities. A parameter sensitivity analysis to model output indicated that the Soil Conservation Service runoff curve number for moisture soil condition II was the most responsive to change for this subwatershed. The SWAT model yielded significantly different soluble P and P leached amounts once the Langmuir model was included as an option to the linear P sorption model. With this new adaptation, SWAT was able to simulate higher P concentrations as validated by laboratory and greenhouse experimental data. The laboratory and greenhouse assessment of the WTR provided insight into the data required to evaluate the incorporation of the Langmuir model into a watershed scale tool. The choice of P model simulation (between the Freundlich option already in SWAT and the Langmuir method) was included in a sensitivity analysis performed to define the model sensitivity to selected parameters. This study provides one case of higher P conditions that SWAT was able to more adequately simulate with the Langmuir model incorporated.