ARS | Publication request: Evaluation of the Langmuir model in the Soil and Water Assessment Tool for high soil phosphorus condition

Submitted to: Environmental Modeling & Software
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 30, 2012
Publication Date: December 1, 2012
Citation: Rossi, C.G., Heil, D.M., Bonuma, N.B., Williams, J.R. 2012. Evaluation of the Langmuir model in the Soil and Water Assessment Tool for high soil phosphorus condition. Environmental Modeling & Software. 38:40-49.

Interpretive Summary: Phosphorus movement in soils and water continues to be evaluated by the scientific community because of the effect it can have on water quality. A combination of plants and P-reducing material were used to determine how much of an impact they could have on removing P from water. The results were surprising in that the depth of P-reducing material was more important than the rate of it applied as long as the entire soil surface was covered. The information obtained from the experiment was used in a soil-plant-water simulation model to determine if the results could be used on a bigger scale to reduce experimental costs. A field scale model (Opus) and a watershed scale tool (Soil and Water Assessment Tool) were evaluated to determine if they could simulate P reductions in the surface runoff. Once the SWAT model was adapted to be able to predict high P concentrations, both models indicated a decrease in P runoff.

Technical Abstract: 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). The objective of this study was to use laboratory and greenhouse experimental phosphorus data to evaluate the performance of a modified version of SWAT for simulating high soil P concentrations. A combination of vegetative filter strips (VFS) and water treatment residuals (WTR) were used to reduce soluble P runoff concentration. To effectively simulate runoff P concentrations measured in the experiments, a nonlinear Langmuir model was incorporated into SWAT. An experimentally determined WTR rate of 64 Mg ha**-1 was used in SWAT as the effective depth of soil:water interaction over a small subwatershed (0.07 km**2). 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 determined that the depth of application was a more sensitive parameter than the soil:water partitioning coefficient. The SWAT model yielded significantly different soluble P loads with the Langmuir model than with the current linear P sorption model. With this new adaptation, SWAT was able to better simulate higher runoff 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. This initial study of one specific case provides a promising indication that incorporation of the Langmuir model can allow SWAT to more accurately simulate a higher range of runoff P concentrations that were erroneously underestimated previously.