Apex Modeling Phosphorus In Tile Drained Systems: Experience and Evaluation Of Modifications In Langmuir's Equation.

New evidence of P losses through tile drained systems in agricultural landscapes is piling. Given that tile usually has direct access to streams and ditches, trying to understand the transfer of P to subsurface soil layers is receiving more attention. The Agricultural Policy/Environmental eXtender (APEX) model is able to predict the loss of nutrients in surface runoff and recent updates include non-linear sorption of P and estimation of P loss in tile flow. Calibration of this variable in tile flow is only possible through the calibration of P in surface runoff. A main parameter in the calibration of surface runoff P is the Langmuir equation (non-linear). We compared the effect of the current linear equation in APEX with an introduced non-linearized model (Langmuir) on the calibration of P in surface and tile flow. Monitored data from two agricultural fields under two conservation practices (No-till and Reduced-till) was simulated in APEX. Preliminary results suggest that improvement in P sorption estimates in surface runoff result in better P estimates in tile flow. Modeling improvements of P partitioning processes in soils can help produce more realistic outputs. In turn, more accurate hydrological models, such as APEX, can be used to better evaluate agricultural conservation practices in order to reduce P losses for water quality purposes.