Intercomparison of Flow and Transport Models Applied to Vertical Drainage in Cropped Lysimeters

Models predicting flow and transport in the vadose zone differ in their conceptual approach, their complexity, and their mathematical formulation. Four models ( MARTHE, TRACE, ANSWERS, and MACRO), which differ significantly in their model concepts and complexity, were applied to a common data set to evaluate and compare the different model approaches. Five free-draining lysimeters, cropped with winter wheat, barley, and oat were used to monitor actual evapotranspiration soil moisture, and drainage for 627 d. One of the five lysimeters was also treated with methabenzthiazuron (MBT), and soil residues as well as leaching were experimentally determined. In Europe, MBT has been commonly used as a herbicide for almost 30 years. Generally, the use of given model inputs is recommended, apart from plant parameters, which need calibration. The use of validation criteria revealed proper simulation of water flow for the four models. After calibration, the Richards' equation - based models MARTHE, TRACE, and MACRO performed better for water flow predictions than the capacity-based ANSWERS. A small amount of preferential flow, which is not included in the model structures of MARTHE, TRACE, and ANSWERS, did not influence the simulation of water flow significantly. But preferential flow was associated with the leaching of 0.0059% of the mass of MBT, causing relevant concentrations in the leachate. Leaching could be described well with MACRO after calibration. The difficulty of estimating parameters for modeling macropore transport is seen as an obstacle to common application