Describing water flow in macroporous field soils using the modified macro model

Preferential soil water movement was simulated using a modified version of the MACRO model. The study contributes to the analysis of the field water regime and the solute balance of two tile-drained no-till agricultural soils influenced by shallow groundwater tables. Previously measured soil matric potentials and groundwater levels as well as Bromide and dye tracer experiments indicated that flow and transport along preferential pathways might be quantitatively important at the heavy clay as well as at the sandy loam site. The objective of this study was to optimally describe the field measured matric potentials and groundwater level fluctuations by using a one- and a two-domain version of the MACRO model and by model calibration using available data. The original MACRO model was modified with respect to simulating effects of groundwater table fluctuations and tile drains in the one-dimensional flow model by including a sink term, based on the potential theory and an empirical approach, that approximately relates the local to the field water regime. We use an implicit finite difference discretization together with a Newton-Raphson iterative scheme for the numerical solution of the model. The coupled matrix and macropore flow equations are solved sequentially. Model parameters were calibrated using laboratory-measured soil hydraulic parameters and field-measured time series of matric potentials and water tables. Comparisons between the measured and simulated water table fluctuations indicate that the consideration of preferential flow in macropores improves the one-dimensional description of the water regime of the aggregated clayey soil. For the sandy loam soil, neither the modified MACRO nor a one-domain model could explain observed preferential flow patterns. The failure at the sandy loam site may possibly be caused by periodically occurring hydrophobic conditions at the soil surface or by the existence of fissures in a relatively low permeable subsoil horizon, effects which could not be considered in the model. Hydraulic parameters of the soil matrix and macropore system could satisfactorily be calibrated. However, the parameters of the exchange term describing water transfer between the inter- and intraaggregate pore system could not be clearly identified using the available field data.