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 held water re gime and the solute balance of two tile-drained no-till agricultural soils influenced by shallow groundwater tables. Previously measured soil matric p otentials and groundwater levels as well as Bromide and dye tracer experime nts indicated that flow and transport along preferential pathways might be quantitatively important at the heavy clay as well as at the sandy loam sit e. 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 avail able data. The original MACRO model was modified with respect to simulating effects of groundwater table fluctuations and tile drains in the one-dimen sional flow model by including a sink term, based on the potential theory a nd an empirical approach, that approximately relates the local to the held water regime. We use an implicit finite difference discretization together with a Newton-Raphson iterative scheme for the numerical solution of the mo del. The coupled matrix and macropore flow equations are solved sequentiall y. Model parameters were calibrated using laboratory-measured soil hydrauli c parameters and field-measured time series of matric potentials and water tables. Comparisons between the measured and simulated water table fluctuat ions indicate that the consideration of preferential flow in macropores imp roves 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 failur e at the sandy loam site may possibly be caused by periodically occurring h ydrophobic conditions at the soil surface or by the existence of fissures i n a relatively low permeable subsoil horizon, effects which could not be co nsidered in the model. Hydraulic parameters of the soil matrix and macropor e system could satisfactorily be calibrated. However, the parameters of the exchange term describing water transfer between the inter- and intraaggreg ate pore system could not be clearly identified using the available field d ata. (C) 1999 Elsevier Science B.V. All rights reserved.