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MACRO (v5.2): Model Use, Calibration, and Validation

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Transactions of the ASABE. 55(4): 1413-1423. (doi: 10.13031/2013.42251) @2012
Authors:   N. Jarvis, M. Larsbo
Keywords:   Calibration, Leaching, MACRO, Macropore, Model, Pesticides, Solute, Water flow

MACRO is a one-dimensional dual-permeability model of variably saturated water flow and reactive solute transport in soil that has been used since the early 1990s as a research tool to investigate the effects of macropore flow on soil hydrology and contaminant transport under transient field conditions. It is also widely used in the form of bespoke versions in pesticide exposure and risk assessments for groundwater and surface waters, e.g., in registration procedures in the European Union (EU). Macropore flow is a highly episodic, fast, non-equilibrium process that can dominate the leaching of reactive solutes in structured soils. This has important consequences for model calibration and validation procedures. Firstly, it means that in addition to water contents and resident solute concentrations, water flows and flux concentrations measured at high time resolution are required. Secondly, it implies that more weight must be placed on flux data obtained during these important but short-lived episodic flow events if parameters controlling macropore flow are to be reliably estimated. Although the choice of approach will vary with the purpose, automatic or hybrid automatic/manual calibration procedures are generally recommended for MACRO, especially global methods that account for uncertainty within a multi-objective framework. Despite the complexity of the processes it attempts to represent, MACRO is a parsimonious model, requiring only five additional parameters to simulate water flow and reactive solute transport, compared to the use of the Richards equation and the advection-dispersion equation. Nevertheless, for practical reasons, the size of the parameter space that can be explored by calibration is often quite limited. This emphasizes the importance of measuring those parameters that can be measured and the role of sensitivity analyses in supporting the choice of parameters to calibrate. This will vary between applications, but at least for structured soils, all five parameters regulating the generation and strength of non-equilibrium flow and transport are usually rather influential.

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