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onstrated under field conditions (e.g., Flury, 1996;Jarvis, 2002).We introduce an improved, one-dimensional, non-steady-state dual-In recent years, our knowledge of the mechanisms that permeability model (MACRO 5.1). The model simulates water flow generate and sustain preferential movement of water and solute transport in the vadose zone of structured soils by coupling and solutes has been incorporated into several simulaa high-conductivity-low porosity macropore domain to a low-conductivity-high porosity domain representing the soil matrix. Mass exchange tion models (Feyen et al., 1998; Jarvis, 1998; Š imů nek between the domains is approximated by first-order expressions. The et al., 2003). Dual-permeability models divide the total numerical solutions are briefly described, focusing on the dual-permesoil pore space into one part (e.g., soil matrix) characterability formulation. The solution method for water flow in macropores ized by a large storage capacity and small flow capacity was verified by comparing simulation results with analytical solutions and another part (e.g., macropores) with a small storage for a "kinematic wave". The model was tested against high timecapacity and a large flow capacity. One example of this resolution measurements of water flow and nonreactive (Cl Ϫ ) solute type of model is MACRO (Jarvis, 1994), which couples transport in transient microlysimeter experiments. The objective was to classical treatments of flow and transport processes in test the identifiability of four key model parameters determining the the matrix (Richards' equation, convection-dispersion degree of preferential flow using the generalized likelihood uncertainty equation) to a macropore region where flow is assumed estimation (GLUE) procedure. The parameters were chosen either beto be gravity-driven. MACRO has been widely used, cause they are difficult or impossible to measure directly or because both as a research tool (e.g., Larsson and Jarvis, 1999; they were considered sensitive on the basis of earlier experience with Kä tterer et al., 2001) and in management (e.g., in pestithe model. The measurements, indicating strong preferential flow, were adequately reproduced by the model simulations (overall model ef-cide regulation in the EU, Forum for the Coordination ficiency ϭ 0.62). The GLUE procedure conditioned the saturated of Pesticide Fate Models and Their Use, 1995), because matrix hydraulic conductivity, the macroporosity, and the mass exit is physically based, numerically robust for all soil change coefficient (diffusion pathlength), indicating that these paramhydrological types (even for long-term simulations, i.e., eters would be identifiable in inverse modeling approaches based decades), and is relatively parsimonious with respect to on microlysimeter experiments. The conditioning of the kinematic parameter requirements (Š imů nek et al., 2003). Despite exponent was poor, which was attributed primarily to correlation with these advantages, a number of limitations of the model the macroporosity.

Dual-permeability models can account for the strong influence of soil macropores on contaminant transport, but their predictive application is hampered by the difficulty in estimating a priori values for the rate coefficient controlling lateral mass exchange between the two flow domains. Our aim was to investigate the possibility of estimating the mass transfer coefficient in the dual-permeability model MACRO from fundamental soil properties. To this end, we calibrated MACRO against transient chloride leaching tests carried out on 33 undisturbed soil columns taken from the topsoils of three agricultural fields, characterized by a wide range of texture and organic matter content. The global search algorithm SUFI was used to derive optimum values of the mass transfer coefficient in each column. A Monte Carlo procedure was carried out on two columns to investigate the stability of the estimates in the presence of potential errors in fixed parameters. Despite such errors, c. 50% of the variation in mass transfer for this data set could be explained by two fundamental soil properties: the geometric mean particle size and the organic matter content. Soils of coarser texture and larger organic matter content were characterized by stronger lateral mass exchange and therefore weaker macropore flow. Harrowing and a 6-year grass ley also reduced the extent of non-equilibrium transport. Our results suggest that a robust functional description of the effects of soil structure on chemical transport is possible for predictive management applications of dual-permeability models across larger areas (i.e. mapping leaching risks at field, farm and catchment scales).

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The objective of this study was to identify the main sources of variation in pesticide losses at field and catchment scales using the dual permeability model MACRO. Stochastic simulations of the leaching of the herbicide MCPA (4-chloro-2-methylphenoxyacetic acid) were compared with seven years of measured concentrations in a stream draining a small agricultural catchment and one year of measured concentrations at the outlet of a field located within the catchment. MACRO was parameterized from measured probability distributions accounting for spatial variability of soil properties and local pedotransfer functions derived from information gathered in field- and catchment-scale soil surveys. At the field scale, a single deterministic simulation using the means of the input distributions was also performed. The deterministic run failed to reproduce the summer outflows when most leaching occurred, and greatly underestimated pesticide leaching. In contrast, the stochastic simulations successfully predicted the hydrologic response of the field and catchment and there was a good resemblance between the simulations and measured MCPA concentrations at the field outlet. At the catchment scale, the stochastic approach underestimated the concentrations of MCPA in the stream, probably mostly due to point sources, but perhaps also because the distributions used for the input variables did not accurately reflect conditions in the catchment. Sensitivity analyses showed that the most important factors affecting MACRO modeled diffuse MCPA losses from this catchment were soil properties controlling macropore flow, precipitation following application, and organic carbon content.

Testing of pesticide leaching models against comprehensive field-scale measurements is necessary to increase confidence in their predictive ability when used as regulatory tools. Version 5.1 of the MACRO model was tested against measurements of water flow and the behaviour of bromide, bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one-2,2-dioxide] and imidacloprid [1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine] in a cracked clay soil. In keeping with EU (FOCUS) procedures, the model was first calibrated against the measured moisture profiles and bromide concentrations in soil and in drain water. Uncalibrated pesticide simulations based on laboratory measurements of sorption and degradation were then compared with field data on the leaching of bentazone and imidacloprid. Calibrated parameter values indicated that a high degree of physical non-equilibrium (i.e. strong macropore flow) was necessary to describe solute transport in this soil. Comparison of measured and simulated bentazone concentration profiles revealed that the bulk of the bentazone movement in this soil was underestimated by MACRO. Nevertheless, the model simulated the dynamics of the bentazone breakthrough in drain water rather well and, in particular, accurately simulated the timing and the concentration level of the early bentazone breakthrough in drain water. The imidacloprid concentration profiles and its persistence in soil were simulated well. Moreover, the timing of the early imidacloprid breakthrough in the drain water was simulated well, although the simulated concentrations were about 2-3 times larger than measured. Deep groundwater concentrations for all substances were underestimated by MACRO, although it simulated concentrations in the shallow groundwater reasonably well. It is concluded that, in the context of ecotoxicological risk assessments for surface water, MACRO can give reasonably good simulations of pesticide concentrations in water draining from cracking clay soils, but that prior calibration against hydrologic and tracer data is desirable to reduce uncertainty and improve accuracy.

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