An Improved Dual-Permeability Model of Water Flow and Solute Transport in the Vadose Zone

onstrated under field conditions (e.g., Flury, 1996; Jarvis, 2002). We introduce an improved, one-dimensional, non-steady-state dualIn 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; Šimůnek 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 ) 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 Kätterer 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 efcide regulation in the EU, Forum for the Coordination ficiency 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 (Šimůnek 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. have also been identified since its introduction more than 10 yr ago. First, explicit numerical solutions are used for the matrix region, leading to long run times T there is increasing concern regarding the difsince extremely small time-steps are sometimes needed fuse pollution threat to surface water and groundto assure stability of the numerical solutions. Second, water posed by the use of agricultural chemicals (e.g., only 22 numerical layers are allowed, restricting applifertilizers, pesticides). The vadose zone acts as a buffer cations to the soil root zone, and leading to a poor spato solute transport and thus is of major importance in tial resolution near the soil surface. Third, the use of the mediating the risk of receiving water bodies becoming Brooks–Corey retention function limits model flexibilcontaminated by diffuse pollutants. Preferential flow is ity in matching to measured hydraulic properties. a generic term for nonuniform infiltration and recharge The main limitation to a much wider adoption of dualprocesses characterized by flow convergence and an inpermeability models in both research and management crease in the effective velocity of the water flow through is the difficulty in parameterization. More parameters a small fraction of the vadose zone. This dramatically inare required compared with classical approaches (Šimůfluences the leaching of surface-applied contaminants nek et al., 2003), some of which cannot be directly meato groundwater because the biologically active and chemisured. The use of inverse modeling techniques may help cally reactive surface soil layers are quickly bypassed. to resolve these difficulties (Hopmans and Šimůnek, 1999). Preferential flow has been frequently observed and demHowever, the numerical limitations in MACRO noted above severely restrict the possibilities of using inverse modeling techniques to derive estimates for parameters M. Larsbo, S. Roulier, F. Stenemo, and N. Jarvis, Dep. of Soil Sciences, controlling macropore flow. Roulier and Jarvis (2003a) SLU, Box 7014, 750 07 Uppsala, Sweden; R. Kasteel, Agrosphere proposed a methodology for estimating macropore flow Institute, ICG–IV, Forschungszentrum Jülich GmbH, D–52425 Jülich, parameters in MACRO on the basis of the global search Germany; S. Roulier, currently, Soil Protection Group, Insitute of Terresterial Ecology, ETH Zurich, Grabenstrasse 11a, 8952 Schlieren, algorithm SUFI (Abbaspour et al., 1997). However, the Switzerland. Received 23 Sept. 2004. Original Research Paper. *Cormethod is time-consuming and demands considerable responding author ([email protected]). computer resources, and it would not have been feasible to implement this methodology on a routine basis with Published in Vadose Zone Journal 4:398–406 (2005). doi:10.2136/vzj2004.0137 the previous version of MACRO. © Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: GLUE, generalized likelihood uncertainty estimation. 398 Published online May 13, 2005