Flow and transport processes in a macroporous subsurface-drained glacial till soil II. Model analysis

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  • Journal of Hydrology

    ELSEVIER Journal of Hydrology 207 (1998) 121-135

    Flow and transport processes in a macroporous subsurface-drained glacial till soil

    II. Model analysis

    K.G. Villholth, K.H. Jensen

    Department of Hydrodynamics and Water Resources. Technical University of Denmark, 2800 Lwgby. Denmnrk

    Received 17 March 1997; revised 30 September 1997; accepted 18 February 1998

    Abstract

    The experimental results from a field-scale tracer experiment in a subsurface-drained glacial till soil were analyzed by the application of a single/dual porosity model (MACRO), optionally accounting for concurrent and interacting flow and transport in the bulk soil porosity as well as in the macropores. The mode1 analysis showed that macropore flow is essential in describing the observed transport phenomenon on a short as well as a longer time scale. The diffusive exchange of solute between the matrix and the macropores was very sensitive and critical for the mode1 prediction of the drainage concentration. The exchange was overpredicted and too rapid when the soil aggregate size (distance between macropores) obtained from an image analysis of soil cores was used in the model. On this basis, the mode1 assumption of instant equilibration of the solute across the matrix porosity, disregarding small-scale concentration gradients, is questioned. Decreasing the domain exchange resulted in an improved model correspondence with the drainage chemograph. The drainage flow pattern was altered between drainage seasons owing to the changes in hydraulic efficiency of surface-vented macropores influenced by the physical disturbance and compaction of the soil surface. Hypothetically introducing fully surface-connected macropores into the calibrated model resulted in a 22% increase in the loss of solute to the drain, indicating the significance of the hydraulic conditions at the soil surface and the model specification thereof. 0 1998 Elsevier Science B.V. All rights reserved.

    Keywords: Structural soils; Macropores; Subsurface-drainage; Conservative tracer: Two-domain model; Mass transfer; Flow and transport processes

    1. Introduction

    Recognition of the prevalence of preferential flow and especially its effects on migration of substances within the terrestrial environment has led to intensi- fied research on quantifying and modeling flow and transport phenomena in natural, structured soils.

    One of the difficulties involved in formulating a stringent, mathematical formulation of the flow

    * Corresponding author

    processes is related to the intricate soil structure superimposed on a texturally heterogeneous soil medium and to the lack of a straightforward relation between easily definable and measurable properties of the soil structure and the actual flow mechanisms. External and initial conditions (Brusseau and Rao, 1990), the physical-chemical properties of the inter- face between the structural pathways and the bulk soil (Gerke and van Genuchten, 1993a; Gerke and van Genuchten, 1993b) as well as the temporal variability of the macropores (Gee et al., 1991; Chen and

    0022-1694/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. /II SOO22-1694(98)00130-9

  • Wagenet. 1992) infhtence the fluid transport and make a straightforward model description difticult.

    A range of model approaches to preferential Ilow and transport phenomena has been suggested in the literature during the last couple of decades. Informa- tive reviews are given by Wierenga ( 1987). Addiscott and Wagenet ( 1985), Nielsen et al. ( 1986), van Genuchten and Jury (1987). Germann (1988), Huyakorn (1988), Brusseau and Rao (1990) and Villholth (1994).

    Modeling of field-scale transport to a subsurface drainage system requires the consideration of the coupled unsaturated-saturated flow system (Vinten et al., 1991). Various approaches to modeling trans- port of non-reactive solutes (van Ommen et al.. 1989a; van Ommen et al., 1989b: Vinten et al., 1991), pesti- cides (van Ommen, 1985a; van Ommen. 1985b; Utermann et al.. 1990), and sediment particles (Bottcher et al.. 1980) in artificially drained soils have been suggested. Common to the work presented is a fragmented technique by which the transport rela- tions are considered separately for the unsaturated and saturated zones. and a coupling is conceptually pro- vided by assuming that output from the former is input to the latter. Simple macroscopic routing or transfer function models are used for the saturated zone, while transport in the unsaturated zone is treated in a more physically based, mechanistic manner allowing for transient flow and preferential flow effects. Usually. no, or very crude, approximations to the evapo- transpiration processes are implemented.

    These models were apparently developed to ana- lyze single drainage experiments (Bottcher et al., 1980; van Ommen et al., 1989a; van Ommen et al.. 1989b) or to study hypothetical cases (van Ommen, 1985a: van Ommen, 3985b; Utermann et al.. 1990), and only little general applicability can be assessed from their use. Vinten et al. ( 1991), however. applied their mode1 to eight adjacent, equally treated drainage plots and concluded that quite variable mobile water fractions were needed to describe the observed drainage chemographs. Little correspondence with observations was found by van Ommen et al. (1989a); van Ommen et al. (1989b), who attributed this to a lack of considering preferential flow effects in the model. Thooko et al. ( 1994) and Singh et al. ( 1996) combined drainage models and one- dimensional pesticide transport models. No preferential flow was considered.

    From the reviewed literature. I! appears that a physically based model that simultaneously describes transient flow and transport in the coupled unsaturated-- saturated soil system. including preferential flow.. rvapotranspiration procrsse\, and drainage intei- ception. could considerably, improve the description of field-scale transport in subsurface drained soils.

    The objectives of the present study were to interpret the experimental held drainage studies presented in Villholth et al. ( 1998) using a model that fultils the requirements stated above. Aside from analyzing the combined flow and transport mechanisms. the aim was to identify and determine the sensitive parameters in the model and to identify model limitations and future research needs. Appropriate question\ arc raised: ( 1 ) is it necessary to include a description of preferential Row to account for the observed pattern of solute dissipation? and (3) can existing models describe/predict how and transport of conservative soluble constituents in highly structured held soils?

    2. Materials and methods

    2.1. Field-scale truer tests

    Field-scale tracer tests involving the surface appli- cation of a conservative chloride ion to macroporous. subsurface-drained glacial till soil in Denmark were conducted (Villholth et al., 1998). The three plots (PLOT 1 to PLOT 3) used for individual tracer tests were located on adjacent drain lines (DRAIN I and DRAIN 2) and their hydraulic behavior was assumed to be similar (Villholth et al.. 1998). The tests were performed over a period of three consecutive drainage seasons (SEASON 1. SEASON 2 and SEASON 3 corresponding to the 1989- 1990. 1990- I 99 1 and 1991- 1992 fall-to-spring-periods respectively). For a detailed description of the site layout. the soil char- acteristics, the experimental conditions and the data collection refer to Villholth et al. (1998).

    2.2. The MACRO model

    The numerical mode1 applied in this study is the MACRO model developed by Jarvis ( 1994). MACRO is a deterministic, finite difference model for one- dimensional, non-steady water and solute transport in

  • K.G. Villholth. K.H. Jensen/Journal of Hydrology 207 (1998) 121-135 123

    macroporous soil. Classified as a two-domain or double- porosity model, MACRO describes the preferential flow as macropore flow in a separately defined pore domain. No a priori assumptions of the detailed geometry of the macropore network are made. This, in combination with the deterministic and one- dimensional approach, means that the small-scale and the larger-scale variabilities are disregarded, and a necessary lumping of properties is performed in order to equate the scale of the model with the experimental plot scale.

    Richards equation is assumed applicable for verti- cal water flow in the micropore domain, whereas flow in the macropore domain is assumed laminar and driven by gravity only. The Brooks and Corey (1964) functional relationship describes the retention properties of the matrix. The hydraulic conductivity in the matrix is given by the Mualem (1976) expression, whereas in the macropores a simple power law function is assumed to represent the hydraulic conductivity relation. Drain flow is included as sink terms in the flow description for saturated layers above the drain depth by use of seepage potential theory (Youngs, 1980). Soil evaporation and evapo- transpiration from a defined crop is accounted for.

    Transport of a conservative solute is described by the convection dispersion equation in both domains. Source/sink terms for the exchange between domains due to water how and solute diffusion are described by linear first-order mass transfer expressions with the first-order rate coefficients being related to the geom- etry and the dimensions of the macropore structure:

    E,=cr,(eb-~mi) (1)

    Es=~s(cm,-cmi) (2)

    where E, (T-l) and E, (M Lm3 T-) are mass transfer of water and diffusive mass transfer of solute from macropores to matrix respectively, orni and eb are the prevailing and saturated volumetric water contents of the matrix, and c,, and c,i are solute concentrations in the macropore and matrix region (M