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Two-Dimensional Dual-Permeability Analyses of a Bromide Tracer Experiment on a Tile-Drained Field

^a Institute for Soil Landscape Research, Leibniz-Centre for Agricultural Landscape Research (ZALF), Eberswalder Strasse 84, D-15374 Müncheberg, Germany
^b Dep. of Hydraulics and Hydrology, Faculty of Civil Engineering, Czech Technical Univ., Prague, Czech Republic
^c Faculty for Agricultural and Environmental Sciences, Soil Physics and Resources Protection, Univ. of Rostock, Justus-von-Liebig-Weg 6, D-18059 Rostock, Germany

View larger version (27K): [in this window] [in a new window]   FIG. 1. . The Bokhorst experimental site as used in the 1996–1997 tracer experiment, including the irrigated plot of 120 m2 and the application strip. The drainpipes are connected to the monitoring station (thick lines with arrows indicating the flow direction). The assumed boundary of the 5000-m2 catchment area of the drain outflow is shown as a hatched line.

View larger version (67K): [in this window] [in a new window]   FIG. 2. . Schematic picture of the two-dimensional (2D) vertical cross-section used in the simulations. Top: 2D single-porosity model simulation results for the pressure head distribution at Day 98.0 (i.e., between the two irrigation events). The colors represent soil matrix potentials, with pink indicating full saturation, and the arrows symbolize flow velocity vectors (colors representing Darcian flux in cm d–1) that are mostly small except in the vicinity of the drainpipe (the latter is indicated by a circle). Bottom: Finite element numerical grid.

View larger version (27K): [in this window] [in a new window]   FIG. 3. . Time series of tile outflow comparing data and model results for a two-dimensional single-porosity model (2D-SPM) and two-dimensional dual-permeability model (2D–DPERM). Drain discharge rates (lines) as well as daily rainfall and plot scale irrigation rates (blue columns at the top) are shown (a) for the total experimental period, and in greater detail (b) for the first and (c) for the second drainage period including the irrigation (Days 97 and 98) and a larger rainfall event (Day 100); rates for (c) are on a 12-h basis.

View larger version (23K): [in this window] [in a new window]   FIG. 4. . Vertical cross-sectional images of water flow simulation results obtained with a two-dimensional dual-permeability model (2D-DPERM) for a nonequilibrium situation at Day 97.74 (i.e., directly after the first irrigation) showing (top) pressure head distribution in the soil matrix (SM) (top left) and preferential flow (PF) domain (top right) with velocity vectors, and (bottom) 2D water transfer rate distribution. Darcian flow velocities in cm d–1 are indicated by the color of the arrows.

View larger version (30K): [in this window] [in a new window]   FIG. 5. . Time course of pressure heads at 35 cm (top) and 50 cm (bottom) depth and 1 m apart from the tile line, comparing measured data with simulated values obtained with two-dimensional single-porosity model (2D-SPM) and the two-dimensional dual-permeability model (2D-DPERM) for soil matrix (SM) and preferential flow (PF) domains. The vertical lines at Day 97.74 indicate the time directly after first irrigation, which is comparable to the 2D cross-sections.

View larger version (36K): [in this window] [in a new window]   FIG. 6. . Vertical cross-sectional images (2-m-wide section below application strip) presenting results of the two-dimensional dual-permeability model (2D-DPERM) solute transport simulation during nonequilibrium situation at Day 97.74 (i.e., shortly after the first irrigation) for (top) 2D distribution of Br concentrations in (top left) soil matrix (SM) and (top right) preferential flow (PF) domain, and (bottom) for 2D vertical distribution of solute mass transfer rates. Here Br was applied in the SM domain only, and solute transfer rate coefficient, ss, was assumed relatively high (Table 2).

View larger version (25K): [in this window] [in a new window]   FIG. 7. . Time series comparing measured and simulated results of (top) Br concentrations in tile outflow and (bottom) cumulative Br mass leaching as in Fig. 7, but here for Br application in the soil matrix (SM) domain only. 2D-SPM, two-dimensional single porosity model; 2D-DPERM, two-dimensional dual-permeability model.

View larger version (26K): [in this window] [in a new window]   FIG. 8. . Time series comparing measured and simulated results of (top) Br concentrations in tile outflow and (bottom) cumulative Br mass leaching. Curves are obtained with two-dimensional single porosity model (2D-SPM) and with two-dimensional dual-permeability model (2D-DPERM) for higher (2D-DPERM high) and lower (2D-DPERM low) values of the solute mass transfer rate coefficient, ss, for the case of bromide application in both domains (Table 2). Blue columns indicate irrigation and a following rainfall event.

View larger version (33K): [in this window] [in a new window]   FIG. 9. . Two-dimensional spatial distribution of Br concentration in the soil solution at the end of the experiments on Day 140 (7 May 1997) of transect 2 for (a) observations (DATA), and simulations obtained with (b) the two-dimensional single porosity model (2D-SPM), (c) the two-dimensional dual-permeability model (2D-DPERM) for the soil matrix (SM) domain, (d) the 2D-DPERM for the preferential flow (PF) domain, and (e) the 2D-DPERM composite Br concentration (related to bulk soil volume, Eq. [1d]). The 2D-DPERM simulations assumed Br application in the SM domain only and relatively low mass transfer rate coefficient.

View larger version (23K): [in this window] [in a new window]   FIG. 10. . Time series comparing simulated plot scale results of (top) drain discharge and (bottom) Br resident concentration at the drain position obtained with single (one-dimensional single porosity model [1D-SPM] and two-dimensional single porosity model [2D-SPM]) and dual-permeability models (one-dimensional dual-permeability model [1D-DPERM] and two-dimensional dual-permeability model [2D-DPERM]), assuming zero Br concentration at Day 97 (higher mass transfer rate coefficient, ss, and Br addition to the soil matrix domain only for DPERM).

View larger version (29K): [in this window] [in a new window]   FIG. 11. . Measured time series of drain discharge and Br concentration in tile effluent (top) for the irrigation period and the following rain event (Days 97–103), and (bottom) measured compared with simulation results for the irrigation period only (Days 97–99.5). 2D-DPERM, two-dimensional dual-permeability model.