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Evaluating Interactions between Groundwater and Vadose Zone Using the HYDRUS-Based Flow Package for MODFLOW

Navin Kumar C. Twarakavi^a,*, Jirka imnek^a and Sophia Seo^b

^a Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521
^b Colorado School of Mines, 1500 Illinois St., Golden, CO 80401

View larger version (30K): [in this window] [in a new window]   FIG. 1. A schematic showing the processes (including the key vadose zone processes) affecting subsurface hydrology.

View larger version (47K): [in this window] [in a new window]   FIG. 2. A discretized aquifer system in MODFLOW and two associated HYDRUS soil profiles. One HYDRUS soil profile is assigned to each MODFLOW zone. Note that the discretization of HYDRUS soil profiles is much finer than that of the MODFLOW domain.

View larger version (48K): [in this window] [in a new window]   FIG. 3. Flowchart describing the coupled modeling approach used in the HYDRUS package for MODFLOW: (a) steps shown in gray correspond to the treatment of variably saturated water flow (i, stress period; j, time step; k, soil profile number; nt, number of time steps; ns; number of stress periods; np, number of HYDRUS profiles); (b) calculations carried out by the HYDRUS package during one MODFLOW time step.

View larger version (19K): [in this window] [in a new window]   FIG. 4. A comparison of measured water contents and corresponding estimates calculated using the HYDRUS, Variably Saturated Flow (VSF), and Unsaturated Zone Flow (UZF1) packages for (a) Day 19 and (b) Day 35 for the Las Cruces trench experiment (data from Wierenga et al., 1991).

View larger version (23K): [in this window] [in a new window]   FIG. 5. A comparison of water table positions calculated using the (a) HYDRUS and (b) Unsaturated Zone Flow (UZF1) packages with the experimental data of Vauclin et al. (1979).

View larger version (38K): [in this window] [in a new window]   FIG. 6. Model domain, spatial distribution of hydraulic conductivities and specific yields, wells (red circles), and general head boundaries for the hypothetical regional-scale groundwater flow problem.

View larger version (56K): [in this window] [in a new window]   FIG. 7. (a) Land surface elevation, (b) depth to bedrock, and (c) water table depth at the beginning of the simulation for the hypothetical regional-scale groundwater flow problem.

View larger version (20K): [in this window] [in a new window]   FIG. 8. Zonation showing the spatial distribution of precipitation for the study area of the hypothetical groundwater flow problem. For any stress period, the actual precipitation rate in the zone is obtained by multiplying the precipitation rates given in Table 2 by the zone precipitation rate factors.

View larger version (39K): [in this window] [in a new window]   FIG. 9. MODFLOW zones used to define HYDRUS soil profiles in the hypothetical groundwater flow problem.

View larger version (16K): [in this window] [in a new window]   FIG. 10. Initial water contents as a function of depth in HYDRUS soil profiles representing selected MODFLOW zones (colors correspond to zones in Fig. 9) in the hypothetical groundwater flow problem.

View larger version (25K): [in this window] [in a new window]   FIG. 11. Groundwater table fluxes (recharge vs. discharge) as predicted by the HYDRUS package at the end of Stress Periods (a) 3, (b) 6, and (c) 12 for the hypothetical groundwater flow problem.

View larger version (29K): [in this window] [in a new window]   FIG. 12. Depths to the water table for the groundwater flow problem at the end of Stress Period 12 as estimated by the (a) Recharge–Evapotranspiration (REC-ET), (b) Unsaturated Zone Flow (UZF1), and (c) HYDRUS packages.

View larger version (24K): [in this window] [in a new window]   FIG. 13. Depth to the water table estimated using the Recharge–Evapotranspiration (REC-ET), Unsaturated Zone Flow (UZF1), and HYDRUS packages at the end of Stress Periods (a) 3, (b) 6, (c) 9, and (d) 12 as a function of the initial water table.