Techniques and Methods 6-A19
U.S. GEOLOGICAL SURVEY
Techniques and Methods 6-A19
The U.S. Geological Survey modular finite-difference ground-water flow model (MODFLOW-2005) has been used to model surface-water and ground-water interaction over a wide range of spatial scales. The capability to simulate vertical unsaturated flow was added to MODFLOW-2005 for the purpose of simulating the delay between infiltration at land surface and recharge at the water table. Unsaturated flow is simulated using a kinematic-wave approximation of Richards’ equation and assuming one-dimensional gravity-driven vertical flow. The method of characteristics is used to solve the kinematic-wave approximation. The approach simulates flow and storage in the unsaturated zone and time-delayed recharge, while maintaining the applicability of MODFLOW-2005 to basin-scale problems.
Unsaturated flow is simulated independently of saturated flow within each model cell in which the water table is below land surface. The relation between the unsaturated hydraulic conductivity and water content in the unsaturated zone is defined on the basis of the Brooks-Corey function. Variables used to simulate unsaturated flow include saturated and initial water content, saturated vertical hydraulic conductivity, and the Brooks-Corey exponent. Additionally, the residual water content is approximated by the aquifer specific retention in the Brooks-Corey function. Specific retention is calculated as the porosity represented by the saturated water content minus specific yield. These variables can be different for each MODFLOW column of cells but cannot change between model layers.
The maximum infiltration rate (flow per unit area) across the land surface is limited to the vertical saturated hydraulic conductivity. Recharge is added to ground water only when water in the unsaturated zone drains to the water table. This differs from the standard Recharge Package, in which the recharge rate must be determined separately from MODFLOW. When the water table rises, the quantity of water stored in the unsaturated zone above the residual water content is added to recharge. Thus, the water table can rise rapidly when the amount of water stored in the unsaturated zone is at or near saturation. A water budget for the unsaturated zone is tracked independently of the ground-water budget in MODFLOW.
Evapotranspiration may be simulated over a specified depth in the unsaturated zone. The volumetric rate of evapotranspiration is dependent on the volume of water stored in the unsaturated zone above the specified extinction depth and extinction water content. Evapotranspiration also may be supplied by ground water when the water table is above the specified extinction depth. Variables that must be specified to simulate evapotranspiration include the evapotranspiration demand rate, the extinction depth, and the extinction water content.
Ground water may discharge to the land surface when using the UZF1 Package. This water may be added to a specified stream segment in the Streamflow-Routing (SFR2) Package or to a specified lake in the Lake (LAK3) Package, provided the packages are being used in the simulation. Water discharging to land surface is added to stream segments or lakes without delay during each time step, regardless of the distance between the cell where ground water is discharged to the nearest stream segment or lake.
Results from the kinematic-wave approximation in the UZF1 Package were compared with results from U.S. Geological Survey’s Variably-Saturated Two-Dimensional Flow and Transport (VS2DT) Model that solves Richards’ equation. The test simulation demonstrates the capability of the UZF1 Package to simulate the advancement of a wetting front assuming three different rates of evapotranspiration. The simulation was in good agreement in the magnitude and downward progression of the wetting front with respect to the results from the VS2DT. An application to a 27 km2 watershed was used to demonstrate the capabilities of the UZF1 Package. A 1-year transient simulation in which infiltration and evapotranspiration demand changed daily demonstrated that simulating flow through the unsaturated zone is important for estimating recharge. Years in which infiltration rates exceed the mean annual ET rate may increase unsaturated-zone and ground-water storage, even in relatively poorly permeable rocks, and can result in increased discharge to streams that may extend well beyond the period of infiltration. A second and simpler problem is included to provide an example of the input-file structure and model output and to provide testing of the package after installation.
U.S.
Department of the Interior | U.S. Geological
Survey
Persistent URL: https://pubs.water.usgs.gov/tm6a19
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Page Last Modified: Friday, 02-Dec-2016 15:45:37 EST
