# Geology 724 - Problem Set 3 - Additional Instructions and Hints for Using Gw Vistas

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Geology 724

Problem Set #5

Hubbertville and The Green Swamp - Transient Analysis

Use MODFLOW with Groundwater Vistas (GV) to solve problem 7.3 a-c from Anderson and Woessner.

In the Solver package, use 1E-4 for the transient head change criterion for closure.

In the stress period information, use 12 stress periods, one for each month, and five time steps per stress period. Set the length of each time step to be the appropriate number of days in the month, e.g., 31 for stress period 1 (January) and 28 for stress period 2 (February). Set the time step multiplier (TSMULT) to 1.5. Set the output to obtain printouts of head for the end of each month. Be sure to request cell-by-cell water budget information by assigning a unit number under the cell-by-cell column in the MODFLOW>packages window in GV.

Storage properties. The GV storage properties option has 3 entries: Storage coefficient, specific yield, and porosity. The entry in the specific yield window is used as the storage parameter for unconfined layers and the entry in the storage coefficient window is used for confined layers. (The storage coefficient window can also be used to enter specific storage by checking the appropriate box in the BCF window.) In our case, we have an unconfined aquifer and therefore, you should enter 0.1 as the specific yield.

Problem 7.3a (Pumping from dynamic steady-state initial conditions): Use 12 stress periods, one for each month of the year, and five time steps per stress period, for a total of 60 time steps. Use a constant recharge rate of 0.001 m/day for the entire simulation. Use initial conditions from problem 4.1a .

In summary, here are the GV options to check.

Modflow>basic package: specify a transient simulation and 12 stress periods.

Modflow>stress period setup: set up the 12 stress periods with five time steps each and a

multiplier of 1.5.

Properties>recharge>property values>copy transient data: set the recharge rate for each

of the 12 stress periods by using the copy transient data option. (You can check the

entries by clicking on Properties>rech/ET stress period, changing the default stress

period as necessary.)

BC>well: insert the pumping well. Note that the check under the well options should

indicate that this is a steady-state boundary condition, i.e., that the pumping rate in the well is constant throughout this transient simulation.

Modflow>initial heads: At the top of the window, set initial heads from whatever file

you are using to store the head output (*.hds) from Problem 4.1a.

Set up a monitoring well on the groundwater divide at x = 2250m, y = 5250 m. You can do this by clicking on Edit>analytic element list and then insert a well. When the monitoring well window appears, enter a pumping rate of zero, the coordinates of the well, and under “well options” designate the well as a monitor well. Enter a name for the monitoring well.

At the completion of the simulation, import results by clicking Plot>import results and then click the browse button near the stress period listing for MODFLOW. Select the last time step in the browse window.

Use Plot>hydrograph>monitoring well to plot the hydrograph for the monitoring well. You can use print options (under the print properties folder) to change from portrait to landscape mode.

Problem 7.3b The objective of this part of the problem is to produce cyclic steady-stateconditions. There is no pumping well in this simulation. You should run the simulation for four years and two months, or 50 stress periods under monthly variable recharge, but without pumping. As before use 12 stress periods per year with 5 time steps to a stress period and a multiplier of 1.5 (250 time steps). This run will produce a lot of output. You should disable the printing of heads to the output file in order to save space on your disk.

In summary, here are the GV options to check.

Modflow>basic package. Designate a transient simulation and 50 stress periods.

Modflow>stress period set up. Set up all 50 stress periods with 5 time steps and a

multiplier of 1.5

Properties>recharge>property values>copy transient data: set the recharge rate for each

of the 50 stress periods by using the copy transient data option. Enter recharge rates

for stress periods 1 through 12. Then copy the recharge rate from stress period 1 to

stress period 13. Copy the recharge rate from stress period 1 to stress period 25, and

then to stress periods 37 and 49. (You can check the entries by clicking on

Properties>rech/ET stress period.)

As in Problem 7.3a, set up a monitoring well on the groundwater divide at x = 2250m, y = 5250 m.

Problem 7.3c (Pumping from cyclic steady-state conditions): Use 12 stress periods, one for each month of the year, and five time steps per stress period, for a total of 60 time steps.

In summary, here are the GV options to check.

Modflow>basic package. Designate a transient simulation and 12 stress periods.

Modflow>stress period set up. Check to make sure the 12 stress periods with 5 time steps and a multiplier of 1.5 are listed.

Modflow>initial conditions. Select the file in which you are storing the heads from

Problem 7.3b. Choose the 48th stress period, 5th time step (end of December) as the

BC>well: insert the pumping well. Note that the check under the well options should

indicate that this is a steady state boundary condition, i.e., that the pumping rate in the well is constant throughout this transient simulation.

As in the other simulations, set up a monitoring well on the groundwater divide at x = 2250m, y = 5250 m.

Write-up. Your write-up should be brief but complete and should include the following sections.

Problem Description. Give the modeling objective for the transient analysis.

Results. Prob. 7.3b: Present the 4 year and 2 month hydrograph at the divide.

Prob. 7.3 a &c: Present N-S profiles of head in a cross section that goes through the pumping node after 1 year of pumping. Present the water-table contour map as calculated by GV, and the discharges to the swamp/marsh after 1 year of pumping. Also present the hydrograph at the divide for both simulations.

Sensitivity Analysis. Compare and contrast the results from 7.3a and 7.3c with those from Problem 4.1c. Discuss the sensitivity analysis.

Discussion. Discuss your results, concentrating on the effect of pumping on the discharge to the swamp/marsh. Compare results from Problem 4.1c, 7.3a and 7.3c.

Conclusion. Make some conclusions about your modeling work as it relates to the modeling objective.