EXERCISE 7a: Execute a Transient Model of the Pumping with Variable Recharge (Establish Initial Heads)

1. Save ex6.gwv

then save files and save project as C:\temp\yourname\modflow\exer\ex7a\ex7a.gwv with root file names of extr7a

be sure to correct the path to files to ex7a and the package root names to extr7a as well

Alter the recharge near the river as 4x10^-8 and away from the river as 2x10^-8

then when we create the new stress periods we will not have to change recharge in the data base of every period
Model>MODFLOW>Basic Package
change the number of stress periods to 15

Model>MODFLOW>Stress Period Setup

assign each 365/2 days = 182.5 * 86400 = 1.5768x10⁷

15 stress periods of 1/2 year each

Model>MODFLOW>Stress Period Setup

assign each 365/2 days = 182.5 * 86400 = 1.5768x10⁷

10 steps each and multiplier of 1.1

Model>MODFLOW>Initial Heads

use exstart5 for starting heads

Model>MODFLOW>Output Control Package

Save heads at every time step

Print as much as possible to the output file for checking purposes

Models>MODFLOW>OutputControl

uncheck the Disable printing box

Models>MODFLOW>PrintFormats

print all relevant items for your problem

Add a thrid zone to the recharge data base that is 0 recharge
Step through the stress periods and check the RCH for the zone near the river is 4x10^-8 for periods 1, 3, 5, ... 15; and the zone away from the river is 2x10^-8 for periods 1, 3, 5, ... 15; all other recharges should be zero

GHB and RIV boundaries need not be considered because the default is that boundary conditions are steady unless specified otherwise

Create MODFLOW files

View the new dis, wel, and rch files in a text editor to confirm that they are correct. Sometimes my first set of files are not saved correctly in spite of the apparently correct input in the GUI due to a problem with paths to models being set wrong. If the results do not fit your intuition, view the files to determine the problem.

Execute MODFLOW, adjust contouring as necessary and compare the end of stress periods 13 and 15 (both time step 10)

Now compare profiles in row 9 and model budget summaries at the end of the "wet" stress periods 1 and 3, and 13 and 15 (all at time step 10)

FIRST! peruse the MODFLOW output file, looking for conditions that you do not expect. Do the budgets make sense? Do you understand the storage terms? If not, talk with me

 

 

View the Mass Balances Plot>MassBalance>ModelSummary
1

3

13

15


Notice that the difference in water out to storage between the first and second years (stress periods 1 and 3) is larger than the difference between the seventh and eighth years (stress periods 13 and 15) which are similar to 7 significant figures and thus the same within the computer accuracy. The lack of change between the seventh and eighth years indicates a cyclic equilibrium has been reached. It may have happened sooner, but using a later time will not change the result.

Also, realize that if we were running a transient simulation until we reached steady state we would be watching for the storage term to go to zero. However, we are striving for a cyclic equilibrium, so we expect the volume to/from storage to be the SAME at the SAME time in each cycle.





This time I did not reset the axes to even values so we could easily see the difference between the profiles because GWV defaults to use the extreme values in the data set. Notice that the difference in head between the first and second years (stress periods 1 and 3) at the right boundary, and the lack of difference between the seventh and eighth years (stress periods 13 and 15) at the right boundary. The fact that there is no difference in head at the same time of year for two years in a row indicates a cyclic equilibrium has been reached. Once this condition prevails, the head will always be the same at that time for all future years unless the cyclic stresses are changed.
Now, we will use the heads at the end of stress period 15, time step 10. Rerun ex7a saving the heads only every 10 time steps so as to create a smaller file, notice the file sizes for .hds and .cbb before and after the changed output control.