MODFLOW – PART 3 File formats, importing models, solvers, trouble-shooting

 Inputs are set of text files  Each package has its own text file  NAM (name) file references other files MODFLOW Input Files

 Each MODFLOW package has a unique format

MODFLOW Input Files  Example RCH and DRN files

 GMS imports MODFLOW  88, 96, 2000, 2005, NWT  GMS exports MODFLOW  2000, 2005, NWT  Inputs saved to a directory with ProjectName_MODFLOW  Right-click on MODFLOW and select Open Containing Folder MODFLOW Input Files with GMS

 Model created outside of GMS  File | Open command and select the NAM file  MODFLOW 88 – open the BAS file Importing MODFLOW Simulations

 GMS stores MODFLOW inputs using text files and an HDF5 binary file  Allows for faster file i/o  Allows for compression of large data sets GMS MODFLOW with HDF5

 MODFLOW | Export Native MF2K text  Takes data stored in HDF5 file and puts it into the text files GMS Native MODFLOW Text

Importing Visual MODFLOW  From Visual MODFLOW  Run MODFLOW from Visual MODFLOW  Modify and rename the “*.mfi” file  Go to the directory where the simulation is saved on your computer  Copy the “*.mfi” file to “*.mfn”  Comment out WHS and NDC lines if necessary  Edit any paths that are referenced in the file  This is necessary if the Visual MODFLOW files were not created on the computer you are on  Open the *.mfn file in GMS

Importing Groundwater Vistas  From Groundwater Vistas  Open the simulation in Groundwater Vistas  Open the “*.gwv” file  Export the native MODFLOW files from Groundwater Vistas  Select “Model | MODFLOW (or MODFLOW 2000) | Create Datasets”  From GMS  Open the NAM file

Importing PMWIN and ModelMuse  From PMWIN  Select Models | MODFLOW (Flow Simulation) | Run  Prior to MODFLOW running the model inputs will be generated  From ModelMuse  Run MODFLOW  This will create NAM file and other input files  From GMS  Open the NAM file

 SIP - Strongly Implicit Procedure  SSOR - Slice successive over-relaxation  PCG2 - Pre-Conditioned Conjugate Gradient  PCGN – PCG with nonlinear controls  GMG - Geometric Multi- Grid  DE4 – Direct Solver  LMG/SAMG - Link-AMG MODFLOW Solvers

Basic Solver Options  Max number of iterations  Iteration stops at this point regardless of convergence  Convergence tolerance  Iteration stops if max head change is less than this amount  Acceleration parameter  Controls the amount of adjustment made to heads at each iteration  Options  =1.0 Default value. Head changes standard amount determined by solver  <1.0 Head change is reduced. Improves convergence but results in slower solution  >1.0 Converges faster but may be unstable

Solver Comparison  Using MODFLOW 2000  Run on Intel Core2 Quad CPU 2.4 GHz  Initial condition the same for all solvers  Starting heads = constant value OR  Starting heads = top of grid  HCLOSE =.001  RCLOSE =.001 (all solvers except SIP)

Solver Comparison Case #1  Woburn model  HUF package  8 layers  152,880 cells SIPPCG2SAMGGMG 980 sec*64 sec39 sec87 sec *had to reduce acc. param to 0.25 in order to converge

Solver Comparison Case #2  One layer model  Unconfined  96,000 cells Injection Wells Extraction Wells Head = 110 m Head = 90 m SIPPCG2SAMGGMG 5 sec*6 sec2.5 sec6 sec *had to reduce acc. param to 0.1 in order to converge, large flow budget error

Solver Comparison Case #3  Same as previous  Increased num cells to 216,000 Injection Wells Extraction Wells Head = 110 m Head = 90 m SIPPCG2SAMGGMG 16 sec * 15 sec6 sec13 sec *had to reduce acc. param to 0.05 in order to converge, large flow budget error

Solver Comparison Case #4  Same as previous  Changed to transient  4 stress periods, 10 time steps Injection Wells Extraction Wells Head = 110 m Head = 90 m SIPPCG2SAMGGMG 50 sec*120 sec55 sec110 sec *had to reduce acc. param to 0.05 in order to converge, large flow budget error

Solver Comparison Case #5  Two layer model  Unconfined  Steady state  600,000 cells SIPPCG2SAMGGMG 587 sec*56 sec29 sec40 sec *had to reduce acc. param to 0.05 in order to converge

Solver Comparison Summary Num. CellsNum. Layers Trans.SIP*PCG2SAMGGMG 152,8808No ,0001No ,0001No ,0001Yes 50* ,0002No *had to reduce acc. param in order to converge, some of the models had large flow budget error

Wetting and Drying  With MODFLOW solvers, if head drops below bottom of cell, the cell becomes dry and is “turned off”  Can occur during any portion (iteration) of solution process, not just at end of time step

 Head overshoot  Model parameters  Transient movement of water table What Causes Cell Drying?

Head Overshoot Initial Condition Starting head “Final” head Bottom elevation Solver guesses how much to adjust head downward Computed head Cell goes “dry”, and is turned off during remainder of simulation “Final” head Bottom elevation After One Iteration

Eliminating Head Overshoot Starting head Starting head is closer to final head Bottom elevation Improve Starting Head Estimates “Final” head Reduce Acceleration Parameter Starting head “Final” head Bottom elevation Iteration #1 Iteration #2 Iteration #3

Model Parameters  In many cases, cells are going dry due to:  Recharge too low  K too high  Etc.  Solution  Adjust parameters to better calibrate model

 Transient conditions will cause heads to fluctuate and cells to go dry  Particularly an issue for multi-layer models Transient Water Table

 New version of MODFLOW released in 2011  Designed to solve nonlinear problems due to unconfined conditions or nonlinear boundary conditions  Best solution for wetting and drying  UPW – Upstream-Weighting Package  Used in place of BCF, LPF, or HUF  Alternative formulation of groundwater-flow equation  NWT – Newton Solver Package  Solves asymmetric matrix MODFLOW-NWT

Trouble Shooting Models  Why won’t my model run?  Improper initial condition  Improper aquifer properties  Improper boundary conditions  Use Model Checker  Look in the LST (listing) file  *.out file for GMS MODFLOW simulations