1. An Introduction to MODFLOW
II. Intro to MODFLOW

2. What is MODFLOW?
Widely used ground-water flow simulation program that runs on any platform (Windows, Sun, Unix, Linux,…).
Mostly written in standard FORTRAN (GMG is C++)
Solves the ground-water flow equation with different possible properties, boundary conditions, and initial conditions
First version, 1983, McDonald and Harbaugh. Written to serve USGS needs. Education emphasized.
MODFLOW escaped!
Public domain (Free)
Open source (Anyone can check and change the source code)
Changed versions are sometimes commercial – it’s up to the developer
Well documented
Modularly constructed (More later)
Latest version: MODFLOW-2005 (Harbaugh, 2005)
II. Intro to MODFLOW

3. What is MODFLOW? Cited in statutes, legally tested
250,000 hits on Google for MODFLOW
Many other programs use results from or are based on MODFLOW:
Public domain/open source
MT3DMS (multi-species solute or heat transport, some reactions, dual porosity) (Chunmiao Zheng, U Alabama)
MODPATH (particle tracking) (Dave Pollock, USGS)
SEAWAT (density-dependent transport using MODFLOW and MT3DMS) (Chris Langevin, USGS)
Phreeqc connections (PHT3D) (Henning Prommer)
Commercial
MODHMS-Surfact (Integrated sw/gw/unsat)
GUI’s: Visual MODFLOW, Groundwater Vistas, GMS, PMWin, …
II. Intro to MODFLOW

4. What is MODFLOW? What is MODFLOW-2005? Program organized into MODules
Activate the capabilities you need; no overhead from other capabilities (execution time, RAM)
The structure is clear and documented for adding additional capabilities such as new equations
Modularity in ‘Processes’ and ‘Packages’
What is MODFLOW-2005?
Latest release of USGS MODFLOW
Internal computer storage redesigned to support storage of multiple models necessary for local grid refinement and facilitate linkages to other models (GSFLOW = MODFLOW + PRMS).
Parameter-estimation, sensitivity analysis, uncertainty now from UCODE_2005
II. Intro to MODFLOW

5. Selected new USGS MODFLOW Capabilities
Available
Transport
Link-MT3DMS (LMT). Produces files for use by MT3DMS (Zheng)
SEAWAT. Variable density using MODFLOW and MT3DMS. Can also be used without density for transport.
Ground-Water Transport (GWT) Package
Solvers
Link-Multi-Grid (LMG) Solver. Typically 10 times faster than PCG2, but uses a lot of RAM. Free to USGS users.
Geometric Multi-Grid (GMG) Solver. Can be very fast!
Multi-Node well (MNW)
Interactions with overlying systems
FARM Process: Sophisticated accounting for ET, diversions, etc related to agricultural demands.
Streamflow-Routing (SFR)
Unsaturated zone using (1) kinematic wave (UZF) and (2) Richard’s equation (VSF)
MODFLOW-LGR. Accurate local grid refinement. Release in 2006 was for a single rectangular area of refinement. New: multiple refined areas.
Hydrogeologic Unit Flow (HUF) Package. Define hydrogeologic units independently of model layers. Variable-direction horizontal anisotropy (VDHA). KDEP: K varies with depth. SYTP: partial accounting for free surface; numerically stable.
Soon
GSFLOW: Integrated surface processes represented using PRMS
LGR with particles tracked between regional and local grids.
II. Intro to MODFLOW

6. Who is MODFLOW?
Collaborative open-source development with roots at the USGS
Some USGS developers
Arlen Harbaugh (MODFLOW, Reston, Virginia, USA)
Ned Banta (MODFLOW-2000, Lakewood, Colorado, USA)
Mary Hill (SA/PE/UA, MODFLOW-2000, UCODE, MMA, Boulder, Colorado, USA)
Steffen Mehl (local grid refinement (LGR), SA/PE/UA, now at CalSU-Chico, USA)
Stan Leake (compaction and subsidence, TMR, Tucson, Arizona, USA)
John Hoffman (compaction and subsidence, TMR, Tucson, Arizona, USA)
Dave Prudic (gw/sw interaction, STR, SFR, GSFLOW, Carson City, Nevada, USA)
Rick Niswonger (gw/sw interactions, SFR, GSFLOW, Carson City, Nevada. USA)
Paul Barlow (ground-water management, MODMAN, Reston, VA, USA)
Randy Hanson (FARM Process, MNW, San Diego, USA)
Alden Provost (HUF, Reston, VA)
Dave Pollock (particle tracking, MODPATH, Reston, Virginia, USA)
Chris Langevin (transport, saltwater intrusion, SEAWAT, Miami, Florida, USA)
Lennie Konikow (transport extended from MOC3D, GWT, Reston, Virginia, USA)
George Hornberger (transport extended from MOC3D, GWT, Reston, Virginia, USA)
Some non-USGS developers
Chunmiao Zheng (transport, MT3DMS, University of Alabama, USA)
Eileen Poeter (UCODE, MMA, Colorado School of Mines, IGWMC, Golden, CO, USA)
Evan Anderman (ADV, HUF, now at EvanAnderman.com, photography)
Henning Prommer (MODFLOW+PHREEQC, CSIRO, Perth, Australia)
Wolfgang Schmid (FARM Process, U. of Arizona, USA)
David Ahlfeld (ground-water management, GWM, U. of Massachusetts, USA)
You…???
II. Intro to MODFLOW

7. MODFLOW-2005 Processes
Processes each solve a fundamental equation. Of importance in this class are
Ground-water Flow (GWF) Kh = S(h/t) …
Observation (OBS) y = y′ + e
II. Intro to MODFLOW

8. GWF Packages
Packages each represent a type of system feature. Of importance in this class are
Package that defines model layers and properties:
Layer-Property Flow (LPF) Package
Packages used to add/remove water at a specified rate:
Well (WEL)
Recharge (RCH)
Packages that add/remove water based on head in the aquifer:
General-Head Boundary (GHB)
River (RIV)
II. Intro to MODFLOW

9. How Processes and Packages Interact
GWF Process
OBS Process
LPF Package
Define K and S properties, possibly using parameters. Calculate contributions to the matrix equations
No observations are now defined for the LPF Package. Possible observations are internal flows.
RIV Package
Define ricer properties, possibly using parameters.
Calculate contributions to the matrix equations.
River gain and loss observations can be defined.
In MODFLOW, subroutines are named using the three-letter identifiers for processes and packages. For example, GWF1LPF6RP

10. Back to the world of users instead of programmers -- What is required for a simulation?
Tell the program what capabilities to use
Name file (NAM)
Package input files for each process (only the GWF Process is always required)
Basic (BAS6) (can define constant head BC’s here)
Discretization (DIS)
Hydrogeologic info (here, LPF)
Solver. Here we use
Preconditioned Conjugate Gradient (PCG)
II. Intro to MODFLOW

11. Activating capabilities
Turn Packages on and define input files using the NAME file
Example:
# GW Flow process input files
bas tc1.bas
lpf tc1.lpf
wel tc1.wel
pcg /data/tc1.pcg .
II. Intro to MODFLOW

12. Basics of Data Input List data Array data
Data input using lists of cells
layer row column ……
Example:
…….
…….
Array data
Data input in arrays with one row for each row of the model grid and one column for each column of the model grid. Sometimes repeat one array for each model layer.
# # # # # ……… .
II. Intro to MODFLOW

13. MODFLOW is a calculation program What is MODFLOW? Input files
(plain text
or binary)
Output files
(plain text
or binary)
Visual modflow is a graphic user interface which serves to make the process of modelling groundwater flow, pathlines, or solute transport much easier. What visual modflow essentially does is to provide a user friendly environment for the user to enter the grid, boundary conditions and other aquifer properties. Visual Modflow then writes the input files that the actual codes Modflow and MT3D need to run. IT runs Modflow etc and allows the results to be visualised and presented effectively.
II. Intro to MODFLOW

14. Often use MODFLOW through a (Graphical) User Interface
Visual modflow is a graphic user interface which serves to make the process of modelling groundwater flow, pathlines, or solute transport much easier. What visual modflow essentially does is to provide a user friendly environment for the user to enter the grid, boundary conditions and other aquifer properties. Visual Modflow then writes the input files that the actual codes Modflow and MT3D need to run. IT runs Modflow etc and allows the results to be visualised and presented effectively.
Results
Maps
Model
II. Intro to MODFLOW

15. MODFLOW  GUI Translate Execute Input MODFLOW Output
Visual modflow is a graphic user interface which serves to make the process of modelling groundwater flow, pathlines, or solute transport much easier. What visual modflow essentially does is to provide a user friendly environment for the user to enter the grid, boundary conditions and other aquifer properties. Visual Modflow then writes the input files that the actual codes Modflow and MT3D need to run. IT runs Modflow etc and allows the results to be visualised and presented effectively.
Output
II. Intro to MODFLOW

16. MODFLOW capabilities used in class
Class exercise
MODFLOW Packages used
Layer-Property Flow (LPF)
Recharge (RCH)
River (RIV)
General-Head Boundary (GHB)
Advective Transport (ADV)
Preconditioned-Conjugate Gradient (PCG)
MODFLOW Processes used
Ground-Water Flow (GWF)
Observations (OBS)
UCODE_2005 capabilities used
Sensitivity
Parameter-Estimation
MODFLOW capabilities used in class
II. Intro to MODFLOW

17. Aspects of flow model creation
Conceptual model
Base map
Grid design
Areal
Model layers (thickness can be variable)
Boundary conditions
Aquifer properties
Pumping wells
Recharge
Time
Here, describe selected aspects of capabilities used in class
II. Intro to MODFLOW

18. Head-dependent boundaries
Generally use many cells to define a feature. Here, shaded cells are used to simulate flow to compare to measured flow Q2-Q1.
Other cells would be used to define the rest of the river.
From Hill+, 2000
II. Intro to MODFLOW

19. Head-dependent boundaries
For each finite-difference cell n:
Qn = (KA/M)n (Hn – hn)
(KA/M)n = Cn = conductance of assumed distinct streambed
Often define Cn with parameters
Cn=FnP1
Additive:
Cn= Fn1P1 + Fn2P2
Hn= water-body stage
hn= simulated head A
Areal view
of typical cell n
Cross-section of typical cell n
Cell center
II. Intro to MODFLOW

20. Packages that represent head-dependent boundaries
GHB
Packages that represent head-dependent boundaries
Positive q n
indicates
flow into
the
subsurface
Negative q
flow out of H q
= 0
Slope = -
Cn = (K A
)/D
flow
out of E h
(C)
(A)
(B)
EXPLANATION
the simulated flow rate at one cell (L 3
/T)
(nega
tive for flow out of the ground
water
system) K
the hydraulic conductivity (L/T) of, for
example, the riverbed or lakebed D
the thickness (L) of, for example, the riverbed
or lakebed
the area of the water body within the finite
difference cell (L 2 ) C
the conductance calculated using K
, D
, and .
is the simulated hydraulic head in the ground
water system adjacent to the head
dependent
boundary (L); and
is the water level in the water body or the
elevation of the drain (L)
is the botto
m of the streambed
GHB
q=C(H-h)
Important here:
GHB: General-Head Boundary
RIV: River
RIV
RIV
DRN
II. Intro to MODFLOW

21. RIV Package with hn below RBOTn (hn<RBOTn)
Cell center
II. Intro to MODFLOW

22. Pumping wells Well (WEL) Package
List input: layer, row , column, rate (negative means flow out of the ground-water system)
Rate can be defined using parameters
Problem: If a well intersects many model layers, how much water comes from each layer?
II. Intro to MODFLOW

23. Flow model creation: Time
Steady state
Inputs = outputs. No change in storage
No time dimension: easier to visualize
Errors in model setup more clear in results
Transient
Requires (often steady-state) initial conditions
Requires a value for storage
Stresses are defined using stress periods (time interval of input)
Each stress period is divided into time steps (time interval of head calculation).
Lengthy calculation times can produce large output files
For some tips on when to “go transient”, see H.M.Haitjema (2006) Role of Hand Calculations in Ground Water Flow Modeling, Ground Water.
II. Intro to MODFLOW

24. Parameters
In the MODFLOW model for the class problem, parameters are used to define the following model inputs
Layer-Property Flow (LPF) Package
Horizontal hydraulic conductivity of model layers (HK)
Vertical hydraulic conductivity of an implicit confining unit (VKCB)
Recharge (RCH) Package
Recharge rate (RCH)
River (RIV) Package
Riverbed conductance (RIV)
Values of defined parameters can be controlled using the PVAL file. This makes it easy for users of UCODE_2005, etc.
II. Intro to MODFLOW

25. Parameters  Model Input
C of river bed equals the value in the package input file times the factor in the package input file.
Here, the factor is C of river bed = 1000 ×
The factor can be different for different cells.
Parameters  Model Input
River Package input file
PARAMETER
MXACTR IRIVCB
K_RB RIV E
. . .
0 1 ITMP NP -- Stress Period 1
K_RB
II. Intro to MODFLOW

26. Parameters and PVAL file
C of river bed equals the value in PVAL times the factor in the package input file.
Here, the factor is C of river bed = 1000 ×
The factor can be different for different cells.
Parameters and PVAL file
River Package input file
PVAL file
PARAMETER
MXACTR IRIVCB
K_RB RIV E
. . .
0 1 ITMP NP -- Stress Period 1
K_RB 6
HK_ E-5
HK_ E-4
VK_CB E-8
K_RB
RCH_
RCH_
Easy to use UCODE_2005 to change parameter values in PVAL file.
II. Intro to MODFLOW

27. Flow model creation: Observations
MODFLOW’s Observation Process allows simulated values to be compared to observations.
Here, use it for the following observations
Head observations
At a cell
Changes in head over time
Flow observations
Over the reach of a feature represented by the RIV Package
II. Intro to MODFLOW

28. Observations Heads (HOB in name file) River Gain
(RVOB in
name file)
Output file (“data 50 ex8._os” in name file)
E+30 NH,MOBS,MAXM,IUHOBSV,HOBDRY
1. TOMULTH (below, lay,r,c,ts,roff,coff,toff,obs)
hd01.ss
hd02.ss
hd03.ss
. . .
hd10.ss
NQxx,NQCxx,NQTx
E TOMULTxx
NQOBxx,NQCLxx
flow01.ss ts,toff,obs
lay,r,c,factor
. . .
"SIMULATED EQUIVALENT" "OBSERVED VALUE" "OBSERVATION NAME"
hd01.ss
hd02.ss
. . .
flow01.ss
II. Intro to MODFLOW

29. Constructing input files
In class we will either use 00-MFI2005.exe or the files will be constructed already.
Instructions for using 00_MFI2005.bat are provided in class.
II. Intro to MODFLOW

30. Execute MODFLOW
Here, we will use 00-MFI2005.exe or already constructed batch files.
Detailed instructions are provided in the exercise instructions.
Basically, need to provide the name file filename on the same line (this is often done in a batch file)
MODFLOW test.nam
II. Intro to MODFLOW

31. Model results
Possible results for class problem (depends on options chosen)
Global budget (check for overall solution accuracy)
Heads at each active cell in the grid at each time step
Flows at each cell face
Simulated equivalents to observations
Often use software to visualize results. In class, use ModelViewer
II. Intro to MODFLOW

32. Pathline Modeling
Advective transport. Used here as a first investigation of transport predictions
Requires:
Flow solution
Porosity – to determine velocity
Starting locations
The particle tracking is calculated using MODPATH, which uses results produced by MODFLOW.
II. Intro to MODFLOW