An Introduction to MODFLOW

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An Introduction to MODFLOW
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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 its
  up to the developer
• Well documented
• Modularly constructed (More later)
• Latest version MODFLOW-2005 (Harbaugh, 2005)

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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)
• GUIs Visual MODFLOW, Groundwater Vistas, GMS,
  PMWin,

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What is MODFLOW?

• 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

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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) Richards 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.

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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)

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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

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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)

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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
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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 BCs here)
• Discretization (DIS)
• Hydrogeologic info (here, LPF)
• Solver. Here we use
• Preconditioned Conjugate Gradient (PCG)

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Activating capabilities

• Turn Packages on and define input files using the
  NAME file
• Example
• GW Flow process input files
• bas6 41 tc1.bas
• lpf 42 tc1.lpf
• wel 43 tc1.wel
• pcg 44 ../data/tc1.pcg
• .
• .
• .

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Basics of Data Input

• List data
• Data input using lists of cells
• layer row column
• Example
• 1 3 43 .
• 2 62 53 .

• 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.
• .
• .
• .

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What is MODFLOW?
MODFLOW is a calculation program
Input files (plain text or binary)
Output files (plain text or binary)
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Often use MODFLOW through a (Graphical) User
Interface
Results
Maps
Model
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MODFLOW ? GUI
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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

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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
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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
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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
• CnFn?P1
• Additive
• Cn Fn1?P1 Fn2?P2
• Hn water-body stage
• hn simulated head

Areal view of typical cell n
Cross-section of typical cell n
Cell center
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Packages that represent head-dependent boundaries
GHB

GHB

• qC(H-h)
• Important here
• GHB General-Head Boundary
• RIV River

RIV
RIV
DRN
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RIV Package with hn below RBOTn (hnltRBOTn)
Cell center
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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?

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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.

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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.

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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 1000. C of river bed
1000 0.0012 The factor can be different for
different cells.

• River Package input file

PARAMETER 1 18 18 54
MXACTR IRIVCB K_RB RIV 1.200000E-03 18
1 1 1 100. 1000. 90. . . . 1 18 1
100. 1000. 90. 0 1 ITMP NP -- Stress Period
1 K_RB
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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 1000. C of river bed 1000
0.00116999 The factor can be different for
different cells.

• River Package input file
• PVAL file

PARAMETER 1 18 18 54
MXACTR IRIVCB K_RB RIV 1.200000E-03 18
1 1 1 100. 1000. 90. . . . 1 18 1
100. 1000. 90. 0 1 ITMP NP -- Stress Period
1 K_RB
6 HK_2 1.523554700000E-5 HK_1
4.619000000000E-4 VK_CB 9.903220000000E-8
K_RB .0011699900000000 RCH_2
38.39840000000000 RCH_1 47.55430000000000
Easy to use UCODE_2005 to change parameter values
in PVAL file.
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Flow model creation Observations

• MODFLOWs 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

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Observations

• Heads (HOB in name file)
• River Gain
• (RVOB in
• name file)
• Output file (data 50 ex8._os in name file)

10 0 0 50 1.E30 NH,MOBS,MAXM,IUHOBSV,HOBDR
Y 1. TOMULTH (below, lay,r,c,ts,roff,coff,to
ff,obs) hd01.ss 1 3 1 1 0.0 0.0 0.0
101.80 hd02.ss 1 4 4 1 0.0 0.0
0.0 128.12 hd03.ss 1 10 9 1 0.0 0.0
0.0 156.68 . . . hd10.ss 2 18 6 1
0.0 0.0 0.0 142.02
1 18 1 50 NQxx,NQCxx,NQTx
1.00000E00 TOMULTxx 1
18 NQOBxx,NQCLxx flow01.ss 1 0.0 -4.4
ts,toff,obs 1 1 1 1.00
lay,r,c,factor . . . 1 18 1 1.00
"SIMULATED EQUIVALENT" "OBSERVED VALUE"
"OBSERVATION NAME" 100.209701538086
101.800003051758 hd01.ss
126.954444885254 128.119995117188
hd02.ss . . . -4.41627883911133
-4.40000009536743 flow01.ss
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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.

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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

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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

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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.