Grid Design, Finite Difference Grids, and an Introduction to MODFLOW

1
Grid Design, Finite Difference Grids, and an
Introduction to MODFLOW
Based on Slides Prepared By Eileen Poeter,
Colorado School of Mines
2
Conceptual Model Defines

• 1) Dimensions of numerical model
• 2) How the grid is designed
• 3) How the grid is oriented

3
Representation of Numerical Model
4
DISCRETIZED HYPOTHETICAL AQUIFER
Layers may correspond to horizontal geohydrologic
intervals

• ---- Aquifer boundary
• ? Active cell
• ? Inactive cell
• ?rj Width of cell in row direction (j indicates
  column number)
• ?ci Width of cell in column direction (i
  indicates row number)
• ?vk Thickness of the cell
• ?rj?ci?vk Volume of cell with coordinates (i,j,k)

5
Representation of Numerical Model

• Choose numbers to define a conceptual object like
  the grid shown below to represent the geometry,
  properties, boundary conditions, initial
  conditions and stresses on a groundwater system
  to build a representation of field conditions

Videos of Flow (Mojave, Santa Clara Transport
Models (Tracy?)
6
Representation of Numerical Model

• Divide space into pieces
• Define one value for each geohydrologic parameter
  to represent the piece
• One value defined for each physical property (ie.
  K and S)
• One value of head and flow is calculated
• Complex geologic material distributions are
  simplified
• Properties vary
• Within a layer
• From layer to layer

7
Representation of Numerical Model

• Some of the model pieces are defined as inactive
  (open circles)
• Take the rectangular form of the mathematics and
  create an odd shaped geometry
• Inactive indicators may continue down through
  every layer of the grid, or not

Example aquifer is bowl shaped then some pieces
that are active in the shallow layers would be
specified as inactive in the deeper layers
8
Grid Design

• Numerical model needs to be divided into pieces
  of space and time for which the solution can be
  linearized and the properties and results
  averaged
• Compromise between accuracy, cost, and effort
• Smaller pieces are more accurate, but require
  more time and effort

9
Grid Design

• Discretize
• Space (plan view and cross section)
• Time
• Difficult Task
• Redesign is a major undertaking

10
Spatial Dimension

• 2D areal
• 2D profile (special class)
• Quasi 3D (confining layers by leakage)
• Fully 3D
• Aquifer viewpoint 2D areal and quasi 3D
• Flow system viewpoint 2D profile and 3D

11
QUASI THREE DIMENSIONAL
Flowlines in sand are nearly horizontal
Flowlines in clay are nearly vertical
Quasi-3d Single model layer maybe used to
represent each sand, while the clay may be
represented by the vertical conductance between
layers
Clay layer is represented by six model layers.
Use if clay storage is an issue.
12
Fully 3D Models

• Simulate confined and unconfined aquifers when
  vertical head gradients are important
• Represent transient release of water from storage
  in confining beds by including confining bed as a
  layer with storage properties
• Parameter arrays specified for each layer of the
  model

13
Parameters

• Transmissivity
• Hydraulic conductivity
• Thickness
• Anisotropy
• Storage properties

14
Laying out the grid

• Types of Grids
• Defining Model Layers
• Orienting the Grid
• Spatial Scales

15
Types of Grids

• Array of Nodes
• Grid Structure
• Finite elements
• Finite difference cells

16
Finite Elements

• Allow more flexibility in designing grid
• 2D elements
• Triangles
• Quadrilaterals
• 3D elements
• Tetrahedrons
• Hexahedrons
• Prisms
• Exact representation of boundaries is possible
• Input of data is generally more laborious than
  finite difference

17
Finite difference cells

• Mesh-centered
• Block-centered
• Easier math for boundaries
• MODFLOW

18
Defining Model Layers

• One layer
• layer represents a single hydrostratigraphic unit
  or aquifer
• Quasi-3D
• Hydrogeologic units horizontal
• Leakance
• Fully 3D
• Dipping units
• Aquifers and Confining units explicit

19
Orienting the Grid

• Grid drawn on an overlay of a map of the area to
  be modeled
• If possible orient the grid so that the x and y
  axes are colinear with Kx and Ky and vertical
  axis is aligned with Kz
• For finite difference, try to minimize the number
  of nodes that fall outside the boundaries of the
  modeled area
• Set boundaries far from the area of interest so
  imposed stresses to the interior of system dont
  reach the boundaries

20
Spatial Scales

• Critical Step
• Based on
• Size of model area
• Changes in head (primary)
• Changes in aquifer properties (secondary)
• Changes in recharge, pumping, surface-water
  interaction

21
Spatial Scales

• Horizontal Node Spacing
• Function of expected curvature in the water table
  or potentiometric surface
• Variations in aquifer properties in horizontal
  dimension typically greater than vertical
• Vertical Node Spacing
• Function of change in head in the vertical
  direction
• Typically one layer per hydrostratigraphic unit
• Significant vertical head gradients may want more

22
Vertical discretization can vary depending on use
of the model
Halford, 1999
23
Spatial Scales

• Overall size of model area also affects the
  selection of model area
• Compromise between accuracy and practicality
• Small number of nodes
• Minimize data handling, computer storage and
  computation time
• Large number of nodes
• Represent system accurately
• Meaningful boundaries may require a large area

24
Variably spaced finite-difference grid allows
good discretization of remediation area, while
allowing model to go to hydrologic boundaries.
Halford, 1999
25
Assigning Parameter Values

• Data Needs (Discussed last week)
• Two Categories
• Physical framework (geometry including thickness,
  extent, and properties of units)
• Hydraulic data (heads and fluxes)
• Transferring field data to the grid
• Scale issues
• Zones (sets of nodes with similar properties)
• Interpolation algorithms such as kriging
• Hydrogeologic judgement
• WHATEVER METHOD, DISTRIBUTIONS MUST BE REASONABLE
  AND MAKE SENSE!!!!

26
FINITE DIFFERENCE METHOD

• The continuous system is replaced by a finite set
  of discrete points in time and space
• The partial derivatives are replaced by terms
  calculated from the differences in head values at
  these points
• The discretization process results in a system of
  simultaneous linear algebraic equationsdifference
  equations
• The solution to the difference equations yields
  values of head at specific points and time

27
Finite Difference MethodsSee handout taken from
Lessons Prepared ByEileen Poeter, Colorado
School of Mines

• Spreadsheet Example
• MODFLOW

28
Discretize Time and SpaceFINITE DIFFERENCE AND
MODFLOW

• Plan View
• Cross Sectional View
• Time

29
Discretize Time and Space

• Plan View
• For a finite-difference grid, lines between cells
  need to orthogonal and extend the entire width of
  the grid
• any detail defined in the interior of the grid is
  extended all the way to the edges
• most finite-difference codes allow the width of
  cells along rows to vary

30
Plan View Grid Considerations

• Problem Domain
• External Inactive Grids
• Flow Direction
• Anisotropy
• Minimize Number of Cells
• Boundaries Between Features
• Stress Areas
• Observation Points
• Symmetry
• Relative Size of Adjacent Grids (1.5)
• Orthogonal Directions (1001)
• Future Solute Transport

31
Plan View Grid Considerations

• Problem Domain
• Use well-defined, permanent natural boundaries
  when possible.
• If a boundary is not permanent (e.g. a
  ground-water divide) anticipate potential future
  variations, and either accommodate them from the
  start or be prepared to monitor appropriately and
  make adjustments later.
• Most approaches to grid development require
  substantial time and effort to make substantial
  changes to the model grid.

32
Plan View Grid Considerations

• External Inactive Grids
• Rotate grid to allow as few nodes as possible
  outside the active model domain
• Minimize input and output file size
• Make data management easier
• Flow Direction
• Orient grid so that the primary flow direction is
  aligned with the rows and columns
• Flow calculations are oriented along rows and
  columns, so diagonal flows are calculated in a
  stair-step manner, thus orienting the rows and
  columns in the direction of flow will reduce
  errors.

33
Plan View Grid Considerations

• Anisotropy
• Orient grid so that the rows and columns of the
  grid coincide with the major axes of the
  hydraulic conductivity ellipsoid.
• Minimize Number of Cells
• Easier to manage
• Executes more quickly
• Tradeoff with accuracy
• Boundaries Between Features
• More detailed grid where conditions change
  abruptly
• May need gradual transition in parameter values
  at a contact, which can reduce calculation errors
  or convergence trouble. If the cells are small,
  such a gradation is a fairly good approximation
  of the actual transition.

34
Plan View Grid Considerations

• Stress Areas (Steep Gradients)
• Gradient between cells represented by a straight
  line.
• Better solution if many small cells are used.
• Observation Points / Areas of Interest
• Head, concentration, or flow rate can be
  interpolated between cells
• More accurate and more convenient to have cells
  at needed locations
• Symmetry
• May allow you to cut your model size in half or
  more
• Common when simulating engineered features
• Relative Size of Adjacent Grids (1.5)
• Orthogonal Directions (1001)
• Future Solute Transport

35
Plan View Grid Considerations

• Relative Size of Adjacent Grids (1.5)
• If adjacent grids have substantially different
  size, then truncation errors may occur in the
  matrix solution.
• To avoid problems maintain a maximum size
  difference of 1.5 for adjacent cells.
• Orthogonal Directions (1001)
• Aspect ratio is less critical than relative size.
  
• Acceptable for the ratio of length to width, or
  width to length, to be 1001.
• Future Solute Transport
• Frequently requires much smaller cells than flow
  modeling
• Often advantageous to start with this
  discretization

36
Discretize Time and Space

• 2) Cross Sectional View
• MODFLOW allows thickness of layers to vary on a
  cell by cell basis
• Each layer must extend across the entire model
• Pinch outs must be dealt with by changing
  properties of the layer

37
Layer Considerations

• One Layer NO VERTICAL flow, flow parallel to
  layer
• Vertical Components stacks of cells, layers
• Two layers upward or downward gradient of one
  magnitude (cannot calculate convergent flow)
• Complicated vertical flow patterns multiple
  layers

38
Layer Considerations

• Purpose of Model
• Regional vs. Local
• Partial Penetration
• Confining Unit Storage
• Future Transport Modeling
• Hydrostratigraphic Units
• Geologic Logs
• Geophysics
• Vertical Hydraulic Gradients
• Dewatering
• Layer Representation Options
• Constant layer thickness (variable properties)
• Variable layer thickness (constant properties)
• Relative size of adjacent grids is not an issue
  in vertical direction

39
Layer Considerations Purpose of the Model

• Regional vs. Local
• Units likely to be grouped or lumped in regional
• More detail in local
• Nature of question will influence
• Partial Penetration
• Layers to define open interval
• Additional layers to define head gradients and
  flow paths
• Confining Unit Storage
• Future Transport Modeling

40
Layer Considerations Purpose of the Model

• Confining Unit Storage
• No layers
• No storage
• Leakage
• Multiple layers
• Water in storage
• Long travel times for pressure gradient
• Future Transport Modeling
• All of above issues
• Travel time requires multiple layers

No cells for confining unit
Multiple layers for confining unit
41
Layer Considerations (cont.)

• Hydrostratigraphic Units
• Geologic Logs
• Build a 3D stratigraphy
• Determine lumping/simplification
• Even homogeneous may have vertical gradients
  because of boundaries
• Geophysics
• Use to add to information from geologic logs
• Vertical Hydraulic Gradients
• Determine if natural gradients are important to
  your problem
• Enough layers to represent variation in gradients

42
Layer Considerations (cont.)

• Dewatering
• The original version of MODFLOW would not allow
  grid cells to "re-wet" if the head had dropped
  below the bottom of a cell in a previous
  iteration. These cells would become impermeable.
  The modern MODFLOW accommodates this feature.
  However there are often convergence issues or
  long run times.
• If not using rewetting, you may have to make
  shallow units thick in order to keep them from
  completely dewatering. Of course, this means that
  you will not evaluate vertical components of flow
  in that zone.permanently impermeable zone if
  re-wetting option is not used, even if the well
  is turned off

43
Layer Considerations (cont.)

• Relative size of adjacent grids is not an issue
  in vertical direction
• MODFLOW connects layers explicitly, consequently
  you do not need to be concerned about truncation
  errors in a matrix solution for vertically
  adjacent cells.

44
Layer Considerations (cont.)

• Layer Representation Options
• Constant layer thickness /variable properties
• Expedites modeling
• Rough approximation
• Compatibility with another function
• Variable layer thickness /constant properties
• More representative of field conditions

45
Discretize Time and Space

• 3) Discretize Time
• TIME STEPS temporal equivalent of grid cells
• Small when stresses change and increase in length
  to a constant, convenient size until the stresses
  change
• STRESS PERIODS groups of time steps during which
  stresses do not change
• Temporal data compiled at these increments

46
Time Discretization
47
Time Discretization Considerations

• Difficult to decide on initial time step size
• MODFLOW requires the time period, number of
  steps and a multiplier to gradually increase steps

Multiplier is typically 1.1 to 1.5
48
How small is small enough?

• YOU KNOW YOUR DISCRETIZATION IS APPROPRIATE WHEN
• THE ANSWER REMAINS THE SAME FORSMALLER TIME
  STEPS, STRESS PERIODS,ANDSMALLER CELL SIZES
• TIME
• Easy to test smaller time steps
• Stress periods require recompiling stress data
  (may be time consuming) and updating any packages
  with stresses specified
• SPATIAL
• Unless you have an automated grid generator /
  input file creator, then the time requirements
  and logistics of rebuilding the model with
  smaller cell sizes renders the task unreasonable
• Important to use smaller grid sizes from the
  beginning of numerical model development because
  you will never be able to test this issue.
• In reality, few if any modelers check this.

49
MODFLOW

• MODFLOW is the world's most used ground-water
  modeling code
• Goal was/is to be
• easy to understand,
• use, and
• modify

50
Versions of MODFLOW

• Trescott, Pinder, and Larson codes
• MODFLOW (much longer name)
• MODFLOW-88 (first version)
• MODFLOW-96
• MODFLOW-2000
• MODFLOW-2005

This class will use the documentation for
MODFLOW-2005 as a primary reference. Class
projects will be done with this version. The
report and program can be downloaded to your
computer from USGS web site
http//water.usgs.gov/nrp/gwsoftware/modflow2005/m
odflow2005.html
51
MODFLOW

• Originally organized in modules
• Modules grouped into packages that perform
  calculations either specific to the behavior of a
  geohydrologic feature or a numerical modeling
  task
• Packages allow
• examination of specific hydrologic features
  independently
• facilitates development of additional
  capabilities
• Originally solely a ground-water flow model

52
MODFLOW

• Scope broadened to allow capabilities such as
  transport and parameter estimation
• Expansion of modular design required
  (MODFLOW-2000)
• addition of Process
• MODFLOW-2005 is similar in design to MODFLOW-2000
• Incorporates different approach for managing
  internal data
• Fortran modules are used to declare data that can
  be shared among subroutines
• MODFLOW subroutines were originally called
  modules
• generic term module has been eliminated and
  replaced by the term subroutine

53
MODFLOW DOCUMENTATION
MODFLOW2005 and associated documentationhttp//
water.usgs.gov/nrp/gwsoftware/modflow2005/modflow2
005.html FOR MUCH MORE DETAIL visit the
USGS Online Guide to MODFLOW
http//water.usgs.gov/nrp/gwsoftware/modflow2000/M
FDOC/guide.html Older versions MODFLOW88 -
http//pubs.usgs.gov/twri/twri6a1/pdf/TWRI_6-A1.pd
fMODFLOW96 - http//water.usgs.gov/software/code/
ground_water/modflow/doc/ofr96485.pdfMODFLOW2000
and associated documentation Overview and
Ground Water Flow Process - http//water.usgs.gov/
nrp/gwsoftware/modflow2000/ofr00-92.pdf
54
ORIGINAL MODULAR STRUCTURE (1988)

• BAS - basic package
• general tasks - gridding, constant head and
  no-flow boundaries, initial conditions, time
  stepping
• OC - output control package
• controls the information and format of results
• BCF - block centered flow package
• layer types, grid dimensions, material properties
  
• WEL - well package
• locations and flow rates of wells
• RCH - recharge package
• recharge rates and locations
• RIV - river package
• locations, river bed material properties, and
  river stages
• DRN - drain package
• location, material properties surrounding drains,
  and elevation of drains
• EVT - evapotranspiration package
• parameters describing evapotranspiration rate
  with depth to water table
• GHB - general head boundary package
• locations, local material properties, and
  elevation of specified heads
• SOLVERS

55
PACKAGES WRITTEN AFTER ORIGINAL MODFLOW

• PCG2 - preconditioned conjugate-gradient 2
  package
• alternative matrix solver
• STR1 - stream routing package
• differs from the river package in that the
  surface water stage varies based on the surface
  water flow and the Manning equation
• BCF2 - block-centered flow 2 package
• allows for re-wetting of cells that have gone dry
  
• BCF3 - block-centered flow 3 package
• a supplement to the BCF2 package, allowing
  alternative interblock transmissivity
  formulations
• HFB1 - horizontal flow barrier package
• simulation of thin, vertical, low permeability
  features that impede horizontal flow
• TLK1 - transient leakage package
• simulates transient leakage and storage changes
  in confining units of quasi-3D models
• GFD1 - general finite difference flow package
• substitutes for the BCF package, allows user to
  enter conductance rather than calculating with
  MODFLOW
• IBS1 - interbed storage package
• simulates compaction of compressible,
  fine-grained units within or adjacent to aquifers
  in response to pumping
• CHD1 - time-variant specified head boundary
  package
• allows time varying specified head

56
PACKAGES WRITTEN MODFLOW-2000 and since

• rapidly growing long list
• earlier packages are listed here,
• refer to the
• USGS MODFLOW and related programs web page, OR
• use the USGS OnLine Guide for MODFLOW
• GWF1 - ground water flow process (GWF in name
  file)
• finite difference simulation of saturated porous
  media flow
• OBS1 - observation process (OBS in name file)
• monitors value of head or flow at specified
  locations
• SEN1 - sensitivity process (SEN in name file)
• calculates the change in simulated head and
  flows at observation locations PES1 - parameter
  estimation process (PES in name file)
• estimates values of parameters by nonlinear
  regression to minimize the weighted sum of
  squared residuals for observations
• DIS - discretization package (DIS in name file)
• gridding, defining division of space and time
  for the numerical solution
• MULT - multiplier file (MULT in name file)
• defines the spatial distribution of multipliers
  in the grid that act on parameter values
  specified in those zone

57
PACKAGES WRITTEN MODFLOW-2000 and
since(continued)
ZON - zone file (ZONE in name file) defines the
spatial distribution of zones in the grid where
specified parameters apply BAS6 - basic package
(BAS6 in name file) constant head and no-flow
boundary conditions and initial conditions OC -
output control package (OC in name file)
controls the information and format of results
BCF6 - block centered flow package (BCF6 in name
file) defines material properties with some
parameters being dependent on grid dimensions
(e.g. transmissivity), thus this package ignores
the discretization information in DIS for some
purposes -- the parameter method of inputting
data cannot be used -- method of interblock
conductance calculations can be selected LPF1 -
layer property flow package (LPFin name file)
an alternative to BCF6 defines material
properties with all parameters independent of
grid dimensions (e.g. hydraulic conductivity) --
the parameter method of inputting data can be
used -- method of interblock conductance
calculations can be selected HFB6 - horizontal
flow barrier package (HBF6 in name file)
represents thin barriers that occur between
model cells by defining their hydraulic
conductivity divided by their thickness and
specifying where they occur
58
PACKAGES WRITTEN MODFLOW-2000 and
since(continued)
WEL6 - well package (WEL in name file)
locations and flow rates of wells RCH6 -
recharge package (RCH in name file) recharge
rates and locations RIV6 - river package (RIV in
name file) locations, river bed material
properties, and river stages STR6 - stream
routing package (STR in name file) differs from
the river package in that the surface water stage
varies based on the surface water flow
(calculated as specified flow and ground water
flux to/from stream) and the Manning equation
DRN6 - drain package (DRN in name file)
location, material properties surrounding
drains, and elevation of drains (this update
allows a fraction 0-1 of the drain outflow to
be returned to a specified cell) EVT6 -
evapotranspiration package (EVT in name file)
parameters describing evapotranspiration rate
with depth to water table GHB6 - general head
boundary package (GHB in name file) locations,
local material properties, and elevation of
specified heads CHD6 - time-variant specified
head boundary package (CHD in name file) allows
time varying specified head
59
PACKAGES WRITTEN MODFLOW-2000 and
since(continued)
SOLVERS (SIP SOR PCG DE4 LMG in name file) SIP5
- strongly implicit procedure package SOR5 -
slice-successive over-relaxation package PCG2 -
preconditioned conjugate gradient package  
DE45 - direct solution by alternating diagonal
ordering package   LMG1 - multigrid solver
speeds execution for large grids and high degree
of heterogeneity ADV2 - advective transport
observation package (ADV2 in name file) allows
use of travel time observations for parameter
observations RES1 - reservoir package (RES in
name file) simulates leakage between reservoir
and aquifer as reservoir area changes in
response to stage changes FHB1 - flow and head
boundary package (FHB in name file) allows flow
and head boundary conditions that vary at times
other than starting and ending times of stress
periods IBS6 - interbed storage (subsidence)
(IBS in name file) simulates compaction related
to hydraulic head decline HUF1 - hydrologic-unit
flow package (HUF in name file) calculates
effective hydraulic properties for cells based on
geometric description of hydrologic units
60
PACKAGES WRITTEN MODFLOW-2000 and
since(continued)

• LAK3 - lake package (LAK in name file)
• allows variation of lake stage based on water
  budgets
• ETS1 - evapotranspiration package with segment ET
  function (ETS in name file)
• allows function describing evapotranspiration
  rate with depth to water table to be piece-wise
  linear
• DRT1 - drain package with return flows (DRT in
  name file)
• allows user to allocate proportions of drain
  flow to be recharge to specified cells
• LMT6 - link to MT3D (LMT in name file)
• allows printing of file to be read by MT3D for
  contaminant transport
• SFR - Streamflow-Routing package (SFR in name
  file)
• is used to simulate streams in a model (provides
  greater flexibility in how streams are specified
  than STR)
• UZF - Unsaturated Zone Flow Package (UZF in name
  file)
• simulates vertical flow of water through the
  unsaturated zone to the saturated zone

61
MODFLOW DOCUMENTATION

• Documentation for MODFLOW-96 and MODFLOW-2000 was
  not complete by itself and referred extensively
  to the MODFLOW-88 documentation
• MODFLOW-2005 is similar to MODFLOW-88
  documentation
• details entirely contained in one report
• fundamental concepts,
• programmer information, and
• user input instructions for ground-water flow
• additional reports for added capabilities
• Processes
• Capabilities to simulate additional hydrologic
  features

62
MODFLOW-2005 DOCUMENTATION

• Purpose is to describe the mathematical concepts
  used in the GWF Process
• program design,
• input needed to use it, and
• programming details
• Outline
• Chapter 1. Introduction
• Chapter 2. Derivation of the Finite-Difference
  Equation
• Chapter 3. Design of the Ground-Water Flow
  Process
• Chapter 4. Basic Package
• Chapter 5. Internal Flow Packages
• Chapter 6. Conceptualization and Implementation
  of Stress Packages
• Chapter 7. Solver Packages
• Chapter 8. Input Instructions
• Chapter 9. Programmer Documentation

63
MODFLOW-2005 DOCUMENTATION

• Get familiar with the MODFLOW code through its
  Documentation
• Read Chapters 1 through 3
• Describes overall program
• Look over the core chapters (Chapters 4 -7)
• 4) Basic Package administrative tasks and
  program design
• 5) Internal Flow Packages how flow is simulated
• 6) Stress Packages physical and mathematical
  concepts
• 7) Numerical Solvers
• Each time you use a new package, stop and read
  the theory section for that package before
  proceeding
• Become familiar with input instructions for
  packages including utility subroutines (Chapter
  8)

64
MODFLOW

• Get an overview of the numerical model
• Note unusual coordinate system
• Sequence of cells rows, columns, and layers
• Origin of numbering top, back, left corner
• Chapters 4-7 discuss theory
• Chapter 8 includes a step by step description of
  the file setup

65
Versions of MODFLOW

• MODFLOW
• MODFLOW-88
• MODFLOW-96
• MODFLOW-2000
• MODFLOW-2005

This class will use the documentation for
MODFLOW-2005 as a primary reference. Class
projects will be done with this version. The
report and program can be downloaded to your
computer from USGS web site
http//water.usgs.gov/nrp/gwsoftware/modflow2005/m
odflow2005.html
66
MODFLOW -2005

• Supports multiple grids so that it is possible to
  incorporate local grid refinement.
• Uses parameter structure to ease the modification
  of data input values.
• Provides expanded data input capabilities.
• Program is designed to minimize changes that
  would impact existing MODFLOW users.

67
MODFLOW -2005 PROCESSES

• Previous MODFLOW tasks (prior to MF2K) are now
  defined as the
• Global Process
• GLO Controls Overall Program Operation
• Equation Solving Processes
• GWF Ground-water Flow Process

68
MODFLOW 2005 PROCESSES

• As Initially released, MODFLOW also includes
  other processes
• OBS - Observation Process
• SEN - Sensitivity Process
• PES - Parameter Estimation Process
• GWT - Ground-water Transport Process
• Other Processes being developed including
• FMP Farm Process
• We are going to concentrate on the GWF
  Ground-water Flow Process and discuss the others
  later in the class

69
GLOBAL PROCESS

• Controls overall program flow
• Activates capabilities (Packages)
• Opens package data files (Input and Output)
• Reads data for space and time discretization (DIS
  file)
• Reads parameter files (Multplier and Zone)
• Has a global level listing file
• The Global Process does not solve an equation

70
GROUNDWATER FLOW PROCESS(abbreviated list)

• GWF Process Packages User Prospective
• BAS6 Basic Package
• Hydrologic Packages
• BCF6 Block Centered Flow Package
• LPF Layer Property Flow Package
• RCH Recharge Package
• RIV River Package
• WEL Well Package
• DRN Drain Package
• GHB General Head Boundary Package
• EVT Evapotranspiration Package
• STR Stream/Aquifer Package
• HFB6 Horizontal Flow Barrier Package
• CHD Constant-Head Package
• Solution Packages
• SOR Slice-Successive Over-relaxation
• SIP Strongly Implicit Procedure
• PCG Preconditioned Conjugate Gradient

71
GROUNDWATER FLOW PROCESS

• GWF Process Procedures Programmer Prospective
• DF Define
• AL Allocate
• RP Read and Prepare
• ST Stress
• AD Advance
• FM Formulate
• AP Solve Equations
• OC Output Control
• BD Calculate Water Budget
• OT Output

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GROUNDWATER FLOW PROCESS Primary Modules

• Example RIV6FM
• The first three characters designate the package
  (river)
• The fourth character is the version number (6)
• The last two characters represent the procedure
  (formulate)

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Example Module Flowchart And Code
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MODFLOW user perspective

• Input Data
• ASCII text files
• Output Data
• ASCII text files
• Binary files
• Graphical user interface (GUI)
• Code Execution

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MODFLOW user perspective

• BASIC INPUT ITEMS
• Grid
• Time stepping
• Solution parameters
• Hydraulic parameters (includes material
  properties)
• Boundary Conditions
• Stresses (source-sinks)
• Output options
• BASIC OUTPUT ITEMS
• Hydraulic Heads
• Drawdown
• Flow rates
• Mass Balance
• Optional info at specified times
• Iteration information
• Binary files containing heads, drawdowns and flow
  rates in compressed form

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GUI

• Allows you to develop a nice image of model
  features on the computer screen and manipulate
  the model inputs graphically
• Creates the text files and executes MODFLOW.
• You never need to see the text files or know the
  commands that are necessary to run MODFLOW ...
  until something goes wrong!

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GUI

• Pros and Cons
• Inevitably something does not work correctly
• Need to have the ability to look at and
  understand the content of the model files and
  control the commands.
• Likely to dislike the tedium associated with the
  portion of the course where we work with text
  files

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File formats

• Original code MODFLOW88 expects to have FORMATTED
  DATA SETS
• exact about placement of data in columns of the
  file
• Occasionally see files in an old format (may have
  no spaces)
• MODFLOW96 provides the option of using either
  FREE or FORMATTED DATA SETS
• Translator was released with MODFLOW-2000
• Takes MODFLOW88 or MODFLOW96 files and convert
  them to MODFLOW-2000
• HUF package allows geometry of the geology
  defined separately from the layers and have code
  simplify it to individual values for each model
  cell

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File formats

• Prior to MODFLOW-2000, MODFLOW required a
  different number in each model cell, now have
  flexibility in populating the cells with
  parameter values
• Parameters
• MODFLOW calculates value for each cell based on
• Parameter file (PVAL) defines values used to
  replace parameters specified in the files where
  parameters are defined (can use SEN in 2000)
• Multiplier files (MULT) - specify multiplier
  arrays which can be used to calculate layer
  variables
• Zone files (ZONE) specify the cells in a layer
  (arrays) that are associated with a parameter

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Next LEARNING MODFLOW

• Global Process
• Overview of Common Packages
• Basic
• Utility Module
• Output Control
• BCF
• WEL
• RIV
• RCH
• EVT
• STR
• DRN
• GHB
• LPF
• Once use to input for several packages, others
  are the same
• Best way to learn is to build a model
• In class, we will build a model for a simple
  problem
• Work on class project