Introduction to MODFLOW

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Introduction to MODFLOW
Part 2
2
Conductance
3
In general, conductance
KA/?L
Also, leakance Kv/thickness resistance (an
analytic element term)
thickness/Kv
4
Vertical Conductance Terms
MODFLOW uses special terms (VCONT) for vertical
conductance, which is required in 3D problems.
Modflow.exe requires the user to supply values
for the VCONT arrays. GW Vistas computes the
values in the VCONT arrays for you.
VCONT Kv/?z Conductance/(?x?y)
leakance
5
See handout from Domenico Schwartz for
MODFLOWs FD equation.
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Examples of solution techniques that
combine matrix solution with iteration IADI
(see chapter 5 of WA) SSOR SIP PCG2 Used
in MODFLOW
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m1
Note m is time level and n is iteration level.
?t
m
m-1
Schematic of solution procedure
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SSOR
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The 3D eqn. with an implicit approximation generat
es a coefficient matrix with 7 diagonals
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SSOR The coefficient matrix has 5 diagonals
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Types of Layers (LAYCON array) Confined Unconfined
Convertible (Useful to think in terms of a
layer transmissivity.)
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Conductance
Confined layer transmissivity (KRi,j-1/2,k)(vk)
Unconfined layer transmissivity
(KRi,j-1/2,k)(hi,j,k-BOTi,j,k)
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Input of Grid Spacing

• MODFLOW asks for values of horizontal grid
  spacing
• (i.e., ?r and ?c) but not vertical grid spacing
  (?v).

• Vertical grid spacing (?v) is calculated by
  MODFLOW
• from user supplied values of the bottom (BOT)
• of each layer. (GwVistas asks for both top and
  bottom of
• each layer.)

TOP
BOT
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Note that a top unconfined layer actually has no
top. The head value in the top layer could
potentially rise to infinity.
Land surface elevation is not used in MODFLOW,
except in the ET package.
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Wooster example from Domenico Schwartz