Transcript Transient Simulations require: Initial Conditions Storage Coefficient

Transient Simulations require:
• Initial Conditions
• Storage Coefficient
• Time step
General 3D equation

h

h

h
h
( Kx ) 
( Ky )  ( Kz )  Ss
W
x
x
y
y
z
z
t
Heterogeneous, anisotropic, transient, sink/source term
Specific storage: Ss = S/b
S is storativity or specific yield
Types of Initial Conditions
• Static steady state
• Dynamic steady state
• Dynamic cyclic conditions
• (Time distant conditions)
Problem set 5: part 7.3b
Generating cyclic steady state conditions
from GWV
Remember: In MODFLOW, a layer is considered “confined”
when the head in the cell is above the top of the cell. (If the top
layer is specified to be unconfined, it has no top elevation.)
MODFLOW allows for two types
of storage coefficients:
• Unconfined storage coefficient (i.e., specific yield)
(used for unconfined layers)
• Confined storage coefficient
(used for confined layers)
• or you may input specific storage, Ss
A “convertible” layer in MODFLOW (laycon type 3)
is one where cells can convert from confined to
unconfined conditions.
(Conversion back to confined is possible with the re-wetting option
but this option doesn’t always work and may give incorrect solutions.
Use with caution, or not at all.)
MODFLOW divides time into Stress Periods.
There are multiple time steps in a stress period.
Initial
t
Parameters in the following packages
can be changed between stress periods:
WEL, RCH, RIV, DRN, EVT, GHB
Rule of thumb: take at least five time steps per
stress period.
MODFLOW divides time into Stress Periods.
There are multiple time steps in a stress period.
Initial
t
Rule of thumb: take at least five time steps per
stress period.
Selecting the time step
Can use:
t = Sa2/4T
for the initial time step of a stress period
or find an appropriate time step by trial and error.