significant additional accuracy.   Time steps as small
as a fraction of an hour are appropriate for a pumping
well while monthly or even yearly timesteps are ap-
propriate for gradual infiltration changes.    Prelimi-
nary hand calculations and sensitivity analyses are
recommended to determine the appropriate timestep and
grid spacing.
Convergence Criteria - The implicit numerical scheme
employed in this program is an approximate solution
which   is  refined   through   successive  iterations.
Therefore, some criterion must be defined to determine
when the solution has converged to an acceptable ac-
curacy. This convergence criterion is implemented by
comparing the difference in heads computed between two
successive iterations. When this difference is within
an acceptable value, convergence    is reached.    This
program presents the difference in heads between iter-
ations two ways.    First, the sum of the change in
heads for all nodes is computed.    Secondly, the maxi-
mum head change at any node is found. Both values are
reported and the program uses the sum of head changes
as its convergence criteria.     The iteration process
stops when the sum of the head changes at every node
point goes below a user defined limit. This method is
most appropriate for regional groundwater simula-
tions.   The program also reports    the value of the
maximum head change at any node and can be easily
modified to use this as a convergence criterion. This
criterion would be more appropriate when simulating
rapid changes to localized portions of an aquifer,
such as pumping or injection wells.

Golder Associates

March, 1982

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786085