User's guide of Proceessing Modflow 5.0

144 Processing Modflow
If a group contains members which are fixed and/or tied you should note that the
values of these parameters are taken into account when calculating parameter increments
using the Rel_to_max option. For the Relative and Rel_to_max options, a DERINC
value of 0.01 is often appropriate. However no suggestion for an appropriate DERINC
value can be provided for the Absolute increment option; the most appropriate increment
will depend on the parameter magnitudes.
- DERINCLB is the absolute lower limit of parameter increments for all group members.
If a parameter increment is calculated as Relative or Rel_to_max, it may become too low
if the parameter becomes very small or, in the case of the Rel_to_max option, if the
magnitude of the largest parameter in the group becomes very small. A parameter
increment becomes "too low" if it does not allow reliable derivatives to be calculated
with respect to that parameter because of roundoff errors incurred in the subtraction of
nearly equal model-generated observation values. DERINCLB is used to bypass this
possibility.
If you do not wish to place a lower limit on parameter increments in this fashion, you
should set DERINCLB to zero. Note that if INCTYP is Absolute, DERINCLB is
ignored.
- FORCEN can be Always_2, Always_3 or Switch. It determines how to calculate
derivatives for group members. If FORCEN is Always_2, derivatives for all parameters
belonging to that group will always be calculated using the forward difference method;
If FORCEN is Always_3, PEST will use central difference method to calculate the
devivatives. In this case, twice as many model runs as there are parameters within the
group will be required; however the derivatives will be calculated with greater accuracy
and this will probably have a beneficial effect on the performance of PEST. If FORCEN
is set to Switch, derivatives calculations for all adjustable group members will begin
using the forward difference method, switching to the central method for the remainder
of the estimation process after the relative objective function reduction between
successive iterations is less than PHIREDSWH as defined in the Control Data (see
below).
Experience has shown that in most instances the most appropriate value for FORCEN
is Switch. This allows speed to take precedence over accuracy in the early stages of the
optimization process when accuracy is not critical to objective function improvement,
and accuracy to take precedence over speed later in the process when realization of a
(normally smaller) objective function improvement requires that derivatives be calculated
with as much accuracy as possible, especially if parameters are highly correlated and the
normal matrix thus approaches singularity.
3.6.5 PEST (Inverse Modeling)