User's guide of Proceessing Modflow 5.0

148 Processing Modflow
If a parameter b is relative-limited, The relative change between optimization
iterations i-1 and i is defined as
(b i&1 & b i )
b i&1
b i&1 /b i if |b i&1 | > |b i |, or
b i /b i&1
3.6.5 PEST (Inverse Modeling)
if |b i | > |b i&1 |
(3.55)
The absolute value of this relative change must be less than RELPARMAX. If a
parameter upgrade vector is calculated such that the relative adjustment for one or more
relative-limited parameters is greater than RELPARMAX, the magnitude of the upgrade
vector is reduced such that this no longer occurs.
If parameter b is factor-limited, the factor change between optimization iterations i-1
and i is defined as
(3.56)
This factor change must be less than FACPARMAX. If a parameter upgrade vector is
calculated such that the factor adjustment for one or more factor-limited parameters is
greater than FACPARMAX, the magnitude of the upgrade vector is reduced such that
this no longer occurs. It is important to note that a factor-limit will not allow a parameter
to change sign. If a parameter must be free to change sign during an optimization
process, it must be relative-limited; furthermore RELPARMAX must be set at greater
than unity or the change of sign will be impossible. Similarly, if a parameter's upper or
lower limit is zero, it cannot be factor-limited and RELPARMAX must be at least unity.
Suitable values for RELPARMAX and FACPARMAX can vary enormously from
case to case. If you are unsure of how to set these parameters, a value of 5 for each of
them is often suitable. For highly non-linear problems, these values are best set low. If
they are set too low, however, the estimation process can be very slow. An inspection of
the PEST run record (by pressing the ESC key) will often show whether you have set
these values too low, for PEST records the maximum parameter factor and relative
changes are recorded on this file at the end of each optimization iteration. If these
changes are always at their upper limits and the estimation process is showing no signs
of instability, it is quite possible that RELPARMAX and/or FACPARMAX are to low
and could be increased.
Note that FACPARMAX can never be less than 1; RELPARMAX can be less than
1 as long as no parameter's upper and lower bounds are of opposite sign. (If necessary,
use OFFSET to shift the parameter domain so that it does not include zero.)
- FACORIG is a criterion for modifying RELPARMAX and FACPARMAX. If, in the