User's guide of Proceessing Modflow 5.0

142 Processing Modflow
parameter estimation process must not be log-transformed; hence if a parameter's lower
bound is zero or less, PEST will disallow logarithmic transformation for that parameter.
Note that by using an appropriate scale and offset, you can ensure that parameters never
become negative. Thus if you are estimating the value for a parameter whose domain, as
far as the model is concerned, is the interval [-9.99, 10], you can shift this domain to
[0.01, 20] for PEST by designating a scale of 1.0 and an offset of -10.0. Similarly if a
parameter's model domain is entirely negative, you can make this domain entirely
positive for PEST by supplying a scale of -1.0 and an offset of 0.0. See the discussion on
the SCALE and OFFSET variables below.
If a parameter is fixed, taking no part in the optimization process, PARTRANS must
be specified as Fixed. If a parameter is linked to another parameter, this is signified by
a PARTRANS value of Tied. In the latter case the parameter takes only a limited role
in the estimation process. However the parameter to which the tied parameter is linked
(this "parent" parameter must be neither fixed nor tied itself) takes an active part in the
parameter estimation process; the tied parameter simply "piggy-backs" on the parent
parameter, the value of the tied parameter maintaining at all times the same ratio to the
parent parameter as the ratio of their initial values. If a parameter is neither fixed nor tied,
and is not log-transformed, the parameter transformation variable PARTRANS must be
supplied as None.
- PARCHGLIM is used to designate whether an adjustable parameter is relative-limited
or factor-limited (See the discussion on RELPARMAX and FACPARMAX, p. 147). For
tied or fixed parameters PARCHGLIM has no significance.
- PARGP is the number of the group to which a parameter belongs. Paramter groups are
discussed in Group Definitions below.
- PARTIED is the number of the "parent" parameter to which a parameter is linked. See
also PARTRANS.
- SCALE and OFFSET: Just before a parameter value is written to an input file of
MODFLOW, it is multiplied by the real variable SCALE, after which the real variable
OFFSET is added. The use of these two variables allows you to redefine the domain of
a parameter. Because they operate on the parameter value "at the last moment" before it
is sent, they take no part in the estimation process; in fact they can "conceal" from PEST
the true value of a parameter as seen by the model, PEST optimizing, instead, the
parameter b where
p
bp ' (b m & o)/s
(3.51)
Here b p is the parameter optimized by PEST, b m
is the parameter seen by the model,
3.6.5 PEST (Inverse Modeling)