User's guide of Proceessing Modflow 5.0

Processing Modflow 141
define the extent of an estimated paramter by assigning a unique parameter number to the
cells of interest. That unique parameter number corresponds to the Number here.
- Active: The value of an estimated paramter will only be adjusted if Active is checked.
Otherwise the user-specified cell value will be used for the simulation. Normally, the
total number of active parameters should not exceed 10, although PMWIN allows 150
parameters.
- Description: A text describing the estimated parameter can be entered here (optional),
for example, “Transmissivity in layer 3”. A maximum of 120 characters is allowed.
- PARVAL1 is a parameter's initial value. For a fixed parameter, this value remains
invariant during the optimization process. For a tied parameter (see PARTRANS below),
the ratio of PARVAL1 to the parent parameter's PARVAL1 sets the ratio between these
two parameters to be maintained throughout the optimization process. For an adjustable
parameter, PARVAL1 is the parameter's starting value which, together with the starting
values of all other adjustable parameters, it is successively improved during the
optimization process. To enhance optimization efficiency, you should choose an initial
parameter value which is close to what you think will be the parameter's optimized value.
You should note the following repercussions of choosing an initial parameter value of
zero.
1. A parameter cannot be subject to change limits (see the discussion on RELPARMAX
and FACPARMAX) during the first optimization iteration if its value at the start of
that iteration is zero. Furthermore FACORIG cannot be used to modify the action of
RELPARMAX and FACPARMAX for a particular parameter throughout the
optimization process, if that parameter's original value is zero.
2. A relative increment for derivatives calculation cannot be evaluated during the first
iteration for a parameter whose initial value is zero. If the parameter belongs to a
group for which derivatives are, in fact, calculated as Relative (see INCTYP and
DERINC below), a non-zero DERINCLB variable must be provided for that group.
3. If a parameter has an initial value of zero, the parameter can be neither a tied nor a
parent parameter as the tied/parent parameter ratio cannot be calculated.
- PARLBND and PARUBND are a parameter's lower and upper bounds respectively. For
adjustable parameters the initial parameter value (PARVAL1) must lie between these two
bounds. For fixed and tied parameters, PARLBND and PARUBND are ignored.
- PARTRANS controls the parameter transformation. By clicking on a cell of the
PARTRANS column, this flag can be set as None, Log-transformed, Tied or Fixed.
Use Log-transformed, if you wish that a parameter be log-transformed throughout the
estimation process. A parameter which can become zero or negative in the course of the
3.6.5 PEST (Inverse Modeling)