Setting up PEST
interaction

Once you have got your model
behaving in a generally believable way, you can use PEST to calibrate your
model parameters in order to match the model's output to a set of measured
data.

Details of the operation of PEST can
be found in its manual, a copy of
which has been placed on Simulistics' website for ease of reference. The
manual is included in the pre-compiled Windows distribution. Only the
basics of PEST operation will be covered here.

The PEST interface is included in Simile as a helper. It is located in the 'Standard tools'
submenu of the helpers menu. When you select it, it will create a tabbed
notebook with four pages inside its top-level window. These are labelled
Inputs, Outputs, Actions and Settings. If using the single-window Model Run Environment, be careful not to confuse these
notebook tabs with those containing other helper pages.

Inputs

On this page you specify which model parameters will be estimated by
PEST. You can add a single parameter by hitting the '+' button at the top,
then clicking on the parameter in the model diagram or explorer tool. The
'-' button allows you to remove a parameter. The last button adds all
suitable parameters that have not already been added. Note that PEST can
only optimize real (floating-point) value parameters.

A minimum and maximum value must be provided for each parameter. For
fixed parameters you can type these into entry boxes. They must reflect the
range of values which are plausible. For variable parameters, PEST uses the
values entered in the parameter's properties before the model was
built.

For variable parameters you can decide whether PEST simply picks a fixed
value for them or generates a series of values. There is a checkbox and an
entry field for your choice. A series will be spaced regularly over the
length of the model run, which is specified in the Actions page. Note that
the total number of values being estimated (including all members of each
array and each series) must not exceed the number of data measurements
being used, or PEST will not have enough information to estimate them
all.

PEST also needs an initial estimate for each parameter value. This
defaults to midway between the parameter's minimum and maximum value, but
you can enter a different estimate. If you select the 'Use current data as
estimates' checkbutton at the bottom of the Inputs page, PEST will use the
current values at the start of the optimization for its initial
estimates.

If an array parameter is used, PEST will use the same specification data
for each array member, but will create a separate estimate for each
member.

Outputs

On this page you specify which model values are to be looked at by PEST
and compared with measured values. Any value in the model can be used,
except values from inside variable-membership submodels. You can add a
value by hitting the '+' button at the top, then clicking on the value in
the model diagram or explorer tool. The '-' button allows you to remove a
value.

For each value you also need to load measurement data for comparison.
PEST displays an input tool similar to the one in the 'File Parameters' dialogue, and data can be
entered directly or loaded from files exactly as described for this
dialogue. The only difference is that when loading measurement data for
PEST, you choose (via a checkbox) whether it is a single value (for each
instance) which will be compared at the end of the run, or whether it is a
time series to be compared with the model value at times throughout the
run.

When you have loaded your measurement data you can save references to it
in a scenario file, as you would for parameter data.

In: Contents >> Running
models >> PEST