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. 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. This indicates a component whose value at the end of each model run is to be compared with measured data. You can also add a value by hitting the graph icon button. This indicates a component whose value is to be sampled at a series of time points during a model run, and each sample compared with measured data. The '-' button allows you to remove a value of either type.

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.

When you have loaded your measurement data you can save references to it in a scenario file, as you would for parameter data. This scenario file will have the extension .smf rather than .spf to indicate that it refers to measurement data rather than parameter data. In other respects the two file formats are interchangeable.

Restrictions

Simile is a much mode expressive modelling tool than the typical command-line-driven models with which PEST is intended to work, so there are some limits on what Simile model constructs can be used when estimating parameters with PEST. In particular,

• The interface does not support multidimensional data arrays, or any arrays that are supplied or sampled at different time points since this effectively involves a multidimensional array.
• Data values must be real, and array indices must be integer (i.e., not enumerated-type).

In: Contents >> Running models >> PEST