To calibrate a model, its parameters must be adjusted so that its outputs match field measurements as closely as possible. If done manually, this can be tedious in the extreme, particularly if the model has more than a few parameters.
If you calibrate a model by hand, you will never know:
PEST calibrates a model automatically.
You, the user, inform PEST of numbers on model input files that it must adjust. You also inform PEST of numbers on model output files that must be matched to field values; you then provide these values. Then you let PEST run the model as many times as it needs to, adjusting parameters all the while, until the fit between model outputs and their field-measured counterparts is reduced to a minimum in the weighted least squares sense.
Before every model run, PEST writes a set of model input files which contain parameter values which PEST would like the model to use on that run. After the model has run to completion, PEST reads from the model output files those numbers which must be matched to field measurements. PEST runs the model through a system call.
What does this mean? Well to start with:
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