The “MODFLOW – Model Calibration” tutorial—which should be completed prior tobeginning this tutorial—described the basic calibration tools provided in GMS. Itillustrated how head levels from observation wells and observed flows from streamscould be entered into GMS and how these data could be compared to model computedvalues. It also described how a trial-and-error method could be used to iteratively adjustmodel parameters until the model computed values matched the field-observed values toan acceptable level of agreement.In many cases, calibration can be achieved much more rapidly with an inverse model. Aninverse model is a utility that automates the parameter estimation process. The inversemodel systematically adjusts a user-defined set of input parameters until the differencebetween the computed and observed values is minimized.GMS contains an interface to an inverse model called PEST. This tutorial illustrates howto calibrate a MODFLOW model using PEST. The model in this tutorial is the samemodel featured in the “MODFLOW – Model Calibration” tutorial. The model includesobserved flow data for the stream and observed heads at a set of scattered observationwells. The conceptual model for the site consists of a set of recharge and hydraulicconductivity zones. These zones will be marked as parameters and an inverse model willbe used to find a set of recharge and hydraulic conductivity values that minimize thecalibration error.This tutorial discusses and demonstrates opening a MODFLOW model and solution,defining conditions, running PEST, and loading optimal parameter values.