PEST DESCRIPTION
PEST98 - Model-Independent Parameter Estimation
New Features in PEST98What's New About PEST98?
PEST98 is the latest version of PEST. It includes Parallel PEST and SENSAN as well as several utilities for automatic generation and checking of PEST input files.
New Features In PEST98
PEST is revolutionizing model calibration. In hydrology, engineering, geophysics, biology, economics and many other fields, PEST is changing the way people work with models. Nonlinear parameter estimation has never been this easy or this robust. For the first time, PEST places this powerful mathematical technique into the hands of all modelers.
Matching model-generated data to field data becomes easy with PEST.
PEST is a nonlinear parameter estimation package with a difference! The difference is that PEST can be used to estimate parameters for just about any existing model whether or not you have the model's source code. PEST is able to "take control" of a model running it as many times as it needs while adjusting its parameters until the discrepancies between selected model outputs and a complementary set of field or laboratory measurements is reduced to a minimum in the weighted least-squares sense.
Most parameter estimation packages have two serious drawbacks that inhibit their ability to optimize parameters for the plethora of computer simulation models that are used today in all fields of study. The first of these difficulties is that a model normally needs to be partially recoded in order to communicate with an estimation program. This usually involves recasting the model as a subroutine which is then called by the estimator each time it needs to run the model. The second disadvantage is that the performance of many commercial and public-domain estimators is seriously degraded when optimizing parameters for large numerical models or for the sometimes complex models used for simulating "messy" environmental processes.
PEST overcomes the first of these difficulties by communicating with a model through the model's own input and output files. Thus PEST adapts to the model; the model does not need to be adapted to PEST. It overcomes the second problem by implementing a particularly robust variant of the Gauss-Marquardt-Levenberg method of nonlinear parameter estimation. Furthermore, through adjustment of a number of control variables, a user is able to "tune" PEST's implementation of the method to suit the model for which parameters are sought.
Complexity is no hindrance to PEST.
Because PEST is model-independent, the "model" can, in fact, be a series of models which PEST runs in succession through a batch file. PEST can estimate parameters for one or all of the models simultaneously. Thus a first model can provide input data for a second model; a single model can be calibrated against a number of different historical datasets at one time; a preprocessor can be run, followed by the model, followed by a postprocessor - the possibilities are endless. The only requirements for the "model" are that it can be run from the command line and that it reads and writes ASCII files.
If a computer model is being used to understand or interpret data pertaining to a natural or man-made system, the chances are that the model's performance will be significantly enhanced through the use of PEST to parameterize that model. PEST has been used successfully in most scientific fields including groundwater and surface-water hydrology, geophysics, geomechanics, chemical, aeronautical and mechanical engineering, biology, and soil science, among others.
SENSAN for Model-Independent Sensitivity Analysis
The latest version of PC PEST includes SENSAN, a model-independent sensitivity analyzer. Like PEST, SENSAN communicates with a model through the model's own input and output files. Thus SENSAN can be used with a model without the user having to make any alterations to the model.
Prior to running SENSAN, a modeler simply identifies the adjustable parameters on model input file(s) and model-generated numbers on model output file(s) that are of particular interest. The user then provides SENSAN with different sets of parameter values, there being no upper limit to the number of sets thus provided. SENSAN runs the model for each set, recording parameter values and model outputs in spreadsheet format for easy later analysis. Relative model output variations as well as sensitivities of model outputs to parameter changes are also recorded.
Parallel PEST for Calibrating Large and Complex Models
Parallel PEST distributes model runs across networked PCs. Where model run times are large and adjustable parameters are many, the saving in overall PEST optimization time through the use of Parallel PEST is enormous. Thus Parallel PEST can be used in the calibration of large and complex models where application of nonlinear parameter estimation techniques would have previously been considered impossible.
Implementation of the Gauss-Marquardt-Levenberg parameter estimation algorithm used by PEST requires that derivatives of model outcomes be calculated with respect to adjustable parameters. By carrying out these model runs simultaneously, each optimization iteration can be completed in a fraction of the time that would have been required had all model runs been carried out in succession on a single machine.
By undertaking simultaneous model runs on different machines on a PC network, enormous savings can be made in PEST optimization time.
Other PEST Features
PEST Utilities for MODFLOW and MT3D Calibration
The PEST MODFLOW/MT3D Utilities are a set of programs which automate construction of PEST interface files for use with MODFLOW and MT3D. Also provided are programs which interpolate MODFLOW and MT3D unformatted output files to borehole locations for direct comparison with field measurements.
The PEST MODFLOW/MT3D Utilities can be used with any version of MODFLOW or MT3D supplied by any vendor. They can never become outdated as MODFLOW and MT3D continue to be upgraded because PEST operation is independent of the model it calibrates.
The Utilities are designed to facilitate calibration of MODFLOW against borehole-measured heads and/or drawdowns, and MT3D against borehole-measured concentrations. However, because of the open-ended nature of the PEST-model interface, this can be extended according to the demands of any particular modeling problem.
Adjustable parameters can include just about anything found on a MODFLOW or MT3D input file including tabulated data such as drain and riverbed conductance as well as array-based data such as transmissivity, recharge and dispersivity. Using PEST to calibrate MODFLOW and MT3D has the added advantage that once you own PEST, you can use it to calibrate your other models as well.
Note: Use of the interpolation utility assumes versions of MODFLOW and MT3D are compiled with a Lahey compiler (as are most commercially-supplied versions of these programs). However, the source code of the interpolator is provided for those who would like to recompile these programs themselves using another compiler.
PEST Utilities for GMS
The PEST GMS Utilities are a set of programs which facilitate the use of PEST in the GMS environment. They are designed to enhance the model calibration functionality already built into GMS by automating construction of PEST input files based on the contents of observation coverages accessed through the GMS Map module (versions 2.1 and later).
At present, the PEST GMS utilities consist of four programs. These are:
PEST PMODFLOW
PEST PMODFLOW is a modification of the version of MODFLOW supplied with GMS. It retains all of the functionality of that version of MODFLOW. However it is also able to read a GMS map file. It searches for the presence of any observation coverages present in a map file and then spatially and temporally interpolates MODFLOW results to the locations and times of measurements cited in those observation coverages.
PEST PSETUP
PEST PSETUP also reads a GMS map file containing one or more observation coverages. It automatically generates two PEST input files via a PEST control file and a PEST instruction file. The control file cites all measurements found in the map file and provides them with weights calculated in accordance with uncertainty levels as previously provided to GMS. The instruction file reads a PMODFLOW output file in which the MODFLOW-generated equivalents to the field measurements are listed.
PEST TEXTREP
PEST TEXTREP assists in the construction of PEST template files from model input files such as MODFLOW input files that can consist of thousands of numbers arranged in arrays.
PEST PARREP
A program that replaces parameter values found in a PEST control file with those found in a PEST parameter value file. Use of PEST PARREP makes it easy to start one PEST run where another left off.
The PEST-GMS Utilities are free! Please contact us if you want to download a copy.