Groundwater Borehole and Soil Stratigraphy

GMS - groundwater borehole and soil stratigraphy software, model calibration

Calibration

Model calibration involves changing input parameters until the model results match field observations. GMS provides a suite of options for model calibration including calibration targets and plots of calibration statistics. Both point and flux observations are supported. GMS also supports automated parameter estimation using PEST and UCODE.

Calibration Targets

For both point and flux observations, GMS provides an option for plotting a calibration target at the location of the observation. The calibration target displays the relationship of the computed value to the measured value in a simple, intuitive manner.

Flux Calibration

The quality of a calibration exercise is greatly enhanced when flux observations are combined with point observations. GMS provides fully automated flux calibration tools. You simply read in the solution and select an object and GMS plots the computed flux through the object. There is no need for an external flow budgeting utility such as ZoneBudget.

PEST/UCODE

Two utilities are provided in GMS for automated parameter estimation with MODFLOW: PEST and UCODE. With automated parameter estimation, either PEST or UCODE is used to iteratively adjust selected parameters and repeatedly launch MODFLOW until the computed output matches field-observed values. Parameter estimation can be used in conjunction with the point observations, flux observations, and calibration display options provided in GMS. Setting up PEST or UCODE to calibrate a MODFLOW model in GMS is extremely simple.

Model Calibration

Calibration is the process of modifying the input parameters to a groundwater model until the output from the model matches an observed set of data. GMS includes a suite of tools to assist in the process of calibrating a groundwater model. Both point and flux observations are supported. When a computed solution is imported into GMS, the point and flux residual errors are plotted on a set of calibration targets and a variety of plots can be generated showing overall calibration statistics. Most of the calibration tools can be used with any of the models in GMS. Automated parameter estimation is supported for MODFLOW models via PEST and UCODE.

Two types of observations can be defined in GMS: point observations and flux observations. Both types of observations are defined in the map module and are associated with points, arcs, and polygons. Point observations represent locations in the field where some value has been observed. In most cases, the points will correspond to observation wells and the value will be the elevation of the groundwater table (the head). However, the calibration tools are designed in a general fashion and the observed value can be anything (concentration, temperature, etc.). Flux observations represent linear or areal objects such as streams and reservoirs where the gain or loss between the aquifer and the object has been measured or estimated. Both point and flux observations can be assigned a confidence interval or calibration target. While point observations can be used with any model, flux observations can only be used with MODFLOW and FEMWATER.

Once a set of observed point and flux values have been entered, each time a model solution is imported, GMS automatically interpolates the computed solution to the observation points and sums the computed flux over each flux object in the conceptual model. A calibration target representing the magnitude of the residual error is displayed next to each observation point and flux object. The size of the target is based on the confidence interval or the standard deviation. In addition to the calibration targets next to the observation points, you can choose to display any of a number of statistical plots.

Calibration Targets and Plots of Calibration Statistics

Another tool is profile arcs. A profile arc is used to plot the variation in a data set vs. distance along a user-defined arc. Profile arcs can only be used with 2D grids or 2D meshes. While observed values are not assigned to a profile arc, they are useful in analyzing 2D data sets.

Borehole Data

The borehole, TIN, and solid modules in GMS can be used to construct 3D solid models of soil stratigraphy. Two types of borehole data are supported: sample data and stratigraphy data. The following figure illustrates sample data from a cone penetrometer test.

Boreholes can also contain stratigraphy data. Stratigraphy data are defined by depths to contacts between adjacent stratigraphic units.

Triangulated Irregular Networks (TINs)

TINs are used to model surfaces, including the interfaces between adjacent stratigraphic units. TINs can be constructed directly from borehole stratigraphy data.

Solids

TINs can be extruded and the resulting solids can be modified with set operations to create 3D solid models of complex soil stratigraphy.

Cross Sections

Cross sections and fence diagrams can be cut through solids to illustrate complex stratigraphic relationships.

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