GMS:MODFLOW-USG Observations

MODFLOW-USG does not support an observation process as does MODFLOW 2000, 2005, NWT, ect. When using MODFLOW-USG in GMS a series of PEST utilities are run to calculate the model computed values at observations. The inputs to these utilities are managed in the MODFLOW-USG Observation dialog. This dialog contains information for Head observations as well as flow observations. The head and flow observations can be steady state or transient depending on the type of MODFLOW-USG simulation.

Generating Observation Data

Observation data is generated from coverages in the conceptual model. The generation of MODFLOW-USG observation data may occur manually by selecting the Generate PEST Obs. Data button or when using the Map->MODFLOW command.

Generate PEST Obs. Data Dialog

The Generate PEST Obs. Data dialog allows the user to select the coverages and settings used to generate observation data. The top of the dialog contains a list of coverages that have MODFLOW Head observations. Any selected coverages will be used to generate the observation data. If the MODFLOW model is steady state then only coverages with the "Head" observation property will be used (as opposed to the "Transient Head" property). Below the list of Head observation coverages the user may specify the interpolation option to be used in calculating the model computed value at the observation points. GMS 10.1 supports IDW interpolation where the user may specify the number of nearest grid cells to use.

NOTE: Determining MODFLOW Model Layer for Observation Points

GMS allows multiple methods for assigning observation point to MODFLOW models including: specifying the layer, using the elevation or Z of the point, or using a well screen. This option is specified in the Coverage Setup dialog under the "3D grid layer option for obs. pts." If the user chooses the well screen option then the middle of the well screen is used to find the layer of the UGrid; the nearest points in that layer are used to interpolate to the observation point.

Map->MODFLOW

When executing the Map->MODFLOW command GMS will generate observation data for any coverage that has Head Observations and any coverage that has Flow Observations.