# HydroGeoSphere/Modifying Zoned Properties

There are a number of instructions which can be used to modify the property values associated with a zone or group of zones. Before these instructions are issued, it is necessary to select the appropriate type of media and then choose the zones which you want to modify.

## Modifying zone property values

For example, suppose that you wished to assign a new porous media hydraulic conductivity to all zones, and thus to all elements in the problem. The following set of instructions, inserted in the prefix.grok file would accomplish this:
In this case, we are applying the instruction k isotropic to zones of type porous media, although it is equally valid to use it with dual-type zones. However, if you try to use this instruction with zones of type fracture or surface, warning messages will be issued to the screen and prefixo.eco file and execution will halt.
The instructions which are valid in specific situations are discussed in the relevant sections of the manual. For example, instructions which can be used for defining saturated flow properties are described in Section 5.8.2.
Another way to define zone properties is through the use of a material properties file, which should be located in the same directory as the prefix.grok file. This file contains lists of media-specific instructions which can be used to define properties for one or more named materials. These material properties can then be assigned to the current set of chosen zones. To assign new properties through the use of a material properties file, we first need to issue the following instruction:

## Properties file

* props_file_name Name of the material properties file.
This file will be searched for materials given as input to the Read properties instruction discussed below.
The Properties file instruction has two benefits: first, it allows you to create sets of material properties and give them meaningful file names and second, it allows you to easily switch between material property sets merely by changing the file name given in the prefix.grok file.
Any line in a material properties file which is completely blank or which begins with an exclamation point (!) is treated as a comment and is ignored by grok. This allows you to include comments whenever required.
Each distinct material in the file is identified by a unique label and may contain instructions which are to be applied to the current zone type. For example, instructions which can be used for defining porous media properties when simulating saturated flow (as described in Section 5.8.2.1) may be included. Figure 5.9 shows an example of a material defined for the verification problem discussed in Section 4.5.1:
    Figure 5.9: Example of a Material Defined in an .mprops File
To make use of the material properties file, you would issue the following instruction:

## Read properties

* mat_name Name of the material.
Chosen zones are assigned properties of the material named mat_name in the current material properties file as defined in a properties file instruction.
So for example, the following set of instructions could be inserted in the prefix.grok file:
The instruction read Properties would, in this case, search the porous media material properties file my.mprops for a material named sand. If found, it would read the instructions defining the material and modify the porous media properties for the current set of chosen zones.

## Example output

A summary of the final data which has been defined for each zone is listed in the prefixo.eco file, and an example is shown in Figure 5.10:
    Figure 5.10: Example Output for a Porous Media Material
In this example, because flow is saturated, no variably-saturated porous media flow properties need to be defined in the material properties file. Also, default values for properties (e.g. bulk density, immobile zone porosity etc.) which have not been modified in the prefix.grok or material properties file are used.