Create a Wilcox Diagram in Grapher

Wilcox diagrams are used to assess the potential hazards of salinity and sodium accumulation in soils from irrigation water.  While Grapher does not directly support the creation of Wilcox diagrams, they can be created quickly using the template and instructions below.

If you use Wilcox diagrams in your work and would like to see this plot type fully supported in Grapher, please let us know.

What is a Wilcox diagram?

A Wilcox diagram is a graphical tool used in hydrochemistry to classify water quality, particularly for irrigation purposes, by plotting sodium percentage against electrical conductivity.  A Wilcox diagram is composed of the following:

• X Axis: Electrical conductivity (EC) measures the total dissolved salts or salinity of the water.  The higher the EC, the higher the salinity, and the higher the risk to crops.
• Y Axis: Sodium percentage (%Na) is the relative proportion of sodium ions compared to other major cations (calcium, potassium, and magnesium).  Higher sodium is linked to reduced soil permeability and poor soil structure which is a risk to crops.
• Hazard zones: The risk posed by different levels of salinity and sodium are labeled and delineated on the graph.  From low the high risk the zones are labeled S1-S4 for sodium and C1-C4 for conductivity.

Create a Wilcox Diagram in Grapher

The Wilcox diagram templates provided below are not true templates because they require some additional data in order to produce the hazard zone lines and classifications. 

Before beginning, ensure your data is formatted in three columns with a header row where Column 1 is the Conductivity data, Column 2 is the SAR data, and Column 3 is the Class data. If you have no Class data, copy one of the other two columns into Column 3. See the attached data.txt for an example data set, if needed.

To use these files with your data, follow the instructions below.

1. Download the attached WilcoxDiagram.grf or and Axes.dat files. Make sure to save the files to the same directory.
2. Click File | Open, select the GRF file, and click Open.
3. Click Yes in the Grapher dialog warning you Unable to open worksheet file.
4. In the Open Worksheet Linked to ... dialog:
   1. Select your data file.
   2. Check the box next to Use this worksheet for remaining items at the bottom of the dialog.
   3. Click Open and the plot is created. 

Once your data is loaded into the file you can customize the look and feel. Some customization options include:

• Turn on the category labels on the X axis by checking the box in the Object Manager next to C1, C2, C3, C4 Labels (Bottom)
• Turn on the category labels on the X axis by checking the box in the Object Manager next to S1, S2, S3, S4 Labels (Right)
• Turn off the colored category bars by unchecking the box next to Horizontal Categories and Vertical Categories in the Object Manager.
• Change the Symbol Shape and Color for a Class Plot in Grapher

An alternative version of the Wilcox diagram can be created using the attached WilcoxDiagram_2.grf and WilcoxData.xlsx.  This version of the graph moves the hazard classifications into a separate legend style display.

Additional Hydrochemistry Resources

• Create a Piper (Trilinear) diagram in Grapher
• Create a Durov Diagram in Grapher
• Create a Stiff Diagram in Grapher
• Create a Schoeller Diagram in Grapher