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Wilcox Plot


A Wilcox plot can be used to quickly determine the viability of water for irrigation purposes (Wilcox, 1955). The Wilcox plot is also known as the U.S. Department of Agriculture diagram (Richards, 1954). An example of the Wilcox plot is shown in the figure below.



The Wilcox plot is a semi-log scatter plot of the "sodium hazard" (sodium adsorption ration [SAR]) on the Y-axis versus the "salinity hazard" (electrical conductivity) on the X-axis. The conductivity is plotted by default in a log scale. The Wilcox plot contains the following zones:

Salinity Hazard Zones: based on electrical conductivity (us/cm)

C1: Low (0-249)

C2: Medium (250-749)

C3: High (750-2249)

C4: Very High (2250-5000)


Sodium Hazard Zones: based on Sodium Adsorption Ratio lines:

S1: Low

S2: Medium

S3: High

S4: Very High


The locations of the SAR lines are determined by the following equations:

S1/2: Line equation: y= -1.5816E-3 x + 10.15816

S2/3: Line equation: y= -2.2959E-3 x + 18.22959

S3/4: Line equation: y= -3.0102E-3 x + 26.30102


Source Data Tab

Data are specified in a Wilcox plot as follows:



Wilcox plots have a total of two required parameters:

X-Axis: typically the Electrical Conductivity parameter

Y-Axis: typically the calculated Sodium Adsorption Ratio (SAR)


Note: the Sodium Hazard lines are added as custom lines and the default values of the slope and intercept of each line may be customized as desired.



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