Van Genuchten–Gupta model

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The Van Genuchten–Gupta model is an inverted S-curve applicable to crop yield and soil salinity relations.^[1] It is named after Martinus Theodore van Genuchten and Satyandra K. Gupta's work from the 1990s.

Equation

The mathematical expression is:

${\displaystyle Y={\frac {Y_{\rm {m}}}{1+(C/C_{50})^{P}}}}$

where Y is the yield, Y_m is the maximum yield of the model, C is salt concentration of the soil, C₅₀ is the C value at 50% yield, and P is an exponent to be found by

As an alternative, the logistic S-function can be used.

The mathematical expression is:

${\displaystyle Y^{\wedge }={\frac {1}{1+\exp(AX^{C}+B)}}}$

where:

${\displaystyle Y^{\wedge }={\frac {Y-Y_{\rm {n}}}{Y_{\rm {m}}-Y_{\rm {n}}}}}$

with Y being the yield, Y_n the minimum Y, Y_m the maximum Y, X the salt concentration of the soil, while A, B and C are constants to be determined by

The third degree or cubic regression also offers a useful alternative.

The equation reads:

${\displaystyle Y=AX^{3}+BX^{2}+CX+D}$

with Y the yield, X the salt concentration of the soil, while A, B, C and D are constants to be determined by the regression.

In the figure: A = 0.0017, B = 0.0604, C=0.3874, D = 2.3788. These values were calculated with Microsoft Excel

The curvature is more pronounced than in the other models.

See also

• Maas–Hoffman model

References

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