Self-starting Nls Power Models for Soil Water Retention

Description

Function to self start the following nonlinear power models for soil water retention:

θ = \exp(a + b*Bd) ψ^c

(Silva et al., 1994)

θ = a ψ^c

(Ross et al., 1991)

where θ is the soil water content.

fitsoilwater() creates initial estimates (by log-linearization) of the parameters a, b and c and uses them to provide its least-squares estimates through nls.

Usage

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fitsoilwater4(theta, psi, Bd, model = c("Silva", "Ross"))

Arguments

theta

a numeric vector containing values of soil water content.

psi

a numeric vector containing values of water potential (Psi).

Bd

a numeric vector containing values of dry bulk density.

model

a character; the model to be used for calculating the soil water content. It must be one of the two: "Silva" (default) or "Ross".

Value

A "nls" object containing the fitted model.

Author(s)

Anderson Rodrigo da Silva <anderson.agro@hotmail.com>

References

Ross et al. (1991). Equation for extending water-retention curves to dryness. Soil Science Society of America Journal, 55:923-927.

Silva et al. (1994). Characterization of the least limiting water range of soils. Soil Science Society of America Journal, 58:1775-1781.

See Also

fitsoilwater4, soilwater, soilwater2, soilwater3

Examples

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# Example 1
ex1 <- with(skp1994,
    fitsoilwater4(theta = W, psi = h, model = "Ross"))
ex1
summary(ex1)

# Example 2
ex2 <- with(skp1994,
    fitsoilwater4(theta = W, psi = h, Bd = BD, model = "Silva"))
ex2
summary(ex2)

# Not run

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