# fitsoilwater4: Self-starting Nls Power Models for Soil Water Retention In soilphysics: Soil Physical Analysis

## 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

 `1` ```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.

`fitsoilwater4`, `soilwater`, `soilwater2`, `soilwater3`
 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14``` ```data(skp1994) # 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 ```