# 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

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: |

### 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

1 2 3 4 5 6 7 8 9 10 11 12 13 | ```
# 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
``` |