HydroMe (version 2.1.1)

SSvgm4: A four-parameter van Genuchten water retention model

Description

This is a four-parameter function to determine water retention hydraulic parameters contained in the van Genuchten water retention model

Usage

SSvgm4(input, Thr, Ths, alp, nscal)

Value

A list of objects returned by standard R regression functions such as nls and lm

Arguments

input

Suction potential/head as contained in the x-column of the xy water retention table or data

Thr

This is the residual moisture content. It's the moisture content when suction potential is very high (almost at the drying point)

Ths

This is the saturated moisture content. It's the moisture content when suction potential is very low (almost at the saturation point)

alp

It's natural logarithm of the inverse of air-entry potential or bubbling pressure

nscal

This is a parameter or index for the pore-size distribution

Author

Christian Thine Omuto

Details

Van Genuchten model is a type of self-starting nonlinear curve fitting model for fitting water retention characteristics using experimental data. In this model, the restriction of m=1-1/n has been imposed on the water retention characteristics. Air-entry potential = exp(alp). If warnings are given during the estimation process, try to increase the iteration limit using control function. Sometimes the warnings can be suppressed using (warn=-1)/ignored since they do not terminate the process

References

van Genuchten MT. 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Science Society of America Journal 44: 892-898

See Also

SSomuto, Brook, Campbel

Examples

data(isric)
library(nlme)
library(minpack.lm)
pfdata=subset(isric,Country=="Benin")
suppressWarnings(
vn4.ns <- nlsList(y ~ SSvgm4(x,thr,ths,alp,nscal)|Sample,
               data = pfdata,
               control = nls.lm.control(maxiter=200)))
coef(vn4.ns)
plot(fitted(vn4.ns)~pfdata$y)
cor(fitted(vn4.ns),pfdata$y)^2