SSvan {HydroMe}R Documentation

A selfstarting function for estimating hydraulic parameters in the van Genuchten model

Description

This selfstarting function estimates the water retention parameters contained in the van Genuchtens function. It has an initial attribute that creates initial estimates of the parameters Thr, Ths, alp, and scal

Usage

SSvan(input, Thr, Ths, alp, scal)

Arguments

input A numeric vector of natural logarithm of the suction potentials
Thr A numeric parameter representing the residual moisture content
Ths A numeric parameter representing the saturated moisture content
alp A numeric parameter representing the inverse of air-entry potential
scal A numeric parameter representing the pore-size distribution index

Details

The dataset should contain a column for logarithm of suction potential, taking care to place zero for log (x) whenever x is zero. The function estimates the parameters in two parts: when the product of alp and input is less than or eaual to unity and when this quantity is geater than unity

Value

a numeric vector of the same length as input. It is the value of the expression Thr+(Ths-Thr)/(1+(input*exp(alp))^scal)^(1-(1/scal)) . It represents the fitted estimates of moisture retained using the van Genuchtens model

Note

This model relates to a case of van Genuchtens model where m is 1 less than the inverse of n. The parameter for inverse of air-entry potential has been transformed using natural logarithm. Thus, the coefficient for this parameter needs to be transformed back using exponential function. It is also important to note that some datasets do perform poorly with this function

Author(s)

Omuto, Christian Thine

References

Van Genuchten, M.T., 1980 A closed-form Equation for predicting the hydraulic conductivity of unsaturated soils Soil Science Society of America Journal 44, 892-898

See Also

SSgard, SSbrook, SScamp

Examples

data(isric)
van.nls <-nls(y~SSvan(x, Thr, Ths, alp, scal), isric)
coef(van.nls)## Obtain the parameter estimates
##       Thr        Ths        alp       scal 
## 0.1747737  0.5010839 -2.9837774  1.3029102 


[Package HydroMe version 1.0 Index]