SSvan {HydroMe} | R Documentation |
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
SSvan(input, Thr, Ths, alp, scal)
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 |
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
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
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
Omuto, Christian Thine
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
SSgard, SSbrook, SScamp
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