soilphysics (version 2.1)

fitsoilwater5: Interactive Estimation of the Modified van Genuchten's Model Parameters

Description

An interactive graphical adjustment of the soil water retention curve via the van Genuchten's formula, modified by Pierson and Mulla (1989). The nonlinear least-squares estimates can be achieved taking the graphical initial values. It may be useful to estimate the parameters needed in the high-energy-moisture-characteristics (HEMC) method, which is used to analyze the aggregate stability.

Usage

fitsoilwater5(theta, x, theta_S, xlab = NULL, ylab = NULL, ...)

Arguments

theta

a numeric vector containing the values of soil water content.

x

a numeric vector containing the matric potential values.

theta_S

an offset; a value for the parameter theta_S, the water content at saturation. See details.

xlab

a label for the x axis; if is NULL, the label "Matric potential" is used.

ylab

a label for the y axis; if is NULL, the label "Soil water content" is used.

...

further graphical arguments; see par.

Value

• A plot of theta versus x and the curve of the current fitted model according to the adjusted parameters in an external interactive panel. Pressing the button "NLS estimates" a nls summary of the fitted model is printed on console whether convergence is achieved, otherwise a warning box of "No convergence" is shown.

Details

The parameter theta_S must be passed as an argument. It is recommended to consider it as the highest water content value in the data set or the water content at saturation.

References

Pierson, F.B.; Mulla, D.J. (1989) An Improved Method for Measuring Aggregate Stability of a Weakly Aggregated Loessial Soil. Soil Sci. Soc. Am. J., 53:1825--1831.

See Also

nls, soilwater5

Examples

h <- seq(0.1, 40, by = 2)
w <- c(0.735, 0.668, 0.635, 0.612, 0.559, 0.462, 0.369, 0.319, 0.296, 0.282, 
	0.269, 0.256, 0.249, 0.246, 0.239, 0.236, 0.229, 0.229, 0.226, 0.222)
plot(w ~ h)

# suggestions of starting values: thetaR = 0.35, alpha = 0.1, n = 10, 
# b0 = 0.02, b1 = -0.0057, b2 = 0.00004 (Not run)

fitsoilwater5(theta = w, x = h, theta_S = 0.70)

# End (Not run)