Modeling unsaturated zone_Van genuchten parameters

Hi everyone,

I am making a 3D model with unsaturated and saturated zone in feflow 6.1. In unsaturated zone, I have 7 layers, while saturated zone have 3 layers.
Unsaturated zone: (sand, sandy loam, loamy sand,clay, sand+ gravels, gravels, boulders). I want to define van-genuchten parameters for each layer of unsaturated zone. But I don't have any idea about fitting coefficent and fitting parameres for gravels and gravels+boulders. Could anyone please suggest me what is the optimum value of these parameters or refer me to any literature.

Thanks :) 

<a href="http://www.ars.usda.gov/News/docs.htm?docid=8953">ROSETTA</a> is a useful reference.

Twarakavi did a statistical analysis over several thousands of probes, determined 12 classes of soils and their van Genughten parameters. He did also an uncertaintay Analysis for that.

The publication is:
Can texture-based classification optimally classifiy soils with resprect to soil hydraulics?
Navin C. Twarakavi
Water resources Research, Vol. 46.

Very helpful because he gives ranges of soil Parameters.

Christian

Thanks Björn and Christian for your useful replies.

Regards.