r.watershed

[Dan Riker]

Hello, I hope all is well

I am presently working on TOPMODEL, a rainfall-runoff model, for my thesis.  What I need GRASS for is a topographic index ln(a/tanB), but I'm not certain how to calculate it.  I would also like to know  how to get the mean, std deviation and skewness of that index. Finally I need to put it in an I*2 form to be used by the model.  
If anyone has any information on any aspect of this I would certainly appreciate it.
Dan Riker
Duke University