Rescaled capillary saturation function by Iden and Durner (2014) 
Scap(suc, par.shp, modality = c("uni"), suc.negativ = FALSE)Suction/pressure heads. Negative if suc.negativ = TRUE
named parameter in list or vector
pore size distribution ('uni' or 'bi')
set TRUE if suction/pressure heads are negative and FALSE if positive
w2 [-]: weigthing between pore space distribution 
alfa2 [1/L]: van Genuchten parameter alfa for second pore space distribution 
n2 [-]: van Genuchten parameter n for second pore space distribution 
m2 [-]: shape parameter (m = 1-(1/n2) if missing) 
Scap(h) = (Gamma(h)- Gamma(h0))/(1 - Gamma(h0)) 
Gamma descripes the capillary saturation function. Here the saturation function of van Genuchten is used: 
gamma(h) = (1/(1 + -suc * alfa)^n)^m (see also Sat)
Iden, S., Durner, W. (2014). Comment to Simple consistent models for water retention and hydraulic conductivity in the complete moisture range by A. Peters. Water Resour. Res. 50, 7530–7534.
Schneider, M., & Goss, K. U. (2012). Prediction of the water sorption isotherm in air dry soils. Geoderma, 170, 64-69.