This function compute water content at given pressure head, using Van Genuchten water retention curve
Arguments
- head
(numeric) suction pressure ([L] or cm of water)
- thetaR
(numeric) residual water content (cm3/cm3)
- thetaS
(numeric) saturated water content (cm3/cm3)
- alfa
(numeric) related to the inverse of the air entry suction, alfa > 0 (1/cm)
- n
(numeric) a measure of the pore-size distribution, n>1, dimensionless