Dear Reinder,

Many thanks for your note, I agree with you that we have to see what have
already  been done and I encourage all the colleagues to point for us what
have been done. I am familiar with the paper you mentioned and looked at it
few years ago. As far as I remember now, Gabrielle et al. (1995) suggested
the use of Darcy's law directly in the water balance of DSSAT instead of
using a simple drainage equation. Again that fits with a mechanistic
approach rather than what we trying to do. As you know, Darcy's law is
enough to simulate vertical drainage when the hydraulic gradient is assumed
one. However, a hydraulic conductivity function and numerical solutions are
needed to use it.

Regards,

Ayman