in the Main Processes window you have to tick to "Root Water Uptake" and then you have to include information necessary to estimate PET (either in a form of meteorological data or you own PET values combnined with the LAI). Then you have to choose from Feddes and S-Shape type of "PET->AET" transfer function and define its parameters. The final step is represented by the definition of the root distribution in a Profile Information window. As far as I remember now, it should be all (at least to run the simulation).