The Living Space of n and Alpha.

Modelling soil water dynamics at the continuum scale is typically based on Richards' equation and with this, on the soil water characteristic and the hydraulic conductivity function. Many parametrizations for these constitutive relations have been proven to provide useful descriptions but the one of Rien van Genuchten is by far the most widely used, so that 'n' and 'alpha' are not just letters in the alphabet of soil physicists. In this contribution we would like to explore the living space of 'n' and 'alpha' and especially the regions where we start to get bumps at the walls. This is done by theoretical considerations, some experimental observations and using numerical case studies.
One bump is obtained when going towards water saturation where the continuous differenziability is numerically convenient but may lead to some physical inconsistencies. We feel another one when we increase the averaging volume too much, so that the characteristic time scale of external forcing becomes too short for the soil to stay on the curve described by n and alpha. We finally conclude that the explored living space of n and alpha might be considerably smaller than often assumed and we will have a glance outside in the garden.