kfact_root

[jgipsl]

Background:
Kfact_root is a variable that change the upper soil water diffusion coefficient to account for the fact that the presence of roots change the porosity and increase the diffusion (and thus the infiltration rate).
The calculation of kfact_root changed in rev 2868. Before, kfact_root was calculated in the initialization phase, at each restart using the variable veget. This means that the kfact_root was different for a simulation with restart every month compared to the same simulation with restart every year. Therefore kfact_root was moved to hydrol_main to be calculated at each time step, see commit [2868], ticket #143. This change was only done to make sure that the code is restarting correct.

Temporary change:
The change above makes kfact_root change in time. Due to ORCHIDEE meeting 15sept it is decided to change the calculation to use veget_max/2 instead of veget. This will make kfact_root nearly constant.

Long term:
We also propose that in the long term Kfact_root should depend on the total_root_biomass and not on the vegetation cover as this is a more logical explanatory variable.
However, this requires to change the empirical function defining Kfact_root and to test wether the "root_biomass" does not vary too much over time as this would introduce "unrealistic" variation of the infiltration/diffusion properties of the soil.

[jgipsl]

The temporary change is done in rev [2916]

[aducharne]

Addition to the background:

The expression of kfact_root as a function of veget that was used before rev 2868 directly comes from D'Orgeval et al. (2008):
​http://www.hydrol-earth-syst-sci.net/12/1387/2008/hess-12-1387-2008.pdf

It can also be found on page 11 of ​http://forge.ipsl.jussieu.fr/orchidee/raw-attachment/wiki/Documentation/eqs_hydrol.pdf