How to prescribe (or impose in ORCHIDEE terminology) the vegetation in a pixel?

Many new ORCHIDEE users will impose their preferred vegetation to a single pixel. This has the advantage that simulations go faster, output files are smaller, data-model comparison is more straightforward and it is easier to understand the effect of specific settings and parameters. To do so you will to make changes to ../config/ORCHIDEE_OL/COMP/sechiba.card and ../config/ORCHIDEE_OL/PARAM/run.def

First, state that you want to impose the vegetation

# IMPOSE_VEG=y/n
IMPOSE_VEG=y

Make sure you don't overwrite your own settings in the next time step, hence, switch of land cover change

# Set LAND_COVER_CHANGE=y if VEGET_UPDATE > 0
LAND_COVER_CHANGE=n

Obviuosly, you don't want to update the land cover map because you're not using one

# VEGET_UPDATE=0Y no change in PFTmap. PFTmap should be set only in InitialStateFiles/List. 
# VEGET_UPDATE=1Y change PFTmap every year in december : PFTmap should be added in BoundaryFiles/List and removed from InitialStateFiles/List.
# This parameter is only used if IMPOSE_VEG=n.
VEGET_UPDATE=0Y

The land cover map should not be specified in Initial state list of the model

[InitialStateFiles]
List=       (${R_INIT}/SRF/${config_UserChoices_TagName}/routing.nc, .),     \
            (${R_INIT}/SRF/${config_UserChoices_TagName}/soils_param.nc, .), \
            (${R_INIT}/SRF/${config_UserChoices_TagName}/cartepente2d_15min.nc, .), \
            (${R_INIT}/SRF/${config_UserChoices_TagName}/floodplains.nc, .)

In this example there are no boundary files

[BoundaryFiles]
List=	()

Because their is no initial state for the model it needs to be specified in the run.def.

For consistency reasons add to the rund.def

impose_veg=y

It is essential to specify

SECHIBA_VEG__01 = x1
...
SECHIBA_VEG__nvm = xnvm

Where the sum of x1 to xnvm should equal 1

Although some branches no longer make use of it, it is better to specify SECHIBA_VEGMAX as well. Just copy the values of SECHIBA_VEG to avoid problems