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

Author: S.Luyssaert
Last revision: 2020/02/28, V. Bastrikov

Objective: 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 prescribe the vegetation you need to make changes to COMP/sechiba.card and PARAM/run.def.

First, in COMP/sechiba.card set that you don't want to update the land cover map (because you're not using one):

# VEGET_UPDATE=0Y : no change in vegetation map. PFTmap.nc should be set only in InitialStateFiles/List.
# VEGET_UPDATE=1Y : change vegetation map every year. PFTmap.nc should be set only in BoundaryFiles/List.
# Note that the model changed behaviour:
# - since rev 2717 in the trunk, the map is read in January. In BoundaryFiles/List, copy the map for current year, use the variable ${year}.
# - for older version, the map is read in December. In BoundaryFiles/List, copy the map for next coming year using the variable ${year_p1}.
VEGET_UPDATE=0Y

The land cover map should not be specified in either "initial state files" or "boundary files" blocks:

[InitialStateFiles]
List=       (${R_IN}/SRF/ROUTING/routing.nc, .),     \
            (${R_IN}/SRF/SOIL/soils_param4.nc, soils_param.nc), \
            (${R_IN}/SRF/SOIL/soil_bulk_and_ph.nc, .), \
            (${R_IN}/SRF/cartepente2d_15min4.nc, cartepente2d_15min.nc), \
            (${R_IN}/SRF/ROUTING/floodplains4.nc, floodplains.nc), \
            (${R_IN}/SRF/reftemp4.nc, reftemp.nc), \
            (${R_IN}/SRF/albedo/alb_bg_modisopt_2D_ESA_v3.nc, alb_bg.nc)

[BoundaryFiles]
List=   ()

Second, in PARAM/run.def state that you want to impose the vegetation:

# Prescribed vegetation (default n)
IMPOSE_VEG = y

And specify the vegetation distribution you need using the keywords SECHIBA_VEG and SECHIBA_VEGMAX:

SECHIBA_VEG__01 = x1
...
SECHIBA_VEG__nvm = xnvm

SECHIBA_VEGMAX__01 = x1
...
SECHIBA_VEGMAX__nvm = xnvm

Note that:

• the number of keywords (from 01 to nvm) should match the number of PFTs used in the ORCHIDEE simulation
• the sum of x1 to xnvm should equal 1
• the values set for SECHIBA_VEG and SECHIBA_VEGMAX should be identical.