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Model architecture

ORCHIDEE consists of three main modules (Figure 1):

• The Soil–Vegetation–Atmosphere Scheme SECHIBA (Ducoudré et al. 1993; de Rosnay and Polcher 1998) which simulates the biophysical exchanges of water and energy between land surface and atmosphere on a short time-scale (20 to 30 minutes). It computes fluxes of momentum, heat, water, and canopy C exchanges, as well as soil water budget and surface energy budget.

• The biogeochemical model STOMATE which describes seasonal C and vegetation dynamics on a daily basis: for instance phenology, C allocation, litter production and decomposition, senescence. This module provides SECHIBA with the physical description of vegetation necessary to compute fluxes (e.g., Leaf Area Index (LAI)). In return it receives the environmental and climatic stresses that affect vegetation development. Note that plant transpiration, respiration and assimilation are computed in SECHIBA since these processes have to be computed at the shortest time-scale.

• A module taken from the Lund–Postdam–Jena (LPJ) model (Sitch et al. 2003) describing the dynamics of the potential natural vegetation (i.e. long-term evolution of vegetation from one type to another). It includes rules of interspecies competition for light, role of fire, appearance and disappearance of different plant types, etc. Time-step here is generally 1 year. This module can be turned off and vegetation distribution prescribed, or read on a land-cover map.

Module description : ORCHIDEE Crop Model

Main function

To account for global vegetation, ORCHIDEE in its standard version uses 10 natural plant functional types (PFTs) (eight evergreen and deciduous trees, C3 and C4 grasses). Two additional PFTs are designed, so as to account for C3 and C4 crops: in the standard version these crops are crudely modeled as, respectively, C3 and C4 grasslands, with enhanced assimilation rates. The global land surface is divided into grid cells, which size is not prescribed but depends on the grid size of climate input. Several PFTs can coexist within the same grid box (but there is no spatial repartition within the cell). They all share the same climate forcing but fluxes are computed separately for each PFT depending on their own properties. Fluxes are then averaged before entering the first atmospheric level: thus the vegetation feeds back on the atmosphere.

Irrigation was integrated in ORCHIDEE by de Rosnay et al. (2003) and tested over the Indian Peninsula . It was then used in global runs to evaluate the impacts of irrigation of the Indian monsoon (Guimberteau et al. 2011). Irrigated fractions of each grid box are prescribed by a digital global map generated by Döll and Siebert (1999, 2000, 2002) and updated by Siebert and Döll (2001). It gives the estimated area of each 0.5° grid box equipped for irrigation around 1995 (up to 1999 for Europe and Latin America). The water requirements for crops (for their optimal growth) are calculated in ORCHIDEE over each irrigated fraction as the difference between maximum transpiration and available water (defined as the difference between precipitation and total runoff).  Water is withdrawn from the reservoirs that transport water to the oceans (Figure 2). This routing scheme was developed by Polcher (2003) and described in Ngo-Duc et al. (2007).

Description of the inputs

Description of the outputs

Various variables can be computed. The main categories are :                      

• Water budget (mm/day): evapotranspiration, sublimation, evaporation, evaporation of water intercepted by the foliage, sweating by the canopy, surface runoff, background runoff.
• Water storage (kg/m²) (snow, soil water content, water stored on leaves (from interception)
• Surface energy balance (W/m²): sensible heat, latent heat, soil heat flux, solar, infra-red and net radiation?
• Carbon budget (gC/m²/day): gross productivity, net productivity, heterotrophic respiration?, maintenance respiration, growth respiration, CO2 total emitted.
• Carbon storage (gC/m²): litter, soil carbon, biomass of various compartments, leaves, hard and sap wood, roots, fruits, reserves.

Condition of acces to the model codes

The code is open source and can easily be used once users have filled up and signed a CeCill licence (http://www.cecill.info/)

References

http://forge.ipsl.jussieu.fr/orchidee

(Krinner et al. 2005)

(Marti et al. 2010)

Ciais et al. (2005),

(Ducoudré et al. 1993; de Rosnay and Polcher 1998)