ORCHIDEE is a process-based ecosystem model developed for simulating carbon, water and energy fluxes in ecosystems, from site level to global scale. ORCHIDEE-GM is a version specifically developed to integrate the management of grassland. The equations describing management in ORCHIDEE are derived from PaSim? model. Accounting for the management practices such as mowing, livestock grazing and fertilizer application on a daily basis, ORCHIDEE-GM proves capable of simulating the dynamics of leaf area index, biomass, and carbon fluxes of managed grasslands. ORCHIDEE-GM v1 was evaluated and some of its parameters calibrated at 11 European grassland sites representative of a range of management practices, with eddy-covariance net ecosystem exchange and biomass measurements. The model successfully simulates the net carbon budget of these managed grasslands. Chang et al. (2015) then added a parameterization of adaptive management through which farmers react to a climate-driven change of previous-year productivity. Though a full nitrogen cycle is not included in ORCHIDEE-GM, the positive effect of nitrogen fertilizers on grass photosynthesis rates, and thus on subsequent ecosystem productivity and carbon storage, is parameterized with an empirical function calibrated from literature estimates (v2.1). ORCHIDEE-GM v2.1 was applied over Europe to calculate the spatial pattern, inter-annual variability, and the trends of potential productivity, i.e. the productivity that maximizes simulated livestock densities assuming an optimal management system in each grid cell. This version was further used to simulate the net carbon budget, budget trends, and the GHG balance of European grasslands during the last five decades at a spatial resolution of 25 km. Chang et al. (2017) recently updated the model to version 2.2 with the general parameterizations from ORCHIDEE Trunk.rev3623 (​https://forge.ipsl.jussieu.fr/orchidee/browser/trunk#ORCHIDEE), and a new parameterization limiting grazing practices under specific conditions such as frost, snow cover and wet soil. ORCHIDEE-GM v2.2 was driven by projected future climate change to provide a European‐wide assessment of the future changes in productivity and phenology of grassland, and their consequences for the management intensity and the carbon balance. At global scale, ORCHIDEE-GM v3.1 is a development of v2.1, includes a parameter adjustment for the C4 grassland biome and implemented a specific strategy for wild herbivores. Combining livestock production information, ORCHIDEE-GM v3.1 was applied to reconstruct a series of global gridded maps containing a time-dependent history of grassland management intensity. These maps are model-dependent, and provide a unique opportunity for models with explicit representation of grassland management to make a more accurate estimate of global carbon and GHG budgets of grassland – as we have here. In this study, ORCHIDEE-GM v3.1 has been updated with the parameterizations of v2.2 to a new version named ORCHIDEE-GM v3.2.

This version of ORCHIDEE has been used in a publication under review.