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Accueil du site → Doctorat → France → 2011 → Including tropical croplands in IPSL’s land surface model, ORCHIDEE : Contribution to the study of cropland/climate interactions

Universite Pierre et Marie Curie (2011)

Including tropical croplands in IPSL’s land surface model, ORCHIDEE : Contribution to the study of cropland/climate interactions

Berg, Alexis M

Titre : Including tropical croplands in IPSL’s land surface model, ORCHIDEE : Contribution to the study of cropland/climate interactions

Including tropical croplands in the IPSL land surface model, ORCHIDEE : contribution to the study of crop/climate interactions

Auteur : Berg, Alexis M

Université de soutenance : Universite Pierre et Marie Curie

Grade : DOCTEUR de L’UNIVERSITE PARIS VI - PIERRE ET MARIE CURIE 2011

Résumé
In this work we introduce a representation of tropical croplands in IPSL’s (Institut Pierre Simon Laplace) land surface/vegetation model, ORCHIDEE, in order to study cropland/climate interactions in the Tropics. The impetus for this work is that the existing version of ORCHIDEE, like most other land surface models, approximated croplands by natural grasslands. However, croplands differ from natural vegetation in terms of seasonality and productivity, thus leading to differences in land-atmosphere interactions and corresponding feedbacks to climate. Moreover, implementing croplands in a global vegetation model also provides an innovative dynamical modeling framework in which to perform large-scale, yet process-based and spatially-explicit, assessments of climate change impacts on agricultural yields.
A parametrization for tropical crops is derived from an existing crop model, SARRAH, which is routinely used by agronomists in Africa for cereals such as millet and sorghum, and implemented into ORCHIDEE. The new version, called ORCHIDEE-mil, is validated at the local and regional scale over West Africa and India. Comparison against field trials and land-atmosphere measurements from eddy-covariance flux towers shows that the new version generally improves upon the default. The analysis of the variability of simulated yields against national data shows that the model correctly simulates the large-scale impact of climate on crop productivity ; however, the model overestimates mean observed yields, as a result of the yield gap in those regions between climatic potential yields – such as simulated by the model – and actual, on-farm yields. We analyze the effects of different biases in precipitation forcing data on the simulation of vegetation, showing that two features of forcing rainfall essential for the model to skillfully simulate interannual variability in productivity at the country scale are cumulative annual variability and intraseasonal distribution. A discussion of large-scale statistical versus process-based modeling is also proposed.
Focusing on West Africa, we then analyze the impact of a more realistic representation of tropical croplands on the regional-scale simulation of land-atmosphere fluxes, and subsequently on climate with an asynchronously coupled simulation. This experiment shows only a limited impact on climate of the modification of land-atmosphere interactions induced by the change in simulated vegetation : the limitations of this experiment are discussed. Finally, ORCHIDEE-mil is used, forced by IPCC climate projections from several models and scenarios, to project the evolution of potential crop yields over Africa and India in the context of climate change. Results show on average a moderately negative impact over the study domain by the end of the century (-6%), with a consistent negative impact of higher temperatures ; however, there is a large spread in individual projections (-29 to +11%), mostly reflecting the uncertainties in precipitation projections over arid regions in the Tropics. This impact must be considered in the light of adaptation possibilities, as well as the current potential for yield increase in those regions.

Mots clés : Agriculture, Changement climatique, Tropiques, Impact, Interactions surfaceatmosphère, Modélisation — Agriculture, Climate change, Tropics, Impacts, Land-atmosphere interactions, modelling.

Présentation (DISCCRS)

Page publiée le 13 janvier 2015, mise à jour le 8 février 2017