Analyse de l'évolution de l'occupation du sol par imagerie Landsat pour aider à la modélisation hydrologique en Afrique de l'ouest

International audienceThe spatial and temporal variability of land cover changes is a fundamental parameter to integrate when modelling water resources in order to reproduce the relations between rainfall and surface flow more precisely. This is particularly important in West Africa, where the land cover has been changing for more than 40 years under the combined impact of climatic effects and human activities. In this study, we evaluated the potential of Landsat imagery to monitor the vegetation cover in the upper Niger watershed (120 000 km²) using archive images from MSS, TM and ETM+ sensors covering three periods of time around 1975, 1985 and 2000. Because of the heterogeneity of the acquisition dates, the spatial and spectral resolution of the images, and the scale of analysis, we chose a simple system of classification. Pretreatments were applied to reduce variations between the images. Vegetation indices (NDVI) were then calculated and subsequently thresholded using the same land cover classification system. The thresholds were then optimized by automated recursive calculations of confusion matrices and control parcels. Our results revealed that although accuracy was not perfect, it was nevertheless possible to estimate changes using an unconventional spatio-temporal scale. The resulting changes were characterized by a moderate trend to deforestation with a corresponding increase in bare soils, soils with sparse vegetation and shrublands. The spatial layers produced were then combined with a soil map to incorporate changes in surface conditions in the hydrological modelling of the Niger River

Simulation of runoff in West Africa : is there a single data-model combination that produces the best simulation results ?

Is there a single combination of hydrologic model and data that yields the best simulations of runoff from a set of catchments in West Africa in the coming decades? To answer this question, a semi-distributed modelling platform was developed. It comprises two models (GR2M and Water Balance Model) and various datasets as inputs to these models (three potential evapotranspiration [PET] grids and four soil water holding capacity [WHC] grids). The platform makes it possible to regionalise runoff flows, which are discretised in grids of mesh size 0.5 degrees x 0.5 degrees. On the basis of these datasets and models, we compare the performance of the various possible data-model combinations. The study area contains 49 catchments located in Cote d'lvoire, Guinea, Mali, Burkina Faso and Niger. The analysis shows that the selected models are hardly sensitive at all to the different PET grids but more sensitive to the soil grids. The GR2M model clearly gives the best results for the set of catchments, but does not perform well over the entire study area, whatever the grid dataset used: It is thus seen to be difficult to define a single data-model combination that is optimal for simulating runoff in West Africa. As to the existence of a "general" model that could be used anywhere, we find that, for the selected study area, neither of the two models selected fully meets the objective

Changement climatique : impacts et adaptations

Ce dossier présente les acteurs de la recherche basés dans la région Languedoc-Roussillon, qui se mobilisent pour répondre aux enjeux liés à l'étude des impacts et des adaptations au changement climatique : 46 unités ; 2 "laboratoires d'excellence" et 1 institut ; 5 infrastructures de recherche, d'envergure nationale ou européenne ; 5 partenaires étrangers ou internationaux. L'étendue des travaux scientifiques menés par les acteurs régionaux sur l'impact ou l'adaptation au changement climatique est très vaste. Loin d'être exhaustif, ce dossier a pour ambition d'offrir au lecteur un panorama de ces travaux, en présentant les acteurs impliqués et quelques exemples concrets de leurs activités de recherche. Ceux-ci sont présentés selon quatre grands champs thématiques : "Changement climatique & ressources, territoires et développement" ; "Changement climatique & écosystèmes et biodiversité (biodiversité et écosystèmes continentaux / biodiversité et écosystèmes marins)" ; "Changement climatique & interactions entre organismes" ; "Changement climatique & systèmes de production agricole et d'élevage"