Agroforesterie et services écosystémiques en zone tropicale

Respectueux de l’environnement et garantissant une sécurité alimentaire soutenue par la diversification des productions et des revenus qu’ils procurent, les systèmes agroforestiers apparaissent comme un modèle prometteur d’agriculture durable dans les pays du Sud les plus vulnérables aux changements globaux. Cependant, ces systèmes agroforestiers ne peuvent être optimisés qu’à condition de mieux comprendre et de mieux maîtriser les facteurs de leurs productions. L’ouvrage présente un ensemble de connaissances récentes sur les mécanismes biophysiques et socio-économiques qui sous-tendent le fonctionnement et la dynamique des systèmes agroforestiers. Il concerne, d’une part les systèmes agroforestiers à base de cultures pérennes, telles que cacaoyers et caféiers, de régions tropicales humides en Amérique du Sud, en Afrique de l’Est et du Centre, d’autre part les parcs arborés et arbustifs à base de cultures vivrières, principalement de céréales, de la région semi-aride subsaharienne d’Afrique de l’Ouest. Il synthétise les dernières avancées acquises grâce à plusieurs projets associant le Cirad, l’IRD et leurs partenaires du Sud qui ont été conduits entre 2012 et 2016 dans ces régions. L’ensemble de ces projets s’articulent autour des dynamiques des systèmes agroforestiers et des compromis entre les services de production et les autres services socio-écosystémiques que ces systèmes fournissent

Actions des surfaces sur l'atmosphère et sur les ressources hydriques et végétales: quelles évolutions au Sahel agro-pastoral ?

International audienc

Evaluating surface and subsurface water storage variations at small time and space scales from relative gravity measurements in semiarid Niger

The acquisition of reliable data sets representative of hydrological regimes and their variations is a critical concern for water resource assessment. For the subsurface, traditional approaches based on probe measurements, core analysis, and well data can be laborious, expensive, and highly intrusive, while only yielding sparse data sets. For this study, an innovative field survey, merging relative microgravimetry, magnetic resonance soundings, and hydrological measurements, was conducted to evaluate both surface and subsurface water storage variations in a semiarid Sahelian area. The instrumental setup was implemented in the lower part of a typical hillslope feeding to a temporary pond. Weekly measurements were carried out using relative spring gravimeters during 3 months of the rainy season in 2009 over a 350 × 500 m2 network of 12 microgravity stations. Gravity variations of small to medium amplitude (≤220 nm s-2) were measured with accuracies better than 50 nm s-2, revealing significant variations of the water storage at small time (from 1 week up to 3 months) and space (from a couple of meters up to a few hundred meters) scales. Consistent spatial organization of the water storage variations were detected, suggesting high infiltration at the outlet of a small gully. The comparison with hydrological measurements and magnetic resonance soundings involved that most of the microgravity variations came from the heterogeneity in the vadose zone. The results highlight the potential of time lapse microgravity surveys for detecting intraseasonal water storage variations and providing rich space-time data sets for process investigation or hydrological model calibration/ evaluation. ©2013. American Geophysical Union. All Rights Reserved

Construction d'une climatologie fondés sur l'observation et la modélisation des bilans hydriques et énergétiques de deux types de cultures dominantes dans le Sahel cultivé. Bilans annuels et saisonnalité

International audienceIn the sub-Saharan Sahel, energy and water cyclingat the land surface is pivotal for the regional climate,water resources and land productivity, yet it is still verypoorly documented. As a step towards a comprehensive climatological description of surface fluxes in this area, thisstudy provides estimates of long-term average annual budgetsand seasonal cycles for two main land use types of thecultivated Sahelian belt: rainfed millet crop and fallow bush.These estimates build on the combination of a 7-year fielddata set from two typical plots in southwestern Niger withdetailed physically based soil-plant-atmosphere modeling,yielding a continuous, comprehensive set of water and energyflux and storage variables over this multiyear period. Inthe present case in particular, blending field data with mechanistic modeling makes the best use of available data andknowledge for the construction of the multivariate time series.Rather than using the model only to gap-fill observationsinto a composite series, model-data integration is generalizedhomogeneously over time by generating the wholeseries with the entire data-constrained model simulation. Climatological averages of all water and energy variables, withassociated sampling uncertainty, are derived at annual to subseasonal scales from the time series produced. Similaritiesand differences in the two ecosystem behaviors are highlighted.Mean annual evapotranspiration is found to represent82-85% of rainfall for both systems, but with differentsoil evaporation/plant transpiration partitioning and differentseasonal distribution. The remainder consists entirelyof runoff for the fallow, whereas drainage and runoff standin a 40-60% proportion for the millet field. These resultsshould provide a robust reference for the surface energy- andwater-related studies needed in this region. Their significanceand the benefits they gain from the innovative data-modelintegration approach are thoroughly discussed. The modeldeveloped in this context has the potential for reliable simulations outside the reported conditions, including changingclimate and land cover

The GHYRAF (Gravity and Hydrology in Africa) experiment : description and first results

This paper is the first presentation of a project called GHYRAF (Gravity and Hydrology in Africa) devoted to the detailed comparison between models and multidisciplinary observations (ground and satellite gravity, geodesy, hydrology, meteorology) of the variations of water storage in Africa from the Sahara and part to the monsoon equatorial part. We describe the various actions planned in this project. We first detail the actions planned in gravimetry which consist in two main surface gravity experiments: on the one hand the periodic repetition of absolute gravity measurements along a north-south monsoonal gradient of rainfall in West Africa, going from Tamanrasset (20 mm/year) in southern Algeria to Djougou (1200 mm/year) in central Benin; on the other hand the continuous measurements at Djougou (Benin) with a superconducting gravimeter to monitor with a higher sampling rate the gravity changes related to an extreme hydrological cycle. Another section describes the actions planned in GPS which will maintain and develop the present-day existing network in West Africa. The third type of actions deals with hydrology and we review the three sites that will be investigated in this joint hydrogeophysics project namely Wankama (near Niamey) and Bagara (near Diffa) in the Niger Sahelian zone and Nalohou (near Djougou) in the Benin monsoon zone. We also address the question of the ground truth of satellite-derived missions: in this context the GHYRAF project will lead to test the hydrology models by comparison both with in situ and satellite data such as GRACE, as well as to an important increase of our knowledge of the seasonal water cycle in Africa. We finally present preliminary results in GPS based on the analysis of the vertical motion of the Djougou site. The resulting absolute gravity changes related to the 2008 monsoon are finally given