Hydrological evidence to guide climate resilient agricultural policies in Senegal

Many agricultural and land conservation programmes could use the latest hydrological evidence and methodology at early stages of programme design to improve long-term climate resilience and sustainability in Sahel region. This report presents stakeholder consultation, awareness (e.g., World Water Forum) and interdisciplinary research conducted in 2022 in Senegal. We evaluate the impact of bench terracing on water recharge and vegetation resilience to drought using satellite NDVI data and AMMA Catch long-term hydroclimate observatory datasets across 34 land conservation sites, covering 6600ha (period 2018-2021). Results show positive impacts from bench terracing but important variability. The causes of variability need to be determined to guide climate adaptation and land restoration

RÔLE DE L'OCCUPATION DU SOL VIS À VIS DE LA MODÉLISATION DES FLUX ENERGÉTIQUES ET HYDRIQUES EN MILIEU URBAIN ET PÉRIURBAIN

National audienceLe projet Rosenhy vise à étudier l’impact de l’occupation du sol sur la modélisation météorologique et hydrologique en termes de flux énergétiques et hydriques, en milieu urbain et périurbain. Trois sites appartenant aux observatoires français OTHU et ONEVU sont au centre de ce projet. Le quartier urbain hétérogène du Pin sec (Nantes), imperméabilisé à environ 45%, a fait l’objet d’une campagne expérimentale durant le mois de juin 2012, visant à estimer les flux de chaleur sensible et latente avec une haute résolution spatiale et temporelle par rapport aux mesures réalisées en continu sur ce site depuis 5 ans. Deux bassins versant périurbains (La Chézine à Nantes et l’Yzeron à Lyon), avec un taux d’imperméabilisation moins important (environ 10%) mais grandissant depuis plusieurs décennies, sont aussi étudiés. Ces deux derniers sites bénéficient d’un suivi hydrométéorologique depuis 10 ans pour la Chézine et 15 ans pour l’Yzeron. Sur ces trois sites, différentes sources de données d’occupation du sol à différentes résolutions sont disponibles :différentes bases de données géographiques communément utilisées par la communauté scientifique et les collectivités et des données télédétectées (multispectrales et hyperspectrales). L’utilisation de ces données en entrée de différents modèles météorologiques et hydrologiques implique un travail d’analyse et de classification pour adapter les informations aux besoins des modèles. Dans ce projet, les différents modèles adaptés au milieu urbain ou périrubain sont évalués et améliorés. Ainsi, les modèles hydrologiques périrubains sont en développement pour prendre en compte les différentes pratiques de gestion des eaux pluviales existantes (noues, toitures végétalisées, ...). L’utilisation conjointe des données simulées par les différents modèles aidera à déterminer le rôle de la part des surfaces naturelles et artificielles sur les bilans énergétique et hydrique en milieu plus ou moins urbanisé. Le milieu périurbain étant en évolution, le projet s’intéressera aussi à des scénarios d’urbanisation prospectifs en regardant d’une part l’impact de la densification sur les scénarios construits pour l’Yzeron lors du projet AVuPUR (ANR-VMCS, 2008-2011) et d’autre part, en réfléchissant conjointement avec Nantes Métropole, aux possibles voies d’évolution sur le bassin de la Chézine

Turbulence spectral-characteristics of pure katabatic flow over a steep Alpine slope

International audienc

Turbulence spectral-characteristics of pure katabatic flow over a steep Alpine slope

International audienc

Buoyancy effect of katabatic flows on turbulence above a steep alpine slope

International audienc

Long-term observations of turbulent fluxes over heterogeneous vegetation using scintillometry and additional observations: A contribution to AMMA under Sudano-Sahelian climate

Based on a 3-year period of infrared scintillometry, soil and meteorological measurements, this study presents an analysis of the surface energy balance partitioning over a heterogeneous savannah, in the Sudano-Sahelian region. The site is located in Northern Benin, meso-site of the African Monsoon Multidisciplinary Analyses (AMMA) project. The 3-year period enables an analysis of several alternate dry and wet periods, as well as the intermediate dry-to-wet and wet-to-dry periods.Infrared scintillometry, coupled with measurements of the available energy (net radiation minus ground heat flux) and a careful analysis of the aerodynamic properties of the scintillometer footprint, are employed to provide robust estimates of the turbulent (sensible and latent heat) fluxes over complex terrain, in terms of the topography and in terms of the spatially and temporally heterogeneous vegetation cover. A characterization of the uncertainties on each term of the energy balance is given at the scale of the scintillometer footprint. These uncertainties strongly depend on the season for the residual latent heat flux.Results point out that the climate of the Sudano-Sahelian region is characterized by a strong seasonal cycle and inter-annual variability, related to changing atmospheric and land surface conditions. The evaporative fraction is found to be relatively constant during the wet period (0.67) and more variable during the dry and intermediate periods. In addition, sensible heat flux and net radiation are well correlated during the dry season. The diurnal cycle shows a predominance of evaporation during the wet season and sensible heat during the dry season. Results point a significant latent heat flux during the dry period, signature of persistent vegetation in the Sudano-Sahelian region. Finally, that data set at hourly time step would provide useful information for modelling and the parameterization of the associated processes for this region

Response of CO2 fluxes and productivity to water availability in two contrasting ecosystems in northern Benin (West Africa)

CO (2) fluxes were measured during 18 months in a forest and a savannah in northern Benin. Higher values of carbon fluxes were found during the wet season at each site. A strong dependency of carbon fluxes on water relations was found in two contrasting sites. The forest sequestered 640 +/- 50 and the savannah 190 +/- 40 g C m (-2) year (-1) . In West Africa, the main mechanisms or factors governing the dynamics of ecosystems, especially the dynamics of the carbon fluxes and productivity, still remain less known. This study reports the carbon fluxes over two contrasting ecosystems, notably a protected forest (lat 9.79A degrees N, long 1.72A degrees E, alt 414 m) and a cultivated savannah (lat 9.74A degrees N, long 1.60A degrees E, alt 449 m) in northern Benin. The two sites were among those equipped by the AMMA-CATCH observatory and Ou,m, 2025 project. Flux data were analyzed at the daily and seasonal scales in order to understand their controlling variables. We discussed the patterns of CO2 fluxes and the characteristics of the two ecosystems. The study also focused on the different water usage strategies developed by the two ecosystems since the alternation between dry and wet seasons highly influenced the seasonal dynamics. Finally, the annual carbon sequestration was estimated together with its uncertainty. The carbon fluxes were measured during 18 months (July 2008-December 2009) by an eddy-covariance system over two contrasting sites in northern Benin. Fluxes data were computed following the standard procedure. The responses of CO2 fluxes to the principal climatic and edaphic factors, and the canopy conductance were studied. A clear CO2 fluxes response to main environmental factors was observed, however with difference according to the seasons and vegetation types. The ecosystem respiration showed the highest values during the wet season and a progressive decrease from wet to dry periods. Also, the carbon uptake values were high during the wet period, but low during the dry period. However, the CO2 fluxes for the protected forest were always higher than that for the cultivated savannah within each defined period. This was due to the seasonal changes not only in phenology and physiology but also to the acclimation to environmental conditions, especially to the soil water availability. The water use efficiency was influenced by VPD during the day conditions for two ecosystems. However, the VPD response curve of water usage was relatively constant for the protected forest during the transitional and wet seasons. In contrary, for the cultivated savannah the VPD response decreased about 46 % from transitional dry-wet to wet seasons and remained relatively constant until transitional wet-dry season. The close relationships between the net CO2 assimilation and the canopy conductance were found for the two ecosystems. This suggests a regulation of the stomata by a partial stomatal closure besides the radiation control. Finally, the forest sequestered at the annual scale 640 +/- 50 and the savannah 190 +/- 40 g C m(-2) year(-1). We conclude with a strong relation between the carbon fluxes and water in the two investigated ecosystems. Apart from the radiation control, the stomata also play an important role in the regulation of the CO2 assimilation in the two ecosystems