Effect of seasonal dynamics of vegetation cover on land surface models: a case study of NOAH LSM over a savanna farm land in eastern Burkina Faso, West Africa

International audienceThe sensitivity of the land surface model of the National Centers for Environmental Prediction (NCEP), the Oregon State University, the Air Force and the Hydrologic Research Lab (NOAH LSM) was evaluated with respect to the seasonal dynamics of the vegetation cover in the savanna area under intensive agriculture in the eastern part of Burkina Faso, West Africa. The data collected during the first long-term measurement of the surface fluxes in this mentioned region was used for this purpose. The choice of NOAH LSM was motivated by the fact that it has already been tested in different environments in West Africa, especially in Ghana. The sensitivity was tested by comparing the simulated surfaces fluxes using a fixed values of the roughness length for momentum as a standard in the model and the true seasonal value of this variable. The results show that NOAH LSM was not sensitive to the change of the roughness length for momentum neither on a seasonal basis nor on a daily basis, which was found to be abnormal. The formulation of the coefficient (Bc) coupling the dry canopy transpiration to the atmosphere was found to be the main reason for this. An improved formulation for this coefficient was given to solve this insensitivity and to improve the performance of the model. Recommendations are also given to enhance the performance of the model in the West African savanna environment

Corrélation entre les flux de chaleur sensible et latente avec la longueur de rugosité de surface. Influence sur la disponibilité des ressources en eau sur les terres intensivement exploitées de l’Est du Burkina Faso.

 L’exploitation des terres, exprimée par la longueur de rugosité de surface zo, est un facteur important qui influence le processus d’échange d’énergie entre la surface de la terre et l’air. Cette étude qui s’intègre dans le cadre des travaux de thèse intitulés « Impact de l’exploitation des terres sur le bilan hydrologique dans les bassins versants de l’Est du Burkina Faso » met en relation les principaux termes du bilan d’énergie (le flux de chaleur sensible et latente) avec la longueur de rugosité de surface zo calculée à l’aide de la loi de similarité de Moninh - Obhukoh.  La méthodologie est basée sur l’observation des données micro-météorologiques qui sont constituées des mesures simultanées des moyennes sur chaque heure de la vitesse du vent, du flux de chaleur sensible et latente et la température. Ces mesures ont été effectuées à 10 m du sol qui constitue la hauteur du mat de la station installée à l’Est du Burkina Faso précisément à Tanyele (département de Kompienga). Un anémomètre sonique triaxial est utilisé pour la mesure du flux de chaleur sensible, de la vitesse du vent et la température de l’air. L’hygromètre Campbell KH2O à krypton est utilisé pour la mesure du flux de chaleur latente. Le traitement des données brutes a été effectué par corrélation de remous en temps réel (chaque heure) à l’aide du logiciel ALTEDDY.  Les résultats montrent qu’il y a une bonne corrélation entre la longueur de rugosité de surface et les principaux termes du bilan d’énergie sur l’ensemble des données. Pour les données collectées sur une année, nous pouvons noter que le flux de chaleur sensible décroît exponentiellement lorsque la longueur de rugosité de surface croit (R2=- 0.84). De même le flux de chaleur latente croit de façon logarithmique avec la longueur de rugosité de surface (R2= 0.79). Lorsque nous analysons les données séparément sur les deux principales saisons qui caractérisent la zone d’étude (saison sèche et saison pluvieuse), nous constatons que la longueur de rugosité de surface n’a pas un impact significatif sur le flux de chaleur sensible et latente. D’autres paramètres climatiques tels que la radiation solaire, l’humidité relative et la température de l’air les déterminent. Cependant en saison pluvieuse, la longueur de rugosité de surface influence très négativement le flux de chaleur sensible (R2= - 0.92) contrairement au flux de chaleur latente qui montre une décroissance qui n’est pas aussi significative (R2= - 0.49). Cette décroissance du flux de chaleur sensible avec la longueur de rugosité de surface en saison pluvieuse peut entraîner une bonne recharge de la nappe si les écoulements de surface ne sont pas assez importants. Nous aurons besoin de beaucoup plus de données pour avoir des informations fiables sur la tendance croissante du flux de chaleur latente avec la longueur de rugosité de surface observée sur l’ensemble des données collectées

Spatial distribution of the chromosomal forms of anopheles gambiae in Mali

<p>Abstract</p> <p>Background</p> <p>Maps of the distribution of malaria vectors are useful tools for stratification of malaria risk and for selective vector control strategies. Although the distribution of members of the <it>Anopheles gambiae </it>complex is well documented in Africa, a continuous map of the spatial distribution of the chromosomal forms of <it>An. gambiae s.s. </it>is not yet available at country level to support control efforts.</p> <p>Methods</p> <p>Bayesian geostatistical methods were used to produce continuous maps of the spatial distribution of the chromosomal forms of <it>An. gambiae s.s</it>. (Mopti, Bamako, Savanna and their hybrids/recombinants) based on their relative frequencies in relation to climatic and environmental factors in Mali.</p> <p>Results</p> <p>The maps clearly show that each chromosomal form favours a particular defined eco-climatic zone. The Mopti form prefers the dryer northern Savanna and Sahel and the flooded/irrigated areas of the inner delta of the Niger River. The Savanna form favours the Sudan savanna areas, particularly the South and South-Eastern parts of the country (Kayes and Sikasso regions). The Bamako form has a strong preference for specific environmental conditions and it is confined to the Sudan savanna areas around urban Bamako and the Western part of Sikasso region. The hybrids/recombinants favour the Western part of the country (Kayes region) bordering the Republic of Guinea Conakry.</p> <p>Conclusion</p> <p>The maps provide valuable information for selective vector control in Mali (insecticide resistance management) and may serve as a decision support tool for the basis for future malaria control strategies including genetically manipulated mosquitoes.</p

Results analysis and validation - D5.3

This deliverable describes the validation processes followed to assess the performance of the algorithms and protocols for the operator governed opportunistic networking as defined in the OneFIT Project. Therefore, this document includes the description of the set-up of the different validation platforms, the design of the test plans for each one of them, and the analysis of the results obtained from the tests. A per-scenario approach rather than a per-platform approach has been followed, so an additional analysis has been performed, gathering the results related to each scenario, in order to validate the premises stated to each one of them. The OneFIT concept has been therefore validated for all foreseen business scenarios

Energy balance closure and footprint analysis using Eddy Covariance measurements in Eastern Burkina Faso, West Africa

International audienceThe quality and the representativeness of the first long-term Eddy Covariance measurements in the savanna zone of West Africa were investigated using the energy balance closure and the footprint analysis. The quality and representativeness of the first long-term Eddy Covariance measurements over the West African savanna were investigated using the energy balance closure and the footprint analysis. The analysis covered four contrasting periods such as the complete dry season (January to March 2004), the dry to wet transition period (April to May 2004), the rainy season (June to September 2004) and the wet to dry transition period (October to November 2004). The results show that the overall energy balance closure can be considered as satisfactory over the whole dataset. The regression fit between (Rn?G) and (H+?E) was significant (Pr2) of 0.80 and a slope of 0.88, while the intercept was 25W/m2. The energy balance closure was affected by rain during the rainy season (r2=0.69), and by sampling problems during the transition periods (R2 were 0.80 and 0.86, respectively). The footprint analysis shows that the fetch ranged between 20 m (daytime) and 800 m (nighttime). This range showed that the fetch was adequate and fluxes sampled were representative, especially during the rainy season when the vegetal cover was dominated by crops and grasses with scale length of a few meters. During the dry season when the surface is free from crops and grasses, the measurements were also representative as about 60% of the trees around the station were contributing to the measured fluxes. However, during the transition periods some sampling problems appeared, less than 30% of the trees were contributing to the measured fluxes. The relevance of the dominant wind direction in the representativeness of the measurements was also discussed

The linkage between security, governance and development: the European Union in Africa

International audienceThe international community currently favours an approach to development that stresses a triangular linkage between security, good governance and economic development. This approach clearly informs the European Union's agenda in Africa, which has progressively integrated governance and security elements. This paper will show that this agenda is at least as much determined by the bureaucratic and national affiliations of the concerned EU actors as it is by African realities and international trends. African security indeed triggers a competition between the different European institutions, eager to be the driving force for a policy that can offer some additional resources and autonomy. The consistency and the credibility of the EU security policy in Africa will therefore depend on the responses provided to these institutional rivalries

Scale effects in Hortonian surface runoff on agricultural slopes in West Africa: Field data and models

This article provides an overview of both experimental and modeling research carried out over the past 15 years by the authors addressing scaling effects in Hortonian surface runoff. Hortonian surface runoff occurs when rainfall intensity exceeds infiltration capacity of the soil. At three sites in West Africa (Côte d’Ivoire, Ghana, and Burkina Faso) runoff was measured from plots of different lengths to assess scale effects. Consistently, longer plots showed much lower runoff percentages than shorter plots. There were large variations in runoff percentages from one rainstorm to the next but there were very good correlations between plots of equal length for each single event. This strongly suggests that temporal dynamics are the cause behind the observed scale effects. In the literature, spatial variability is often proffered as explanation for such scale effects without providing a mechanism that would cause consistent reduction in runoff percentages with increasing slope length. To further examine whether temporal dynamics can indeed provide the explanation, Hortonian runoff was simulated using models with increasing levels of complexity. The simplest model was already able to reproduce the observed scale effects. Also more complex models were used that accounted explicitly for spatial variability. The conclusions remained the same regarding the role of temporal dynamics. Finally, a dimensional analysis was developed that helps predict under which circumstances one can expect scale effects similar to the ones observed in West Africa