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Tagesson T.,Copenhagen University | Ardo J.,Lund University | Guiro I.,Cheikh Anta Diop University | Cropley F.,Lund University | And 8 more authors.
Geografisk Tidsskrift - Danish Journal of Geography | Year: 2016

Africa is a sink of carbon, but there are large gaps in our knowledge regarding the CO2 exchange fluxes for many African ecosystems. Here, we analyse multi-annual eddy covariance data of CO2 exchange fluxes for a grazed Sahelian semi-arid savanna ecosystem in Senegal, West Africa. The aim of the study is to investigate the high CO2 exchange fluxes measured at the peak of the rainy season at the Dahra field site: gross primary productivity and ecosystem respiration peaked at values up to −48 μmol CO2 m−2 s−1 and 20 μmol CO2 m−2 s−1, respectively. Possible explanations for such high fluxes include a combination of moderately dense herbaceous C4 ground vegetation, high soil nutrient availability and a grazing pressure increasing the fluxes. Even though the peak net CO2 uptake was high, the annual budget of −229 ± 7 ± 49 g C m−2 y−1 (±random errors ± systematic errors) is comparable to that of other semi-arid savanna sites due the short length of the rainy season. An inter-comparison between the open-path and a closed-path infrared sensor indicated no systematic errors related to the instrumentation. An uncertainty analysis of long-term NEE budgets indicated that corrections for air density fluctuations were the largest error source (11.3% out of 24.3% uncertainty). Soil organic carbon data indicated a substantial increase in the soil organic carbon pool for the uppermost .20 m. These findings have large implications for the perception of the carbon sink/source of Sahelian ecosystems and its response to climate change. © 2016 The Royal Danish Geographical Society Source


Labat D.,Geosciences Environnement Toulouse GET | Sivakumar B.,University of New South Wales | Sivakumar B.,University of California at Davis | Mangin A.,French National Center for Scientific Research
Stochastic Environmental Research and Risk Assessment | Year: 2015

One of the major challenges in hydrology consists in the conception of models to predict runoff evolution in time, as this is of crucial importance in water resource assessment and management. These models are required to provide estimations of high flows and low flows, so that appropriate short-term (flood) emergency measures and long-term (drought) management activities can be undertaken. However, due to the inherent nonlinearity of climate inputs (e.g. rainfall) and the heterogeneous nature of watersheds, understanding and modeling the catchment hydrologic response is tremendously challenging. This is particularly the case for karstic watersheds that are generally highly nonlinear and also sensitive to initial conditions. Investigation of the dynamic nature of hydrologic response is an important first step towards developing reliable models for such watersheds. To this end, this study examines the dynamic nature of streamflow discharge from karstic watersheds, especially the short-term variations. A nonlinear dynamic method, the correlation dimension method, is employed to unique long, continuous, and high-resolution (30-min) streamflow data from two karstic watersheds in the Pyrénées Mountains (Ariège) of France: the Aliou spring and the Baget spring. The results reveal the presence of deterministic chaos in the streamflow dynamics of the two watersheds, with attractor dimension values below 3. These results have great significance regarding the presence of deterministic chaos in karstic flows and in the issue of data size regarding chaos studies in hydrology. © 2015 Springer-Verlag Berlin Heidelberg Source


Breton G.,rue des Reservoirs | Breton G.,French National Center for Scientific Research | Bilotte M.,Geosciences Environnement Toulouse GET | Eychenne G.,Clot Del Moulinie
Annales de Paleontologie | Year: 2013

The amber of Le Mas d'Azil (Ariège, France), fashioned by the Magdalenian people of Le Mas d'Azil cave, was collected in clay levels rich in Cupressinoxylon Göppert, of the Campanian Labarre Sandstone Formation, which is a large deltaic set, infilling the sub-Pyreneean trough. The amber pieces are small and resemble modern resin exudates on coniferous trunks. We describe following micro-inclusions. Actinomycetes: Cardonia stellata, nov. gen., nov. sp., located close to the surface of amber pieces, is abundant and displays chains of conidia and isolated aleuriospore. Nocardiopsis ? sp. D is rare. Actinomycete "de type Salignac" is abundant. Its filaments often display a tendril shape, which seems to prelude to a mycelium fragmentation. Other bacteria: Leptotrichites resinatus Schmidt (Schmidt and Schäfer, 2005), poorly represented, is more variable than the already known material; cf. Sphaerotilus sp., very abundant, also displays differences with the Cenomanian "Sphaerotilus sp.". Eukaryotes: one fungal filament, and a group of spores, pollens or cysts. Inorganic inclusions: gas bubbles, pseudo-protists of B and C? types, and tiny, transparent, cubic crystals. It seems that most of the quoted prokaryotes were resinicolous organisms, able to settle on the surface of the exudate, and grow in the resin, after inoculation either by a contact with the substrate, or by an anemophilic dispersion of spores. This "taphonomic way" seems here to be more general than trapping. © 2013 Elsevier Masson SAS. Source


Lesnoff M.,CIRAD - Agricultural Research for Development | Corniaux C.,CIRAD - Agricultural Research for Development | Hiernaux P.,Geosciences Environnement Toulouse GET
Ecological Modelling | Year: 2012

Dynamics of cattle populations in arid and semi-arid tropical Africa are highly influenced by droughts, which can create dramatic drops in herd sizes as well as disturbances in sex-and-age structures. The Sahel region is particularly affected by such climate shocks. Successive major droughts are assumed to have effectively decreased the cattle stock and strongly influenced the evolution of pastoral and agro-pastoral systems in the past 40 years and probably before. Demographic resilience, i.e. the ability to recover from significant losses, is a key parameter for the sustainability of livestock populations in systems regularly perturbed by demographic shocks, and thus for prospective studies of the livestock sector, in particular for cattle that are more vulnerable to feed shortage than small ruminants and have a slower biological turnover. Here, a simple mathematical herd growth model is used to simulate the post-drought dynamics of a hypothetical Sahelian cattle population taken as example. A set of scenarios describing the drought severity, herd performances and management practices was considered in a global sensitivity analysis. The resilience was measured by the probability to recover, the recovery time of the population and its annual growth rate during the recovery period. An important finding of the study was the extreme variability of the recovery time. This variability challenges the common postulate according to which 10-15 years are needed for a cattle population to recover after a 'severe' drought. This also emphasizes the difficulty of obtaining reliable recovery predictions from inaccurate estimates of demographic parameters. Simulations showed that the proportion of population size decline due to the shock, the calving rate and farmers' offtake strategy after the drought are overall the most influential factors for recovery dynamics. Simulations also showed that the recovery time can be highly influenced by the transient regime of the herd growth model when the post-drought sex-and-age structure is non-equilibrated. In particular, the sensitivity analysis confirmed that large losses of breeding females during shocks considerably delay herd regeneration. © 2012 Elsevier B.V. Source


Becerra S.,Geosciences Environnement Toulouse GET | Peltier A.,University of Toulouse II - Le Mirail | Antoine J.M.,University of Toulouse II - Le Mirail | Labat D.,Geosciences Environnement Toulouse GET | And 4 more authors.
Hydrological Sciences Journal | Year: 2013

Urbanized areas subjected to moderate risks to human lives from flooding are often underestimated in studies on hydrological vulnerability, despite the roles they play in terms of crisis management. How do riverine residents face flood risk and what do they consider "good reasons" to take action? Based on a socio-geographical analysis of two recent floods (in 2000 and 2003) near Toulouse (southwest France), we show that the characteristics of the hazard in the studied watersheds influence the hydrological risk perception and thereby people's motivation to take action. Confronted by a major risk, society's vulnerability is clearly increasing. In order to improve the resilience of the population, we show that it is necessary to adapt the communication on risk: by personalizing information, improving the hydrological explanation of the flooding process, and initiating new forms of mediation between water management or administration and the riverine inhabitants. © 2013 Copyright 2013 IAHS Press. Source

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