Land clearance and hydrological change in the Sahel: SW Niger   总被引：1，自引：0，他引：1  

Marc J. Leblanc  Guillaume Favreau  Sylvain Massuel  Sarah O. Tweed  Maud Loireau  Bernard Cappelaere 《Global and Planetary Change》2008,61(3-4):135-150

In the West African semiarid belt of the Sahel, for the second half of the XXth century, lasting droughts (1970s–1980s) and one of the World's highest population growths have resulted in major land cover and hydrological changes that can be quantified using aerial photographs. This paper aims to provide one of the longest combined observations of land cover and hydrological changes for semiarid areas using a time series of normalised mosaics of aerial photographs dating back from 1950, field inquiries, and updated groundwater data. The 500 km² study area in southwest Niger was chosen (i) for its rural environment representative of the rain-fed agriculture belt of the Sahel and (ii) to encompass the main hydrological study sites investigated in this region over the past two decades (Hapex-Sahel and AMMA experiments, 1990–2000s). Results have significant implications for future freshwater availability and food security in the Sahel.Between 1950 and 1992,  80% of the study area has been cleared, firstly to open new areas for agriculture and secondly for firewood supply (59% of the plateaux, 42% of the valley bottoms, and 87% of the hillslopes). Intermediate aerial photograph surveys (1960, 1975) attest an accelerated loss in the woody savannah that could not be recovered on the short term. A strong, indirect impact of land clearance is observed on the water resources. Land clearance has resulted in a modification of the soil properties and infiltration capacity and has led to an increase in Hortonian runoff collected in numerous gullies and ponds. Between 1950 and 1992, aerial photographs show a  2.5 fold increase of the drainage density with the development of large drainage systems and new ponds. Groundwater data also indicate a continuous rise in the water table, mostly noticeable since the 1980s with a mean groundwater level rise of  4 m for the 1963–2005 period (+ 15% in aquifer reserves). The relatively short  30 year time-lag between the onset of land clearance and the beginning of the water table rise is linked to the process of indirect groundwater recharge and is timed with the connectivity of the drainage network and the formation of new ponds. Finally, the sustained increase in surface runoff and groundwater recharge during the past four decades indicates that the indirect impact of land clearance on the terrestrial water balance has been stronger than that of the long-lasting Sahelian drought. As the rate of land clearance increased for the past century in semiarid Africa, its main hydrological effects may not yet be fully perceptible.  相似文献   

Edge Flow and Canopy Structure: A Large-Eddy Simulation Study   总被引：4，自引：4，他引：0  

Sylvain Dupont  Yves Brunet 《Boundary-Layer Meteorology》2008,126(1):51-71

Sharp heterogeneities in forest structure, such as edges, are often responsible for wind damage. In order to better understand the behaviour of turbulent flow through canopy edges, large-eddy simulations (LES) have been performed at very fine scale (2 m) within and above heterogeneous vegetation canopies. A modified version of the Advanced Regional Prediction System (ARPS), previously validated in homogeneous conditions against field and wind-tunnel measurements, has been used for this purpose. Here it is validated in a simple forest-clearing-forest configuration. The model is shown to be able to reproduce accurately the main features observed in turbulent edge flow, especially the “enhanced gust zone” (EGZ) present around the canopy top at a few canopy heights downwind from the edge, and the turbulent region that develops further downstream. The EGZ is characterized by a peak in streamwise velocity skewness, which reflects the presence of intense intermittent wind gusts. A sensitivity study of the edge flow to the forest morphology shows that with increasing canopy density the flow adjusts faster and turbulent features such as the EGZ become more marked. When the canopy is characterized by a sparse trunk space the length of the adjustment region increases significantly due to the formation of a sub-canopy wind jet from the leading edge. It is shown that the position and magnitude of the EGZ are related to the mean upward motion formed around canopy top behind the leading edge, caused by the deceleration in the sub-canopy. Indeed, this mean upward motion advects low turbulence levels from the bottom of the canopy; this emphasises the passage of sudden strong wind gusts from the clearing, thereby increasing the skewness in streamwise velocity as compared with locations further downstream where ambient turbulence is stronger.  相似文献   

Suitability of airborne laser scanning for the assessment of forest protection effect against rockfall     

Jean-Matthieu Monnet  Franck Bourrier  Sylvain Dupire  Frederic Berger 《Landslides》2017,14(1):299-310

Rockfall simulation models are now able to quantify the protective effect of forest with the integration of rock impacts on trees. Those models require spatially explicit forest characteristics which are costly to acquire in operational conditions. The present study compares rockfall simulation results obtained with different forest input data sources: field data with different levels of spatial detail and two methods based on airborne Lidar data. Three different forest stands are tested with several virtual terrain configurations. When rockfall energies are below 200 kJ, the forest protection effect is significant. For higher energies, it also exists but it is minor compared to the effects of topography and rock volume. For all forest input data sources, the estimated rockfall intensity is within ?13 and 16 % of the reference value, whereas the frequency is generally overestimated. Both Lidar methods yield a satisfactory forest protection effect evaluation, but single tree detection tends to underestimate it. Improvements are possible regarding the spatial heterogeneity of stem density and the diameter distribution by tree species.  相似文献   

Topical Collection: Groundwater-based agriculture in the Mediterranean     

Marcel Kuper  Christian Leduc  Sylvain Massuel  Sami Bouarfa 《Hydrogeology Journal》2017,25(6):1525-1528

This essay introduces a collection of articles that explore the future of groundwater-based agriculture in the Mediterranean from an interdisciplinary perspective, in a context of declining water tables due to intensive groundwater use. The imminent crisis that many groundwater economies face due to very rapid and intense global change may have severe irreversible social, economic and environmental consequences, but could also be the opportunity to make a clear break with current agricultural development models and move towards more sustainable agricultural practices. The Mediterranean region is, therefore, an interesting case for the future of intensive groundwater use, as innovative ideas and practices may emerge and inspire similar groundwater-based agricultural systems around the world.  相似文献   

Reassessment of historical sections from the Paleogene marine margin of the Congo Basin reveals an almost complete absence of Danian deposits     

Floréal Solé  Corentin Noiret  Delphine Desmares  Sylvain Adnet  Louis Taverne  Thierry De Putter  Florias Mees  Johan Yans  Thomas Steeman  Stephen Louwye  Annelise Folie  Nancy J.Stevens  Gregg F.Gunnell  Daniel Baudet  Nicole Kitambala Yaya  Thierry Smith 《地学前缘(英文版)》2019,10(3):1039-1063

The early Paleogene is critical for understanding global biodiversity patterns in modern ecosystems. During this interval, Southern Hemisphere continents were largely characterized by isolation and faunal endemism following the breakup of Gondwana. Africa has been proposed as an important source area for the origin of several marine vertebrate groups but its Paleogene record is poorly sampled, especially from sub-Saharan Africa. To document the early Paleogene marine ecosystems of Central Africa, we revised the stratigraphic context of sedimentary deposits from three fossil-rich vertebrate localities: the Landana section in the Cabinda exclave(Angola), and the Manzadi and Bololo localities in western Democratic Republic of Congo.We provide more refined age constraints for these three localities based on invertebrate and vertebrate faunas, foraminiferal and dinoflagellate cyst assemblages, and carbon isotope records. We find an almost complete absence of Danian-aged rocks in the Landana section, contrary to prevailing interpretations over the last half a century(only the layer 1, at the base of the section, seems to be Danian). Refining the age of these Paleocene layers is crucial for analyzing fish evolution in a global framework, with implications for the early appearance of Scombridae(tunas and mackerels) and Tetraodontiformes(puffer fishes). The combination of vertebrate fossil records from Manzadi and Landana sections suggests important environmental changes around the K/Pg transition characterized by an important modification of the ichthyofauna. A small faunal shift may have occurred during the Selandian. More dramatic is the distinct decrease in overall richness that lasts from the Selandian to the Ypresian. The Lutetian of West Central Africa is characterized by the first appearance of numerous cartilaginous and bony fishes. Our analysis of the ichthyofauna moreover indicates two periods of faunal exchanges: one during the Paleocene, where Central Africa appears to have been a source for the European marine fauna, and another during the Eocene when Europe was the source of the Central Africa fauna. These data indicate that Central Africa has had connections with the Tethyian realm.  相似文献   

Coupes Sismiques des Structures Superficielles dans les Petites Antilles—II: Martinique     

Sylvain Eschenbrenner  Jacques Dorel  Jean-Pierre Viode 《Pure and Applied Geophysics》1980,118(2):807-822

Résumé En 1977, deux profils sismiques ont été réalisés en Martinique, un dans la région volcanique de la Montagne Pelée, l'autre dans la partie est de l'île. Le premier couvrait des distances de 3 à 23 km et le second de 2 à 50 km.Les structures superficielles du volcan sont interprétées par un modèle à trois couches. Sur une couche déjà connue, à 6,1 km/s nous trouvons, avec une épaisseur de 0,4 à 2,6 km, une vitesse de 4,3 km/s. En surface et jusqu'à une profondeur de 0,8 à 1,4 km la vitesse est d'environ 2,7 à 2,9 km/s.Dans la partie est de la Martinique, il existe une vitesse élevée-6,5 km/s—à partir d'une profondeur faible, de l'ordre de 3 km.

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In 1977, two seismic profiles were carried out in Martinique, one in the area of Montagne Pelée volcano, and the second in the eastern part of the island. The length of the first profile ranges from 3 to 23 km and the second from 2 to 50 km. The interpretation shows that the superficial structures of the volcano may be characterized by a three layer model: in the first layer the compressional velocity is about 2.7 to 2.9 km/s, the thickness varying from 0.8 to 1.4 km. The second layer has a velocity of 4.3 km/s and the thickness goes from 0.4 to 2.6 km. The deeper layer known at Leeward West Indies has a 6.1 km/s velocity.The second profile shows that the eastern part of Martinique is marked by a high velocity of 6.5 km/s, found at a depth of 3 km.

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Sources of fine-sized organic matter in North Atlantic Heinrich Layers: δC and δN tracers     

Sylvain Huon  Francis E. GroussetDidier Burdloff  Gérard BardouxAndré Mariotti 《Geochimica et cosmochimica acta》2002,66(2):223-239

Organic carbon (OC) and total nitrogen (TN) concentrations and stable isotope ratios (δ¹³C, δ¹⁵N) of fine (<50 μm) size fractions of deep-sea sediments from the central North Atlantic were employed to identify changes in sources of organic matter over the past 50 ka BP. Ambient glacial sediments are characterised by values that reflect mixtures of marine and terrestrial inputs (averages ± 1σ: OC/TN = 7.6 ± 0.8; δ¹³C = −22.8 ± 1.0‰; δ¹⁵N = 5.5 ± 0.6‰). δ¹³C, OC, and TN concentrations shift to higher values during the Holocene, indicating a gradual decrease of fine terrigenous supply to the North Atlantic. The unchanged δ¹⁵N record between last glacial and Holocene stages indicates that the central North Atlantic region remained oligotrophic at least during the past 50 ka BP, but additional studies are required to support this result in terms of nitrogen oceanic budget. During the phases of enhanced ice-rafted detrital supply corresponding to prominent Heinrich events (HL₁, HL₂, HL₄, and HL₅), fine-sized sedimentary organic matter has lower OC and TN concentrations, contrasting sharply with those of ambient glacial sediments. Lower δ¹³C (down to −28‰) and δ¹⁵N (down to 1.6‰) values and high OC:TN ratios (up to 14.7 ± 1.1) are found for HL₁, HL₂, and with lesser extent for HL₄. These values reflect enhanced detrital supply originating from poorly differentiated soil horizons that characterise periglacial climate conditions and from organic matter-bearing rock sources of the underlying geological basement. During HL₅, only the δ¹³C offset records the input of fine size ice-rafted organic matter. Gradually changing soil development conditions during the time interval covering HL₅ to HL₁ (marine isotope stages 5 to 2), as well as varying erosion levels, have been hypothesized on the basis of constant δ¹³C, increasing OC/TN and decreasing δ¹⁵N values.  相似文献   

Simulation of Meteorological Fields Within and Above Urban and Rural Canopies with a Mesoscale Model   总被引：3，自引：4，他引：3  

Sylvain Dupont  Tanya L. Otte  Jason K. S. Ching 《Boundary-Layer Meteorology》2004,113(1):111-158

Accurate simulation of air quality at neighbourhood scales (on order of 1-km horizontal grid spacing) requires detailed meteorological fields inside the roughness sub-layer (RSL). Since the assumptions of the roughness approach, used by most of the mesoscale models, are unsatisfactory at this scale, a detailed urban and rural canopy parameterisation, called DA-SM2-U, is developed inside the Penn State/NCAR Mesoscale Model (MM5) to simulate the meteorological fields within and above the urban and rural canopies. DA-SM2-U uses the drag-force approach to represent the dynamic and turbulent effects of the buildings and vegetation, and a modified version of the soil model SM2-U, called SM2-U(3D), to represent the thermodynamic effects of the canopy elements. The turbulence length scale is also modified inside the canopies. SM2-U(3D) assesses the sensible and latent heat fluxes from rural and urban surfaces in each of the computational layers inside the canopies by considering the shadowing effect, the radiative trapping by the street canyons, and the storage heat flux by the artificial surfaces. DA-SM2-U is tested during one simulated day above the city of Philadelphia, U.S.A. It is shown that DA-SM2-U is capable of simulating the important features observed in the urban and rural RSL, as seen in the vertical profiles of the shear stress, turbulent kinetic energy budget components, eddy diffusivity, potential air temperature, and specific humidity. Within the canopies, DA-SM2-U simulates the decrease of the wind speed inside the dense canopies, the skirting of the flow around the canopy blocks, warmer air inside the vegetation canopy than above open areas during the night and conversely during the day, and constantly warmer air inside the urban canopy. The comparison with measurements shows that the surface air temperature above rural and urban areas is better simulated by DA-SM2-U than by the `standard version' of MM5.  相似文献   

Seismic Microzonation: A Comparison between Geotechnical and Seismological Approaches in Pointe-à-Pitre (French West Indies)   总被引：1，自引：0，他引：1  

Benoit LeBrun  Anne-Marie Duval  Pierre-Yves Bard  Olivier Monge  Myriam Bour  Sylvain Vidal  Hubert Fabriol 《Bulletin of Earthquake Engineering》2004,2(1):27-50

Pointe-à-Pitre, the main city of Guadeloupe in the French West Indies, has on several occasions been partially destroyed by major historical earthquakes. Moreover, a post-seismic assessment of the damage from the 1985 Montserrat earthquake indicates that the town is prone to site effects. Consequently, from 1996 to 1998, BRGM conducted a seismic microzonation study based on geotechnical and geological data. At the same time, three seismological studies were being conducted – two based on earthquake recordings using a time-series analysis and the classical spectral ratio (CSR) method (CETE/LCPC and BRGM), and the third based on noise measurement at 400 points using the horizontal-to-vertical noise ratio (HVNR) method (CETE/LCPC). The objective of this paper is not to carry out a new microzonation study by taking into account all the results, but rather to show in what respects the results of these different methods are in agreement or not. A comparison of the results of the seismological studies with the geotechnical microzonation shows that they are in fairly good agreement, albeit with some discrepancies. The results indicate that the seismological methods and the geotechnical data are highly complementary and should be used together in compiling seismic transfer-function microzonation maps. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

The surface energy balance and the mixing height in urban areas—activities and recommendations of COST-Action 715     

Martin Piringer  Sylvain Joffre  Alexander Baklanov  Andreas Christen  Marco Deserti  Koen De Ridder  Stefan Emeis  Patrice Mestayer  Maria Tombrou  Douglas Middleton  Kathrin Baumann-Stanzer  Aggeliki Dandou  Ari Karppinen  Jerzy Burzynski 《Boundary-Layer Meteorology》2007,124(1):3-24

The specific problems of determining and simulating the surface energy balance (SEB) and the mixing height (MH) over urban areas are examined. The SEB and MH are critical components of algorithms and numerical models for the urban boundary layer, though the constituent parts of the SEB and the MH are not routinely measured by national weather services. Parameterisations are thus needed in applications. In this investigation, several recently developed algorithms and models for estimating the SEB and MH were applied to new datasets and assessed. Results are discussed in terms of the need for spatial resolution and the parameters needed to describe the urban atmosphere. Limitations of models are identified and recommendations for further development and observations are given. Having identified gaps in knowledge, key findings from new urban experiments and numerical modelling for the SEB and MH are given. The diurnal cycle for the SEB is significantly different from rural conditions—urban heat storage is needed in urban parameterisations. The urban MH is increased over the rural MH, as shown by several numerical schemes and careful sodar analyses. This work has been carried out within the COST-715 Action “Meteorology applied to urban air pollution problems (1998–2004). COST 715 reached a consensus proposing representatively sited measurements of meteorological parameters and turbulent fluxes above roof-tops, and recognised that such data are needed to improve numerical models of the urban surface processes.  相似文献