Monitoring the evolution of irrigated areas with Landsat images using backward and forward change detection analysis in the Kou watershed,Burkina Faso

The Kou watershed is characterized by important water resources used for drinking, agriculture (especially in the irrigated areas), industry and the preservation of aquatic fauna and flora. For several decades, there has been increasing pressure on the Kou's water resources, partly because of the expansion of the irrigated agricultural areas. This study was conducted to examine this issue, focusing on one specific irrigated area. In order to monitor the expansion of irrigated areas in developing countries, a low-cost remote sensing method based on Landsat images and aerial photographs was developed. The method is based on maximum-likelihood classification, followed by backward and forward change detection analysis requiring agronomic expertise. Using pixel trajectory analysis, the method connects all pixels to their consecutive states in order to correct their current states. The study showed that the irrigated area has expanded by almost 70% over 20 years, with most of this expansion occurring in the past 10 years. The method, if validated, could be used to obtain better information on past occupation in the rural irrigated areas for which there is currently no archived data, making temporal analyses impossible.     

Magnetotellurics applied to the study of the crustal structures in Senegal (West Africa)

Abstract

In Senegal, magnetotellurie (MT) method has been used in an attempt to resolve the principal structural features by their electrical response. On the basis of numerical modelling of data, an unified model of possible crustal structure is presented for the West african margin. The results are in agreement with other independent geophysical and geological information.     

La couverture océanique des ultrabasites de Lanzo (Alpes occidentales) : arguments lithostratigraphiques et pétrologiques

Résumé

De nombreuses analogies entre les évolutions pétro-logiques et métamorphiques des ultrabasites de Lanzo et des ophiohtes liguro-piémontaises ont été soulignées récemment. Ces analogies concernent: la nature des ultrabasites (Iherzolites à plagioclase peu appauvries), la présence de gabbros et de filons basaltiques superficiels intrusifs dans les péridotites, l’existence de reliques de paragenèses métamorphiques de haute température attribuables à un épisode océanique, l’empreinte du métamorphisme alpin de haute pression. Ces faits ont conduit certains auteurs à proposer que les péridotites de Lanzo représentent un fragment du plancher océanique téthysien.

Nous avons tenté de vérifier cette hypothèse en recherchant la couverture océanique (volcanique et/ou sédimentaire) des péridotites de Lanzo, dans la partie occidentale du massif où des pincées de métabasites et de schistes lustrés s.l. sont visibles associées aux serpentinites.

Aux environs du village de Richiaglio, à l’Est de la cicatrice de Viù, les successions lithostratigraphiques reconstituées comprennent schémati-quement : les ultrabasites serpentinisées du corps de Lanzo, des métabasites, des quartzites manganésifères (comparables à celles du Mont Viso par exemple), et des calcschistes. Ces successions s’apparentent à celles décrites dans les couvertures d’autres ophiolites piémontaises, ce qui conduit à proposer que ces ensembles métavolcaniques et Dimetaires dimentaires représentent des résidus de la couverture océanique originelle des péridotites. L’analyse des paragenèses éclogitiques des métasédiments et des métabasites permet de montrer que l’évolution P,T alpine de ces lambeaux est compatible avec celle du corps de Lanzo. Cette interprétation et ses implications sont discutées.     

Karst et évolution des rivières : le cas de l’Ardenne

Résumé

Dans les régions karstiques comprenant des vallées épigénétiques, les phénomènes tectoniques ont deux influences majeures : a) la surrection donne de l’énergie potentielle en créant des différences d’altitude, générant à la fois le creusement des vallées et celui du karst ; b) une tectonique est nécessaire pour que le processus de karstification débute. L’opposition entre le développement privilégié soit des vallées, soit des réseaux karstiques dépend ainsi du type dissipée. Les réseaux karstiques de l’Ardenne se situent surtout entre 5 et 15 m au-dessus de la surface piézométrique pour les réseaux secs, et dans la zone saturée pour les parties actives. La structuration de ces réseaux karstiques est favorisée par un ralentissement de la surrection et une activité tectonique. Cette période se situe dans le Pléistocène moyen et récent, d apres les datations U/Th de spéléothèmes. © Elsevier, Paris     

Sedimentary diatoms along a temporal and spatial gradient of metal contamination

We examined changes at the community and population level of sedimentary diatoms over a wide temporal and spatial gradient of metal pollution encountered in cores from three lakes in the Abitibi mining region (Québec, Canada). Diatom communities on the whole appeared to be very tolerant of metal contamination, as shown by diatom cell accumulation rates decreasing only under the most severe conditions of contamination, which were found from the 1930s to the 1980s in Lac Dufault (cadmium, up to 94 μg/g dry sediment; Cu, up to 8600 μg/g; Zn, up to 9000 μg/g). Under the moderate conditions of contamination observed in the other two lakes and in the most recent sediment of Lac Dufault, diatom cell accumulation rates tended to increase over values typical of the pre-mining period. However, there were increasing rearrangements of the community composition along the contamination gradient. Under moderate metal enrichment, the diatom community of Lac Vaudray experienced only subtle changes, with Cyclotella stelligera, albeit decreasing, remaining the dominant taxon. In the intermediately contaminated Lac Caron, several benthic taxa, noticeably Cymbella silesiaca and several Fragilaria species, rose in taxonomic importance. The most extreme contamination observed in Lac Dufault led to a severely impoverished community almost entirely represented by Achnanthes minutissima and Brachysira vitrea. With increasing levels of contamination, there was a shift from planktonic to benthic taxa and morphotypes and a consistent decrease in the siliceous stomatocysts/diatom frustules ratio. These trends suggest that littoral zones may represent an important refugium under conditions of high contamination. Responses to metal stress were in general more evident at the population than at the community level. Cyclotella stelligera and B. vitrea had a consistent negative and positive response, respectively, along the gradient and are the most promising indicators of metal pollution for this region.     

Hydro-climatic analysis of mechanical breakups reconstructed from tree-rings, Necopastic watershed, northern Québec, Canada

We used a 54-year (1950–2003) ice scar chronology constructed from damaged trees to investigate relationships between large scale hydro-climatic conditions and mechanical breakup magnitude in the Necopastic River watershed (James Bay, northern Québec). Our objectives were: (1) to identify hydrologic and climatic variables that explain variations in mechanical breakup magnitude at the watershed scale, (2) to organize these variables in terms of importance and (3) to construct a predictive model for ice-floods in the Necopastic watershed. We used parametric correlation analysis to measure the degree of linear association between variables and classification trees (CT) coupled with a cross-validation approach to construct the predictive model and to organize variables according to their importance. In the Necopastic watershed, the type and magnitude of breakups are determined by an interaction between forces that maintain the ice cover in place and forces that tend to dislodge it. In contrast with lakes, early and rapidly rising floods, rather than abnormally high flood discharge, create conditions that favour intense mechanical breakups. Moreover, cold and snowy spring conditions during high positive Arctic Oscillation (AO) indices delay the thermal degradation of the ice cover and preserve its mechanical properties over longer time periods.     

Factors Influencing the Stream‐Aquifer Flow Exchange Coefficient

Knowledge of river gain from or loss to a hydraulically connected water table aquifer is crucial in issues of water rights and also when attempting to optimize conjunctive use of surface and ground waters. Typically in groundwater models this exchange flow is related to a difference in head between the river and some point in the aquifer, through a “coefficient.” This coefficient has been defined differently as well as the location for the head in the aquifer. This paper proposes a new coefficient, analytically derived, and a specific location for the point where the aquifer head is used in the difference. The dimensionless part of the coefficient is referred to as the SAFE (stream‐aquifer flow exchange) dimensionless conductance. The paper investigates the factors that influence the value of this new conductance. Among these factors are (1) the wetted perimeter of the cross‐section, (2) the degree of penetration of the cross‐section, and (3) the shape of the cross‐section. The study shows that these factors just listed are indeed ordered in their respective level of importance. In addition the study verifies that the analytical correct value of the coefficient is matched by finite difference simulation only if the grid system is sufficiently fine. Thus the use of the analytical value of the coefficient is an accurate and efficient alternative to ad hoc estimates for the coefficient typically used in finite difference and finite element methods.     

BIOB: A mathematical model for the biodegradation of low solubility hydrocarbons

Modeling oil biodegradation is an important step in predicting the long term fate of oil on beaches. Unfortunately, existing models do not account mechanistically for environmental factors, such as pore water nutrient concentration, affecting oil biodegradation, rather in an empirical way. We present herein a numerical model, BIOB, to simulate the biodegradation of insoluble attached hydrocarbon. The model was used to simulate an experimental oil spill on a sand beach. The biodegradation kinetic parameters were estimated by fitting the model to the experimental data of alkanes and aromatics. It was found that parameter values are comparable to their counterparts for the biodegradation of dissolved organic matter. The biodegradation of aromatics was highly affected by the decay of aromatic biomass, probably due to its low growth rate. Numerical simulations revealed that the biodegradation rate increases by 3–4 folds when the nutrient concentration is increased from 0.2 to 2.0 mg N/L.     

Twenty years of groundwater evolution in the Triassic sandstone aquifer of Lorraine: Impacts on baseline water quality

The Lorraine Triassic Sandstone Aquifer (LTSA), which has already been the subject of a chemical and radioisotopic study (1979), is used to investigate the impacts of 20 a of large scale pumping on baseline water quality. In parallel, new sampling of the aquifer (2001) provides new inorganic geochemical data (including trace elements) that allow improving the knowledge of baseline conditions and hydrochemical functioning of a major sandstone aquifer. The good correlation between ¹⁴C activities, temperature and depth along the main flow line indicate regular downgradient trends and possible water stratification. Unreactive tracers, mainly stable isotope ratios ¹⁸O and ²H, as well as C isotopes are used to define a timescale for the aquifer, showing two groups of groundwater, namely of modern and Holocene age, and late Pleistocene age, with a mixing zone. Baseline quality is then represented by a wide range of concentrations, mainly the result of time-dependent water–rock interaction, as already observed elsewhere in Triassic sandstone aquifers. Some trace elements such as Li, Rb, Cs, which are not limited by solubility constraints, show linear trends. During saturated flow downgradient, the chemistry is also specifically characterised by a regular increase in Na and Cl (and locally SO₄) as a result of evaporite dissolution related to overlying or basement limits. The aquifer is mostly oxidising with a redox boundary marked by U decrease, some 40 km from outcrop.