Risques D’inondation Et Proposition D’un Plan D’évacuation Des Eaux De Pluie Dans La Ville De Kantché, Région De Zinder Au Niger

Le Niger connait une hausse exponentielle du nombre d’inondations consécutives à de fortes pluies. Ce nombre est passé en moyenne à moins de deux (2) par an avant 1990 à plus de huit (8) par an au cours des années 2000. Ainsi, ces inondations surviennent chaque année dans l’ensemble des villes nigériennes mais aussi les zones rurales. La présente étude porte sur le cas de la ville de Kantché dans la région de Zinder qui, avec une population de 27615 habitants en 2012, est sujette aux inondations récurrentes et aux problèmes d’assainissement liés à la mauvaise gestion des eaux pluviales. A titre d’exemple cette commune a connu des inondations graves au cours des années 1998, 2007, 2012 et tout récemment en 2016, 2017 et 2018. Ces inondations ont occasionné de nombreux dégâts matériels : Effondrement d’habitations, des puits, ruptures des digues, perte de bétail, et perte en vies humaines. L’objectif global de l’étude vise à analyser les facteurs de l’augmentation du ruissellement et de la recrudescence des inondations dans cette commune afin de proposer un plan d’évacuation des eaux pluviales. A travers une approche diachronique, il s’agit d’analyser les cartes d’occupation de sol dans le bassin versant de cette ville afin de faire un lien entre celles-ci et l’augmentation de ruissellement et du ravinement. Les données topographiques et géomorphologiques couplées aux observations de terrain, plus les informations collectées auprès des populations de Kantché permettent d’établir une cartographie des zones à risque d’inondation afin de proposer un plan d’évacuation des eaux pluviales efficace. Niger faces very high raise in number of floods resulting from heavy rains. The average passes from two (2) per year before 1990 to over than eight (8) in the years 2000. These occur not only in towns, cities but also in Nigerien rural areas. This study deals with in the town of Kantche in Zinder region with a population of 27,615 people in 2012 and subjected to recurrent floods and sanitation problems in connection with the mismanagement of rain water. The commune has witnessed severe floods in 1998, 2007, 2012 and very recently in 2016, 2017 and 2018 the floods have engendered a number of damages: house and wells collapses, levee ruptures, kettle and even human loss. The overall objective of this study is to analyze factors that enhance the streaming and the recrudescence of floods in that community in to suggest vital solutions. Through this diachronic approach, land occupation maps are analyzed in the town basin in order to establish a link between the land occupation and the high streaming and furrow. Topographic and geomorphological data combined with field observations and information obtained from Kantché populations will enable us to mapping flood risk zone in order to work out an efficient rain water draining map

Role of a cyanobacterial cover on structural stability of sandy soils in the Sahelian part of western Niger

Microbiotic soil crusts, mostly formed by cyanobacteria, are widespread on the surface of fallow land in western Niger. They lie adjacent to completely bare soils. We have investigated the structural stability of these soils by testing aggregate breakdown under fast-wetting, slow-wetting and mechanical breakdown. The tests were effected on aggregates ranging from 3 to 5 mm in size. These experiments were completed by micromorphological examinations under light and scanning electron microscope. For all tests, the fragment size distribution and the mean weight diameter (MWD) revealed the great stability of aggregates from soils with a cyanobacterial cover (MWD 1.82 to 3.10 mm), compared to those from soils devoid of cyanobacterial cover (MWD 0.25 to 1.26 mm). Fast- and slow-wetting of microbiotic soil crust material induced a lesser disaggregation of aggregates compared to mechanical breakdown. On the contrary, fast-wetting and mechanical breakdown of aggregates from soils devoid of cyanobacterial cover induced a greater disaggregation than slow-wetting. Microscopic examination of microbiotic crusts revealed an intricate network of filamentous cyanobacteria and extracellular polymer secretions, which binds and entraps mineral particles on the soil surface. Organo-mineral aggregates ranging from 0.10 to 0.12 mm were observed. Below the superficial crusts, filaments and residual organic matter bind tightly soil particles, thus giving a compact structure. The great stability of aggregates of soil with cyanobacterial cover is likely related to the binding and gluing effect of cyanobacteria and derived organic matter. This is consistent with the positive correlation between MWD values and organic carbon content. The present results thereby confirm the resistance to erosion of soil with microbial cover as indicated by field measurements

Microbiotic soil crusts in the Sahel of Western Niger and their influence on Soil POROSITY and water dynamics

International audienceMicrobiotic soil crusts are common features of the surface of fallow land in Western Niger. We investigated the interaction between these microbial covers and the porosity and water dynamics of soils at the surface of a Sahelian landscape. The soil pore system was examined by microscopic observations and mercury porosimetry. The soil water retention capacity was measured using a Richard pressure membrane apparatus. Runoff measurements were performed in situ at a one m2 scale under natural rainfall. Samples with dense cyanobacterial cover had microbially-originated pores ranging from 0.04 and 50 μm in size. These samples also showed higher total soil porosity and retained two to four times more water than samples with thin microbial cover. These properties are closely related to the hydrophobic nature of cyanobacterial components. Final runoff values obtained on densely covered surfaces were significantly higher compared to those measured on surfaces with thin microbial cover due to the geometry of the microbially-originated pore system and its functioning

Apports du cryo-microscope électronique à balayage à émission de champ à l'étude des matières organiques et des relations organo-minérales naturelles. Application aux croûtes microbiotiques des sols Field emission cryo-scanning electron microscopy of organic matter and organomineral associations. Application to microbiotic soil crusts

The cryo-SEM (SEM equipped with a freeze-drying sample preparation system) allows the micromorphological transformations occurring during desiccation-rewetting cycles of microbiotic crusts from Sahelian soils to be visualised, like the swelling up of mucilaginous envelopes of the constituent micro-organisms through water absorption. When the cryo-SEM is equipped with a field emission gun, which permits observations at low voltage operation (1 kV) without coating, the natural surface of the samples may be revealed, in particular the presence, at the surface of mineral grains, of microbe-derived organic meshes that probably play a role in the resistance of these crusts to erosion. Résumé Le cryo-Meb (Meb équipé d'un système de préparation des échantillons par congélation-lyophilisation) permet de visualiser les modifications micromorphologiques se produisant lors des cycles de dessiccation-réhydratation des croûtes microbiotiques de sols sahéliens, comme le gonflement, par absorption d'eau, des enveloppes mucilagineuses des micro-organismes constitutifs. Lorsque le cryo-Meb possède un canon à émission de champ, qui autorise des observations à basse tension d'accélération (1 kV), sans metallisation, c'est l'état de surface naturel des échantillons qui peut être révélé, notamment la présence, autour des grains minéraux, de réseaux organiques d'origine microbienne, jouant vraisemblablement un rôle dans la résistance à l'érosion manifestée par ces croûtes

Long-term effect of forest and landscape restoration practices on soil organic carbon stock in semi-arid Burkina Faso

In semi-arid areas, forest and landscape restoration (FLR) practices are being implemented to reverse the land degradation process. The objective of this study was to investigate the long-term effect of FLR practices on soil organic carbon stock (SOCs) under different land uses in the semi-arid region of Burkina Faso. The study was conducted on degraded land under rehabilitation practices for 45, 27, 18 and 11 years, which were compared to similar land without specific rehabilitation measures. The soil was collected in 2018 in 35 sampling plots of 30 m x 30 m. Soil analysis concerned bulk density, soil particle size, soil pH, soil organic carbon content, and respiratory activity of microorganisms. SOCs increased by 150%, 98% and 29% over 0-10 cm depth in 45-, 27- and 11-year of FLR practices, and decreased by 6% in 18-year of FLR practices compared to their respective control. SOCs were not linearly increased with the duration of the implementation of FLR practices because the variation of SOCs depends on several other parameters such as soil texture, and types of combination of FLR practices. The highest SOCs were recorded for 27 years (9.5 t.ha-1) and 45 years (8.5 t.ha-1) of FLR practices. This study revealed the importance of including Assisted Natural Regeneration (ANR) as one of the combined FLR practices, for improvement of SOCs

Gully erosion prediction in a Sahelian context

In Sahelian region, concentrated overland flow often leads to the formation of gullies. Although this phenomenon is widespread in those regions, research efforts are still needed to be able to model their spatial distribution and the role of the different parameters involved in this process. In this context, the objectives of this study are twofold. The first step is to investigate to what extent the role of Sahelian soil surface crusts (biological and/or physical) on soil surface infiltrability and detachment affect the formation and development of gullies. The second step is to integrate the results of these investigations in a simple geomorphological model to predict gully location at the watershed scale. The evaluation of the resulting model on two test catchments demonstrated that the integration of soil crusting is a key parameter to insure the quality and relevance of gully prediction. The model is able to distinguish between two types of gullies, those whose width range between 0.5m and 4m and those whose width exceeds 4m. The application of the model at the regional scale is however limited by the resolution of available regional digital elevation model (i.e. the 90m resolution SRTM DEM) which only permits the prediction of large gullies

The pervasive and multifaceted influence of biocrusts on water in the world's drylands

The capture and use of water are critically important in drylands, which collectively constitute Earth's largest biome. Drylands will likely experience lower and more unreliable rainfall as climatic conditions change over the next century. Dryland soils support a rich community of microphytic organisms (biocrusts), which are critically important because they regulate the delivery and retention of water. Yet despite their hydrological significance, a global synthesis of their effects on hydrology is lacking. We synthesized 2,997 observations from 109 publications to explore how biocrusts affected five hydrological processes (times to ponding and runoff, early [sorptivity] and final [infiltration] stages of water flow into soil, and the rate or volume of runoff) and two hydrological outcomes (moisture storage, sediment production). We found that increasing biocrust cover reduced the time for water to pond on the surface (−40%) and commence runoff (−33%), and reduced infiltration (−34%) and sediment production (−68%). Greater biocrust cover had no significant effect on sorptivity or runoff rate/amount, but increased moisture storage (+14%). Infiltration declined most (−56%) at fine scales, and moisture storage was greatest (+36%) at large scales. Effects of biocrust type (cyanobacteria, lichen, moss, mixed), soil texture (sand, loam, clay), and climatic zone (arid, semiarid, dry subhumid) were nuanced. Our synthesis provides novel insights into the magnitude, processes, and contexts of biocrust effects in drylands. This information is critical to improve our capacity to manage dwindling dryland water supplies as Earth becomes hotter and drier.This work was conducted as part of the Powell Working Group “Completing the dryland puzzle: creating a predictive framework for biological soil crust function and response to climate change” supported by the John Wesley Powell Center for Analysis and Synthesis, funded by the US Geological Survey. J.B. and S.R. were funded by USGS Ecosystems and Land Use Change Mission Areas, by the US Department of Energy (DESC-0008168), and by the Strategic Environmental Research and Development Program (RC18-1322). J.D. is supported by grants from the Holsworth Wildlife Research Endowment & The Ecological Society of Australia, and a scholarship from China Scholarship Council (No. 201706040073). B.C. is supported by grants from the National Science Foundation (award DEB-1844531) and DePaul University. M.A.B. is supported by a grant from the National Science Foundation (award DEB-1638966). B.W. was supported by the Max Planck Society and a Paul Crutzen Nobel Laureate Fellowship. E.H.-S. was supported by CONACYT grant 251388 B. F.T.M. was supported by the European Research Council (ERC grant agreement 647038 [BIODESERT]) and Generalitat Valenciana (CIDEGENT/2018/041)

Impact of Drought and Land – Use Changes on Surface – Water Quality and Quantity: The Sahelian Paradox

International audienceAfrique de l'Ouest a connu des conditions de sécheresse depuis la fin des années 1960. Cette tendance a été particulièrement évident dans le Sahel, mais semble avoir atténué dans la dernière décennie dans les régions orientales et centrales de cette région. D'autre part, la pluviométrie annuelle reste très faible dans la partie ouest du Sahel [ 1 ]. Une diminution correspondante a également été observé dans le débit moyen annuel des fleuves Sénégal et Niger, qui sont le plus grand dans la région et principalement alimenté par l'eau provenant de régions tropicales humides. Toutefois, le pourcentage de diminution du débit annuel moyen était presque deux fois plus grande que la diminution des précipitations [ 2 ] pour la période 1970-2010. Des tendances similaires ont été observées sur des réseaux hydrographiques plus petits. En revanche, même si le Sahel et la plupart de l'Afrique de l'Ouest ont connu la sécheresse aussi importante au cours des 40 dernières années, les coefficients de ruissellement et des débits d'eau ont augmenté dans la plupart des régions du Sahel. Ce phénomène a été appelé «Le Sahel Paradox" après l'augmentation de la nappe phréatique au Niger depuis les années 1960, a été nommé le paradoxe de Niamey et attribués à des changements importants dans l'utilisation des terres. Le les (Afrique multidisciplinaire de la mousson d'analyse) programmes AMMA HAPEX-Sahel (hydrologique et Expérience atmosphérique pilote) et ont fourni, parmi de nombreux résultats complets, les mesures de valeur portant sur les variations spatiales et temporelles de la teneur en eau du sol sahélienne ainsi que de l'infiltration de l'eau à travers les couches profondes du sol de la zone non saturée. Le but de ce chapitre est de fournir un aperçu du comportement hydrologique en Afrique de l'Ouest basée sur le point, locale, méso et échelles régionales observations

Soil erosion processes in european vineyards: a qualitative comparison of rainfall simulation measurements in Germany, Spain and France

Small portable rainfall simulators are considered a useful tool to analyze soil erosion processes in cultivated lands. European research groups in Spain (Valencia, Málaga, Lleida, Madrid and La Rioja), France (Reims) and Germany (Trier) have used different rainfall simulators (varying in drop size distribution and fall velocities, kinetic energy, plot forms and sizes, and field of application) to study soil loss, surface flow, runoff and infiltration coefficients in different experimental plots (Valencia, Montes de Málaga, Penedès, Campo Real and La Rioja in Spain, Champagne in France and Mosel-Ruwer valley in Germany). The measurements and experiments developed by these research teams give an overview of the variety of methodologies used in rainfall simulations to study the problem of soil erosion and describe the erosion features in different climatic environments, management practices and soil types. The aims of this study are: (i) to investigate where, how and why researchers from different wine-growing regions applied rainfall simulations with successful results as a tool to measure soil erosion processes; (ii) to make a qualitative comparison about the general soil erosion processes in European terroirs; (iii) to demonstrate the importance of the development of standard method for measurement of soil erosion processes in vineyards, using rainfall simulators; and (iv) and to analyze the key factors that should be taken into account to carry out rainfall simulations. The rainfall simulations in all cases allowed infiltration capacity, susceptibility of the soil to detachment and generation of sediment loads to runoff to be determined. Despite using small plots, the experiments were useful to analyze the influence of soil cover to reduce soil erosion, to make comparisons between different locations, and to evaluate the influence of different soil characteristics. The comparative analysis of the studies performed in different study areas points out the need to define an operational methodology to carry out rainfall simulations, which allows us to obtain representative and comparable results and to avoid errors in the interpretation in order to achieve comparable information about runoff and soil los