Issue 3FIGHTING WIND EROSIONone aspect of the combatagainst desertification                    Comité Scientifique Français ...
Les dossiers thématiquesdu CSFD Issue 3Managing EditorRichard EscadafalPresident of CSFDResearch Director at CESBIO (Centr...
ForewordM          ankind is now confronted with an issue                             These reports are devoted to differe...
PreambleD        esertification and land degradation give rise                  This readiness to provide decisionmakers a...
ue t                 an d M . M ai ng    © F. Du m ay                                                                     ...
Wind erosion a complex environmental degradation mechanismW          i nd erosion is one of t he most ser ious           T...
Arrival of a sandstorm in the area of Mare d’Oursi, Oundalan, Burkina Faso.Note that behind the huts the red color of the ...
Wind erosion onset thresholds and land degradation triggering factors highlighted by remote sensingI    t is now recognize...
Oblique aerial view of the centre of the city of Nouakchott,                                                              ...
> FOCUS | Regional wind actionsystem (RWAS) on the easternside of the Aral Basin    Aral Basin (1.8 million km²) accounts ...
> EXAMPLE | Using aerial photographs to monitor sand dunes                          in Mauritania On an aerial photograph ...
Children carrying fuelwood collected at dawnon the dyke of the Sologo barrier lake (Côte d’Ivoire).                       ...
> FOCUS | Variations in the meaning                                                            The UNDP definition was ame...
> FOCUS | Uncontrolled building (gazra in Hassania) on longitudinal dunes on the outskirts of Nouakchott (Mauritania)     ...
Southeastern part of the fixed Erg Aouker on the Togbha-Tamchekket road, Mauritania, December 2001. Violent rains during th...
> FOCUS | Impact of soil              conditions: erosivity              and erodibility     The wind velocity at the soil...
> FOCUS | Soil drought—the result of                       wind erosion        In the 1950s, natural vegetation cover, esp...
Wind erosion mechanisms at the Man-Nature interfaceW         ind act ions t hat ta ke place at t he soi l-         atmosph...
J.J. Lemasson © IRDAeolian particle source area                              Longitudinal dunes are also found in particle...
> FOCUS | Wind impacts according                 to the substrate                 in the source area     • Desert pavement...
Seif dune reactivation in the Trarza sandsea on the eastern outskirts of Nouakchott (Mauritania, 2005). Reactivated seif d...
Sand accumulation area                                      their path. When dune movements are slowed down               ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. ...
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Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. CSFD/Agropolis International, Montpellier, France. 44 pp.

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Mainguet M. & Dumay F., 2011. Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. CSFD/Agropolis International, Montpellier, France. 44 pp. - Wind erosion—alone or combined with other physical or socioeconomic causes—is a mechanism that may induce desertification, i.e. severe or irreversible degradation of water and soil resources. Now that this phenomenon is better understood, the model of the 1970s based on three distinct stages (causes, mechanisms, consequences) has been discarded, in view of the many feedbacks and insidious links generated by wind erosion. Timely detection of wind erosion onset thresholds with remote sensing tools (satellite images and aerial photographs), and spatial delimitation and positioning of the phenomena observed are essential to be able to efficiently combat the damaging effects of wind erosion. No field operations can be effective without prior knowledge of wind erosion mechanisms at the land-atmosphere interface.

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Fighting wind erosion. one aspect of the combat against desertification. Les dossiers thématiques du CSFD. N°3. May 2011. CSFD/Agropolis International, Montpellier, France. 44 pp.

  1. 1. Issue 3FIGHTING WIND EROSIONone aspect of the combatagainst desertification Comité Scientifique Français de la Désertification French Scientific Committee on Desertification
  2. 2. Les dossiers thématiquesdu CSFD Issue 3Managing EditorRichard EscadafalPresident of CSFDResearch Director at CESBIO (Centre for the Studyof the Biosphere from Space, Toulouse, France) for IRD French Scientific Committee(Institut de recherche pour le développement, France) on DesertificationAuthorsMonique Mainguet The creation in 1997 of the French Scientific Committee onMember of the Institut universitaire de France Desertification (CSFD) has met two concerns of the Ministries in chargeand Director of the Laboratoire de géographie of the United Nations Convention to Combat Desertification. First,zonale pour le développement (LGZD), Researcherassociated with the Groupe d’étude sur les CSFD materializes the will to involve the French scientific communitygéomatériaux et environnements naturels et versed in desertification, land degradation, and development of arid,anthropiques (GEGENA²), University of Reims semi-arid and sub-humid areas, in generating knowledge as well asChampagne-Ardenne, France guiding and advising policymakers and stakeholders associated inmonique.mainguet@univ-reims.fr this combat. Its other aim is to strengthen the position of this FrenchFrédéric Dumay community within the international context. In order to meet suchResearch Engineer at GEGENA² expectations, CSFD is meant to be a driving force regarding analysisUniversity of Reims Champagne-Ardenne, France and assessment, prediction and monitoring, information and promotion.frederic.dumay@univ-reims.fr Within French delegations, CSFD also takes part in the various statutory meetings of organs of the United Nations Convention to CombatScientific editing and iconography Desertification: Conference of the Parties (CoP), Committee on ScienceIsabelle Amsallem and Technology (CST), Committee for the Review of the ImplementationAgropolis Productionsinfo@agropolis-productions.fr of the Convention. It also participates in meetings of European and international scope. It contributes to advocacy activities concerningDesign and production dryland development, in connection with civil society and media. It cooperates with DesertNet International (DNI).Olivier PiauAgropolis Productionsinfo@agropolis-productions.fr CSFD includes a score of members and a President, who are appointed intuitu personae by the Ministry for Higher Education and Research,Photography credits and come from various specialties of the main relevant institutions and Danièle Cavanna and Christelle universities. CSFD is managed and hosted by the Agropolis International Mary (INDIGO picture library of Association that gathers, in the French town of Montpellier and the Institut de recherche pour le Languedoc-Roussillon region, a large scientific community specialiseddéveloppement) as well as the authors of the in agriculture, food and environment of tropical and Mediterraneanpictures shown in this report. countries. The Committee acts as an independent advisory organTranslation without decisionmaking powers or legal status.David Manley Its operating budget is financed by subsidies from the French MinistriesEditing, production and distribution of Les dossiers of Foreign and European Affairs and of Ecology, Energy, Sustainablethématiques du CSFD are fully supported by this Development and the Sea in charge of Green Technologies and ClimateCommittee through the backing of relevant French Change Negotiations as well as the French Development Agency. CSFDMinistries as well as the French Development Agency.Les dossiers thématiques du CSFD may be freely members participate voluntarily to its activities, as a contribution fromdownloaded from the Committee website: the Ministry for Higher Education and Research.www.csf-desertification.orgPrinted with solvent-free inks on certified chlorine-free More about CSFD:bleached paper derived from sustainably managed www.csf-desertification.orgforests.Printed by: Les Petites Affiches (Montpellier, France)Copyright registration on publicationISSN: 1772-6964 • 1,500 copies Également disponible© CSFD / Agropolis International, May 2011 en version française originaleFor reference: Mainguet M. & Dumay F., 2011. Fighting Mainguet M. et Dumay F., 2006.wind erosion. one aspect of the combat against Combattre l’érosion éolienne : un voletdesertification. Les dossiers thématiques du CSFD. N°3. de la lutte contre la désertification.May 2011. CSFD/Agropolis International, Montpellier, Les dossiers thématiques du CSFD. N°3.France. 44 pp. Avril 2006. CSFD/Agropolis International, Montpellier, France. 44 pp.
  3. 3. ForewordM ankind is now confronted with an issue These reports are devoted to different themes such of worldwide concern, i.e. desertification, as global public good, remote sensing, wind erosion, which is both a natural phenomenon and agroecology, pastoralism, etc, in order to take stocka process induced by human activities. Our planet of current knowledge on these various subjects. Theand natural ecosystems have never been so degraded goal is also to set out ideological and new conceptby our presence. Long considered as a local problem, debates, including controversial issues; to expounddesertification is now a global issue that affects us all, widely used methodologies and results derived fromincluding scientists, decisionmakers, citizens from both a number of projects; and lastly to supply operationalthe South and North. Within this setting, it is urgent and intellectual references, addresses and usefulto boost the awareness of civil society to convince it to websites.get involved. People must first be given the elementsnecessary to better understand the desertification These reports are to be broadly circulated, especiallyphenomenon and the concerns. Everyone should have within the countries most affected by desertification,access to relevant scientific knowledge in a readily by e-mail (upon request), through our website, andunderstandable language and format. in print. Your feedback and suggestions will be much appreciated! Editing, production and distribution ofWithin this scope, the French Scientific Committee Les dossiers thématiques du CSFD are fully supportedon Desertification has decided to launch a new series by this Committee thanks to the backing of relevantentitled Les dossiers thématiques du CSFD, which is French Ministries and the French Development Agency.designed to provide sound scientific information on The opinions expressed in these reports are endorseddesertification, its implications and stakes. This series by the Committee.is intended for policymakers and advisers from theNorth and South, in addition to the general public Richard Escadafaland scientific journalists involved in development and President of CSFDthe environment. It also aims at providing teachers, Research Director at CESBIO (Centre for the Studytrainers and trainees with additional information of the Biosphere from Space, Toulouse, France) for IRD (Institut de recherche pour le développement, France)on various associated fields. Lastly, it endeavours tohelp disseminate knowledge on the combat againstdesertification, land degradation, and poverty tostakeholders such as representatives of professional,nongover n menta l, a nd i nter nat iona l sol ida r it yorganisations.Une évaluation indispensable de l’état actuel de la dégradation des terres 1
  4. 4. PreambleD esertification and land degradation give rise This readiness to provide decisionmakers and other to several phenomena (or processes) whose stakeholders with access to this pool of expertise should severity can vary depending on the region and be applauded. Our network of francophone researchersprevailing conditions: soil fertility loss, reduction in specialized on erosion and sustainable water and soilplant and tree cover, loss of soil water retention capacity, management (EGCES Network), supported by the Agencerunoff, water and wind erosion. The impacts include universitaire de la francophonie, is in full agreementincreased environmental fragility, reductions in crop with the conclusions of this Dossier and praises theyields and thus in inhabitants’ income, increased initiative. This Network, which developed out of afood insecurity and increased overall vulnerability former network that was very active at ORSTOM, i.e.of societies to climatic risks and economic crises, thus now the Institut de recherche pour le développementpromoting outmigration. (IRD), would like to play an active role, while working alongside communities of francophone researchers,This Dossier thématique du CSFD is of considerable developers and teachers in developed and developinginterest as it is focused on one of the most f lagrant countries to boost awareness on the serious globalsigns of desertification—wind erosion. As pointed out problem of soil erosion. The EGCES Network alsoby the authors at the outset, winds and dust storms welcomes researchers from various scientific fieldsare currently worsening, thus fostering wind erosion. who could fuel the soil erosion debate: methodsThis situation is especially worrisome in Mauritania (spat ia l biophysica l indicators), socioeconomicand along the Niger Loop. aspects, conventional and modern erosion control strategies, water and water quality management, landThe Dossier highlights wind-driven mechanisms, rehabilitation and biodiversity.factors that trigger erosion and land degradation, aswell as monitoring strategies. It also provides a crucial The EGCES Network is close to the activities of CSFD inupdate on what is currently known about techniques semiarid regions where vegetation cover degradationfor combating sand invasion and this form of erosion. gives rise to most problems concerning land erosionThe need for an integrated approach is also stressed. (since the wind is highly active), dam silting andMany examples are presented which demonstrate access to good quality water supplies. It is essentialboth the advantages and shortcomings of different to overcome these problems in order to be able tocontrol methods, while also providing the cost ranges fulfil the basic needs of a growing population. Thesefor carrying out control operations. issues were stressed during the Network’s scientific seminars that were held within the framework of the last International Soil Conservation Organization (ISCO) Conference, in May 2006, Marrakech, Morocco. Éric Roose President of the EGCES Network Georges De Noni Scientific Secretary of the EGCES Network Frédéric Dumay (1970-2011) In 1993, wind erosion phenomena in Mauritania was the topic of his postgraduate diplôme d’études approfondies Frédéric Dumay passed away on 18 February 2011, he was PhD qualification thesis. In 2007, he became a Research 41 years old. He has been a very fine, excellent, intelligent, Engineer at GEGENA². devoted and efficient collaborator for the last 20 years. His university studies and professional functions were based Frédéric was highly proficient in his field work, underpinned at the University of Reims Champagne-Ardenne from 1989 by his preliminary satellite image analysis skills. until his passing. His research was carried out in Africa (Morocco, Mauritania, After beginning his university studies in Geography, Senegal, Niger, northern Cameroon) and Asia (Israel, China). Frédéric immediately took a passionate interest in tropical From 14 to 27 May 2010, he launched a collaboration with dryland environments. In 1992, he became a Technical Hunan University in Changsha (China). Frédéric was always Collaborator for the French Laboratoire de géographie ready to invest time and energy in helping others. zonale pour le développement (LGZD). In his MSc thesis in 1993, he analysed the negative Vincent Barbin, Professor of Geoscience, Director of GEGENA² environmental impacts of the war in Koweit. In 1993, he successfully passed the French national Monique Mainguet, competitive design engineering examination and became Professor associated with GEGENA² one of the pillars of LGZD. 2 Fighting wind erosion—one aspect of the combat against desertification
  5. 5. ue t an d M . M ai ng © F. Du m ay © F. Dum ay u m ay © F. D Table of Contents Wind erosion—a complex environmental degradation mechanism 4 Wind erosion onset thresholds and land degradation triggering factors highlighted by remote sensing 6 Wind erosion mechanisms at the Man-Nature interface 16 Wind erosion control techniques 24 Hope for the future of wind erosion control 40 List of acronyms and abbreviations 41 For further information… 42 Glossary 44Table of Contents 3
  6. 6. Wind erosion a complex environmental degradation mechanismW i nd erosion is one of t he most ser ious To combat wind erosion more effectively, decision- environmental degradation mechanisms, makers should be aware of the space-time concept so inducing soil textural and mineral depletion, as to be in the right mindset to detect erosion triggeringas well as the mobilization of high volumes of sand thresholds and to monitor new changing boundariesand dust. The first imperceptible stages are insidious, of areas threatened by wind erosion.and nonspecialists will only notice the process onceit has reached a severe hard to control stage. This The functional units delineated under the global windphenomenon is illustrated by the sand encroachment action system (GWAS, concept discussed hereafter),history of ancient Mauritanian caravan trading city of ranging from the sand source area to the depositionChinguetti (designated as a UNESCO World Heritage area, must be taken into account. It is essential to setSite in 1996), which was partially buried under sand up wind erosion control initiatives and make strategicbut has now been exhumed through a rescue project. choices of intervention areas on this functional unitHowever, in 2005, there was still 5 m of sand in some scale (continental scale).alleyways within the city. Thresholds for the onset of environmental degradationIn dry ecosystems (southern Algeria, Mauritania, etc.), induced by wind along with other key wind erosionwind erosion, boosted by the sudden change in the pace triggering factors (natural and human) could be clearlyof degradation over the last four decades, is such that it perceived using remote sensing tools * (satellite imagesnow cannot be assessed using the conventional three- and aerial photographs). Once clarified, the complexstage model (causes, mechanisms and consequences) interlaced wind erosion mechanisms should be placeddeveloped in the late 1970s. For instance, the 800% in the GWAS context. The physical and biological windincrease in dust-bearing winds, and thus in wind erosion control and resource preservation measureserosion in Nouakchott (Mauritania) between 1960 and that have been proposed highlight that this initiative is1995, clearly illustrates that the process is accelerating. part of the desertification combat and that it cannot beIt is now impossible to separate these three stages and successful without the involvement of the internationaldetermine the chronological pattern because of the community.many feedbacks and insidious links between stages.These three stages must therefore be analysed together * Terms defined in the glossary (page 44) are highlighted in blue and underlined in the text.to ensure their contextual relevance. Note that eachcase is different and that a solution that works in onesituation cannot be directly applied elsewhere withoutadjustment to the local prevailing conditions—the‘green barrier model’ developed in Algeria in the mid-1970s is striking evidence of this failure! 4 Fighting wind erosion—one aspect of the combat against desertification
  7. 7. Arrival of a sandstorm in the area of Mare d’Oursi, Oundalan, Burkina Faso.Note that behind the huts the red color of the sandstorm is due to the origin of the winnowedparticles (surface bedrock or red sand). The higher particles are derived from a different originresulting from deflation of an aeolian sandsheet. This highlights the long trajectory that asandstorm may have. The sand deposited on the surface is thus of different origins.F. Sodter © IRDWind erosion—a complex environmental degradation mechanism 5
  8. 8. Wind erosion onset thresholds and land degradation triggering factors highlighted by remote sensingI t is now recognized that anthropogenic factors (socioeconom ic, popu lat ion pressu re, such a s excessive tappi ng of nat u ra l resou rces,deforestation, overgrazing, excessive irrigation and,consequent ly, sa linization, etc.) are major landdegradation triggering factors in dryland areas. Theimpacts of these factors are worsened when they overlaprecurrent drought periods or severe rainstorms.People, including farmers, cattle breeders, tow n DETECTION OF THE FIRST SIGNS OF WIND EROSIONdwellers and policymakers—through their activitiesand developments—play a key role in wind erosion The wind erosion onset threshold is the point at whichonset. This process may also be accelerated by severe the balance* between natural resources and humanclimatic events (drought, flooding, peaks in water and pressure, aggravated by severe climatic conditions,wind erosion). is upset. It is essential to determine this threshold so as to be able to rapidly (on temporal and spatialIn dryland environments (which in Africa, according scales) identify premonitory signs of environmentalto our observations, reaches isohyet 600 mm/year in degradation before the degradation stage and ultimatethe southern Sahel), aeolian mechanisms are the main desertification stage. Improvements are needed withfactors that determine the vulnerability of farmland respect to detecting these thresholds and the differentand urban areas located in sandy environments (e.g. in degrees of preliminary degradation in order to be ableNouakchott, rainfall 100 mm/year), sometimes leading to rapidly boost awareness on wind erosion onset.to desertification. Precursory signs detected by remote sensing includeThis phenomenon is under way in the groundnut overdeepening of the surface layer (blowouts), increasesgrowing area in Senegal. The extent of degradation in the density of small aeolian sand deposits or nebkasnoted in the northern part of this area around the end (sand arrow leewards of a bush or stone), the emergenceof the first decade of the 21st century was similar to that of irregular mobile sand deposits, reactivation ofobserved at the latitude of Nouakchott, Mauritania, in the crests of sand dunes, changes in soil reflectance.the early 1970s (Fall, 2001). It would thus be essential Monitoring tools are available that should be tailoredto analyse satellite images and aerial photographs, to different monitoring scales and supplemented bybefore and after ground checks, to quickly determine field surveys and quantified data. These signs mustthe wind erosion onset threshold in order to develop also be clearly and quickly interpreted so as to enableand implement preventive control initiatives. rapid and effective interventions.All concerned countries should have several permanent * In dryland ecosystems, this refers to a steady state in soils and in the naturally open vegetation cover.teams working on this issue. 6 Fighting wind erosion—one aspect of the combat against desertification
  9. 9. Oblique aerial view of the centre of the city of Nouakchott, Y. Boulvert ©IRDMauritania, 1996. This photograph shows that there is very little vegetationin the town, and progressive sand and dust invasion, which has been steadilyincreasing since 1996 under the effect of harmattan sandstorms.SATELLITE IMAGES AND THE GWAS CONCEPT The smallest scale images (METEOSTAT, NOAA) are used to identify different wind system units: sourceSatellite images are key tools for env ironmenta l area, transit areas and deposition areas. Scales mayinvestigations. They provide users with access to overlap within these units depending on the presencein-depth detailed and repetitive information. The of megaobstacles (mountains and plains, Mainguet,observations may be combined to obtain a multiscale 1976) and the roughness of the local landscape (relief,view, ranging from the synoptic scale with METEOSAT vegetation, buildings). Defined subunits may, forand NOAA images, to the regional scale with LANDSAT instance, be an erosion area within a deposition unitand SPOT images. or, conversely, a deposition area within a unit in which erosion prevails, as detected on large-scale satelliteBased on these satellite images, the global (or regional) images (SPOT or LANDSAT satellites).wind action system (GWAS [or RWAS]) concept wasdeveloped and the limits were determined (Mainguet, When assessing a regional system, an image mosaic can1984). A GWAS (or RWAS) is a series of particle-laden be constructed, e.g. the regional wind action system inwind currents marked out by open interlinked ergs the Aral Basin in Central Asia (Mainguet, Dumay and(sand seas) that form erg chains like those linking the Létolle, 2002). Aerial photographic coverage is essentialSahara and the Sahel (Mainguet and Dumay, 1995). when highly detailed observations are required forGeostationary satellites (having a stationary orbit, i.e. infrastructure protection (buildings, roads, irrigationMETEOSAT for Africa or circumpolar regions, NOA A canals, etc.).for Eurasia) generally covering an entire GWAS.Wind erosion onset thresholds and land degradation triggering factors highlighted by remote sensing 7
  10. 10. > FOCUS | Regional wind actionsystem (RWAS) on the easternside of the Aral Basin Aral Basin (1.8 million km²) accounts for 4% of the global temperate dryland area. This RWAS, which is studied in the field and on Cosmos, NOAA and SPOT satellite images, begins at the southern outlet of the Tourgai Corridor at the interface of the Siberian Plain and Aral Basin. At its centre, there is a deflation area where satellite images reveal a dense paleohydrographic network, Sir Daria and Amou Daria Basins, the only permanent allogenic streams and, in the south, the temporary Tedjen and Mourgab streams. It ends at the northern foot of Khopet Dag (3 349 m) with a 375 km by 40 km loess strip. The wind releases its load of sand at the outlet of the Tourgai Venturi, which has led to the formation of four ergs north of Aral Lake: Grand Barsouki, Small Barsouki, Barsakoum and Karakoum, which should not be mistaken for the Grand Karakoum erg. At their southern edge, shifting barchan dunes are invading the villages of Kulandy and Akespe (46°5 N, 60°3 E). Grand Barsouki (46° à 48°N, 58°5 à 60°E) is 250 km long by 25-50 km wide. At the point of contact with Aral Lake, near Koulandy (59°3 N and 46°1 E), it is the only erg west of the lake within the RWAS. In its downwind part, it comes up against Tchink Escarpment. Small Barsouki is 110 km long by 30 km wide, starting at 47°N and 61°E. These two ergs blanket the early Quaternary paleovalleys. Their dunes This RWAS clearly highlights the regional aspect of aeolian currents that form part of the shores of the Gulfs of Boutakov and determine the surface features in sandy regions covering an area of 1 500 km Touchibas. The sand derives from glaciofluvial deposits from north to south. extending from the Mougodjar hills in the southern Ural 1. Aral • 2. Sand sheets and ergs • 3. Paleo-wadis • 4. Western boundary of riverine region to the Tourgai Corridor plains (Irgiz, etc.). paleochannels not blocked by aeolian material • 5. Major wind directions in the regional wind action system (RWAS) in Aral Basin • 6. Sheltered areas on the leeward side of obstacles • 7. Erg boundary A 1957 geological map (at 7 500 000 scale) indicates that the material is of tertiary paleogenic fluvial origin Zana Daria paleostreams crossed the Kyzylkoum erg and the result of wind erosion. The RWAS extends along until the late Bronze Age, whereas the Amou and Sir the eastern side of Aral Basin. Deflation in the Kazakh Daria deltas converged. The western part consists of steppe south of the Siberian plate is the source of sand longitudinal dunes that cross the network of paleovalleys for the NNW-SSE longitudinal dunes in the northern visible on satellite images. Karakoum erg. Sir Daria interrupts the continuity of the ergs and supplies sand for the Kyzylkoum erg that begins Less clearcut dunes are found on the older eastern just south of it. The general NNW-SSE orientation of the part. Kyzylkoum sandseas extends to the right banks dunes continues south of Sir Daria and then shifts 44°N to of Syr Daria river and, simultaneously, to the left banks. become NNE-SSW as the dunes encompass Aral Basin, The wind streams cross the river carrying the sand that which in turn serves as an obstacle to the wind. has emerged during the dry season. This could explain why alluvial material blends fine carbonate-rich (20- Sand deposits thus do not accumulate to a great extent 30%) aeolian material from Kyzylkoum sandsea with in the sheltered lake basin but rather on the first reverse alluvial silts. In this sector, the wind currents form a slopes. The inflexion can be seen on satellite images up to divergence which is responsible for the formation of 150 km east of the lake, at the northern edge of the Amou Grand Karakoum sandsea, which is followed downwind Daria paleodelta where the wind direction measured in by the Kyzylkoum sandsea. The Karakoum sandsea is the field is NNE-SSW 10°. Between Sir Daria delta and the formed north of the loess strips in the southern part southeastern Aral Basin (former Akpekti Archipelago), the of the RWAS. Kyzylkoum erg sand particle size decreases to become silt in the Sir Daria delta region. From Mainguet M., Dumay F. and Létolle R., 2002.8 Fighting wind erosion—one aspect of the combat against desertification
  11. 11. > EXAMPLE | Using aerial photographs to monitor sand dunes in Mauritania On an aerial photograph taken in 1991, there N. Nouakchott • W. Water tower• S. Seif dunes • F. Fence is a clear increase in the number of seif 1. Aerial photograph IGN MAU 1991 12/150 n°58 © IGN 1998 dunes and in the extent of sand invasion in 2. Interpretation sketch drawn by LGZD. the vicinity of the water tower northwest of Nouakchott. ESSENTIAL GROUND CHECKS The combined analysis of satellite images and aerial photographs is an essential support for field surveys. Preliminary overall knowledge of sectors investigatedAERIAL PHOTOGRAPHS AND CLOSE MONITORING OF in the field enhances the in situ understanding ofSAND DUNE TYPES AND THEIR SURFACE STATUS sand encroachment and wind erosion areas and their dynamics. In the field, geographers’ expertise—anW hen u se d a longside satel l ite i ma ge s, aer ia l essential tool—and ability to consider landscapesphotographs prov ide more detailed information, in terms of morphodynamic units , enable them toespecially on types of individual sand dunes and pinpoint and sample key wind dynamics sectors.on their surface status. They are essential tools for Assessments of aeolian dune types ( barchan dunes ,erosion control studies in urban areas (Nouakchott seif dunes, longitudinal dunes, etc.) inform geographersin Mauritania, Draa Oasis, or the Kénitra-Larache on the nature of the local aeolian sediment budget : ismotorway in Morocco). the area in a particle loss (erosion, with a negative sediment budget) or accumulation period (deposition,In wind erosion control, they are excellent indicators for with a positive sediment budget)? All collected soildetecting premonitory signs of degradation: blowouts samples are then sent to the laboratory for particle size(aeolian overdeepening) of the landscape surface, (concerning the size of sand grains), morphoscopicformation or extension of nebkas , increased sand (concerning the shape and surface status of particlespowdering or sand veils, and neogenesis of small aeolian and transport processes) and mineralogical analyses,dunes that are not visible on satellite images. Finally, which determine the geographical (and associatedaccurate detailed maps can be drawn up from aerial transport mechanisms) and geological origins ofphotographs and used to combat sand encroachment particles, thus enabling confirmation of the findingsand monitor individual aeolian dunes. and hypotheses put forward.Wind erosion onset thresholds and land degradation triggering factors highlighted by remote sensing 9
  12. 12. Children carrying fuelwood collected at dawnon the dyke of the Sologo barrier lake (Côte d’Ivoire). C. Lévêque © IRDHUMAN ACTIVITIES—THE MAIN WIND Population pressure—a wind erosion inducerEROSION TRIGGERING FACTOR During the 1990s, the driest regions on Earth—most of which are located in Africa—had a populationContrary to the desertification definition put forward in growth rate of over 3%/year (Evers, 1996). In manyAgenda 21 where climatic variations are placed ahead dry countries, over 50% of the population is underof human factors, we rank humans (or societies) as the 25 years-old, i.e. the runaway population growthmain initiators of aeolian degradation mechanisms, phenomenon is not under control. In agricultural areasand then they represent causes, mechanisms and of many countries, this population pressure, which isconsequences once t he process has been set in common in countries during development periods,motion. is still synonymous with labour and thus prosperity. Population growth, however, aggravates environmentalCornet (2002) stresses that “land degradation occurs degradation via the constant high pressure placed onwhen humans modify the balances or natural dynamics environmental resources.[…]. Human activities are generally voluntary, sometimesdue to ignorance and often determined by an increase in Population concentrations have harmful environmentalneeds in a setting of insufficient technological development impacts regardless of the type of ecosystem. Dr yand an absence of resource access regulations.” We would ecosystems have a lower resilience than temperatealso add resource use without accounting for climatic ecosystems, w ith more sustained env ironmentalvariations. degradation, especially since they are hampered by droughts and the environment is not protected byDuring unusually rainy periods (1950-1960), Sahelians vegetation cover. Population concentrations lead to(the people most affected by wind erosion in the overgrazing and excessive consumption of timber andworld) expanded their cropping area at the expense fuelwood. For fuelwood, nomadic people generally useof the Saharo-Sahelian region because of the growing wood that they collect during their movements, sopopulation and declining soil fertility. This situation their pressure on the environment is dispersed alongbecame catastrophic during subsequent dry periods, carefully chosen routes that they follow in responseespecially since they spanned two decades (1970- to seasonal variations, contrary to settling, which1990). leads to overgrazing and depletion of edible species. This results in the expansion of wood collection andRainfall conditions improved in the 1990s, but the grazing areas, thus creating centrifugally degradedresilience potential of these ecosystems (natural patches that eventually merge (i.e. become linkedimprovement) seemed to be negatively impacted by and therefore broader). These patches representhuman pressure and population growth. desertification phases, forcing people to migrate towards cities, as already noted in Niger in 1976 (Mainguet, 1976). Field work carried out in Mauritania in October and November 2009, just after the rainy season, revealed rings of degradation around villages along the Nouakchott–Kiffa road. 10 Fighting wind erosion—one aspect of the combat against desertification
  13. 13. > FOCUS | Variations in the meaning The UNDP definition was amended by UNCED as follows in July 1992: “Desertification is land degradation of ‘desertification’ in arid, semiarid and dry subhumid areas resulting from various factors, including climatic variations and Deser tification takes its meaning from the Latin human activities.” According to this definition, the etymology: responsibility of humans is watered down and degrees of degradation severity are not taken into account, • ‘desert’ has a dual Latin origin: from the adjective especially the ultimate irreversible degree, which should desertus (meaning ‘unmanned’) and from the substantive be considered since this is the essence of the meaning desertum (abandoned area). of the word ‘desertification’. In practice, degradation • ‘fication’, meaning ‘making or causing’, comes from is irreversible when the soil no longer contains seeds, fieri, the passive form of the active verb facere (meaning or when (according to Dregne, 1984) the soil is so ‘to make’ or ‘to do’) degraded that it has lost its water retention capacity, so any seeds it does harbour would be unable to In 1971, in J. Gilbert’s Dictionnaire des mots nouveaux germinate. (Hachette-Tchou, p. 158), the term ‘desertification’ means: “...the almost complete withdrawal of all human If desertification is synonymous with land degradation, activity in a region that has been gradually abandoned then it is currently the most dramatic environmental by its inhabitants.” problem on Earth. If the irreversibility concept is included in the current technical end economic context—since In 1977, the United Nations Conference on Desertification it is the endpoint of a series of processes leading proposed : “Deser tification is the diminution or to definitively barren environment—then it is almost destruction of the biological potential of the land, and nonexistent and, according to Dregne (1984), only can lead ultimately to desert-like conditions. It is an 0.2% of the globe would be affected. aspect of the widespread deterioration of ecosystems, and has diminished or destroyed the biological potential, We proposed (Mainguet, 1991) the following definition i.e. plant and animal production […] at a time when based on three decades of observations in dryland increased productivity is needed to support growing parts of Africa and Asia: “Desertification, as revealed by populations in quest of development.” drought, is due to human activities when the carrying capacity of the land is surpassed. It proceeds by In the ecological concept of a balance between humans natural mechanisms that are exacerbated or induced and the natural environment, humans are responsible by humans. They lead to plant and soil degradation for desertification: the ultimate stage of environmental and—on a human timescale—to a decrease or degradation has reached an irreversible point, but irreversible destruction of the biological potential of the notion of irreversibility warrants discussion. In the land or its capacity to support people living on December 1989, the UN General Assembly, asked the it.” Within a few decades, human activities achieve UN Environment Programme (UNEP) to reevaluate the what natural evolution takes a few millennia to achieve term for subsequent submission to the UN Conference on a geological scale. This definition stresses human on Environment and Development (UNCED) held in Rio causes, with climatic parameters like drought simply de Janeiro in 1992. UNEP then proposed the following revealing the process. It was proposed (UNEP, 1991) definition to UNCED in June 1992: “Desertification is that the term ‘desertification’ be replaced by ‘land land degradation in arid, semiarid and dry subhumid degradation’ or ‘environmental degradation’ to avoid ecosystems resulting mainly from the impact of adverse inconsistencies. human activities.” By this definition, land degradation encompasses declining crop yields, vegetation cover Environmental degradation associated with human deterioration, exacerbation of physical mechanisms at activities occurs in all ecozones, but it is especially the soil surface, qualitative and quantitative decreases in in the driest areas that humans most contribute to water resources, soil degradation, and air pollution. turning environments into desert-like wastelands. This is why the term ‘desertification’ should be limited to arid, semiarid and dry subhumid areas.Economic pressure—another wind erosion inducer Competition between farmers and nomadic people worsens this situation and often leads to armed conf licts. There is also high pressure for access toPopulation growth in a limited area systematically water resources and wells, which are increasinglyincreases the economic pressure and competition for deep and require very heav y investment.resources. In dry countries, this pattern is reflected bythe trend towards settling on new land at the expense Economic pressure then forces many farmers andof fragile and marginal areas (e.g. Nouvelles Terres in cattle breeders to find other subsistence solutions,Eastern Senegal). Clearing, tilling and cropping these often leading to migration to capital cities or settlementlands increases their vulnerability to wind erosion. along main roads.Wind erosion onset thresholds and land degradation triggering factors highlighted by remote sensing 11
  14. 14. > FOCUS | Uncontrolled building (gazra in Hassania) on longitudinal dunes on the outskirts of Nouakchott (Mauritania) In 1991, a difference was noted between temporary inhabitations (1) built on dunes (2), which is surmounted by seif dunes (3), and organized neighbourhood areas (4) built in the depression and which were relatively well protected. The newly built inhabitations destabilised the dune and increased the risk of sand scouring and encroachment, while also creating rough surfaces and enhanced vorticity. In 1991, new observations highlighted that seif dunes were derived from sand barchanic domes arising in the urban environment. Source: LGZD research Aerial photograph IGN mission MAU 1991 12/150. Photograph n°55 © IGN 1998Migration and settling—causes especially by sharp hooves of goats, which are otherwiseand consequences of wind erosion very useful) and of the sparse surrounding vegetation cover. These surfaces are then vulnerable to erosionFor farmers—especially their sons—the option of as the wind lifts and carries away the loose sand.moving to the capital to work is often a preliminarystep towards a more complicated migration further Dairy cattle are being reared in the longitudinal duneabroad. Outmigration and settlement are sometimes system south of Nouakchott, extending between thepromoted by policymakers and are also the result of capital and Rosso, to supply milk to markets in theriskier regional development policies. In Mauritania, city. This production, developed by nomad settlers,for instance, the Route de l’Espoir (Nouakchott-Néma is very environmentally damaging since these formertrans-Mauritanian highway crossing the country from cattle breeders have maintained their transhumantwest to east), which was built to open up the eastern herding habits but on much smaller transhumancepart of the country and promote political unification, rangelands. The grazing areas are highly degraded.has become a settlement belt for nomads seeking to Wit hout any rehabilitation initiatives, t he activeimprove their food security situation. dunes will eventually extend to the latitude of Rosso where, however, annual rainfall levels are as high asThis settlement of a new fringe population group 280 mm. A field trip in November 2009 confirmed thathas placed greater pressure on the economy and the grassy and shrubby plant cover on longitudinalled to further departures towards the capital. Here and transverse dunes located south of Nouakchottagain, the flow of migrants towards the capital is self- did not recover and that wind erosion is ongoing, asmaintained. Uncontrolled installations (gazra or kebbe shown by the giant blowouts/def lation basins. Thein Hassania) in interdunal corridors between NNE- population boom is responsible for land and resourceSSW longitudinal dunes create new rough areas in the degradation, thus forcing people to migrate again.landscape which naturally trap the mobile sand blown The waves of successive settlements and migrationsthrough these corridors and initiate sand deposition a re a lso responsible for u nsusta i nable nat u ra land encroachment mechanisms. resource use, especia lly when they involve sma ll population concentrations. When these populationsMoreover, population concentrations, which are f lee to urban centres, especially to capital cities,generally in the vicinity of a well or medical dispensary they are generally crippled by poverty and structuralalong this road, degrade the soil structure (trampling, breakdown. 12 Fighting wind erosion—one aspect of the combat against desertification
  15. 15. Southeastern part of the fixed Erg Aouker on the Togbha-Tamchekket road, Mauritania, December 2001. Violent rains during theJuly–September rainy season induce concentrated runoff, which etches widerills (here 2 m. deep) in compacted and ferruginous (turning red) paleosolsfrom fixed transverse dunes. Sand particles loosened by water erosion can bemore readily mobilized by wind erosion (deflation). © F. Dumay and M. MainguetUrbanization and impoverishment—starting ENVIRONMENTAL VULNERABILITY FACTORSpoints of wind erosion INHERENT TO DRY ECOSYSTEMSSince the late 1960s, developing countries have been The first environmental vulnerability factor, whichaffected by runaway urbanization and many countries should normally be a source of natural wealth, is theare impeded by oversized cities, which has given rise to soil. The high diversity of soils in dryland ecosystemsserious problems of economic rebalancing and clearcut is a key resource, but their thinness and vulnerabilitypopulation distribution disparities. favour wind erosion, especially with respect to sandy soils. This vulnerability is heightened by water erosionMetropolitan Nouakchott (800 000 inhabitants) pools in clayey soils, i.e. pa leosoils of ancient genesis.30% of the population of the Republic of Mauritania, Blowouts and gullies are very common in Sahelianwhich has led to problems of delinquency and an increase landscapes.in numbers of beggars and homeless people. This urbanboom also has impacts on the countryside—there is Vulnerability of soils to wind erosionincreased pressure on farmers to practice commercialfarming to supply the expanding urban markets, to The v ulnerabilit y of dr yla nd ecosystem soils isthe detriment of conventional, more environmentally associated with the very long time required for theirfriendly, subsistence farming. genesis because of the absence of chemical weathering of rocks. Edaphic drought promotes the formationThe profit lure has prompted farmers to produce more, of single grain structures (i.e. that may be easilywhile ruining those who are unable to do so. Moreover, powdered). These soils are initially degraded bythese profit seekers are sterilizing their lands. This aeolian winnowing of sand particles and silts. Deflationtrend is boosting the f low of migrants to the cities. (wind-induced drifting) gathers the particles, whileShelters are required for these people and the quest winnowing sorts them and a new generation of mobilefor materials to build these shelters is a further source dunes is created via their deposition. These organicof environmental degradation—the offshore barrier matter depleted soils can soon become barren throughsandbar east of Nouakchott, which has been largely overexploitation and aeolian winnowing, or when theyreactivated, has been so overexploited that it is no can no longer be cultivated, except under irrigation,longer completely effective in protecting the city during due to the prevailing climatic conditions. Irrigationvery strong tides, i.e. sea water penetrates at certain often causes excessive soil salinization, especially whenpoints and floods low-lying urban neighbourhoods. All sprinkling is ill-timed and the washing and drainagethe anthropogenic pressures described earlier affect conditions are poor. When there is a balance betweenecosystems that have been fragilised by severe climatic crop planting and soil conservation, it often falters as aconditions. The resulting ecological catastrophes result of seasonal degradation under specific climaticaccentuate imbalances between the anthropogenic variations, e.g. rainfall delays, droughts or, conversely,pressure, available resources and social disparities. ill-timed excessive rainfalls.Wind erosion onset thresholds and land degradation triggering factors highlighted by remote sensing 13
  16. 16. > FOCUS | Impact of soil conditions: erosivity and erodibility The wind velocity at the soil surface varies according to the roughness of the topography. It starts declining at an altitude tenfold above the height of the vegetation cover, natural and anthropogenic roughness. The wind is slowed down by a frictional effect in contact with the sandy surface or mobile particles. The lower air layer has zero velocity on a thickness of around 1/30 th of the particle diameter. The effects of a wind system at the soil surface vary depending on the rain and wind erosivity and the soil erodibility. Its erosion capacity decreases when the values of some variables increase and when those of other variables decrease : • EROSIVITY (capacity of rain and wind to induce sufficient erosion to remove the top soil layer, Mainguet and Dumay, 2005). Wind parameters: • Velocity ( ) • Frequency ( ) • Duration ( ) • Area ( ) • Shear ( ) • Turbulence ( ) • ERODIBILITY (dependent on the soil properties) Wind velocity profiles close to the ground represented in Cartesian Sedimentological parameters: coordinates (a) and semi-logarithmic coordinates (b). • Grain size ( / ) • Height ( ) Zm: height in metres; Um.s-1: wind velocity; U*: breaking power of the soil; Zo: roughness parameter at zero wind velocity; k: Von Karman constant [= 0.35] • Orientation ( ) • Abradibility ( , capacity to be eroded and carried away) • Transportability ( , wind transport potential) • Organic matter ( ) Surface parameters: • Vegetation: Residue ( ), Height ( ), Orientation ( ),Density ( ), Smoothness ( ), Extent of cover ( ) • Soil moisture ( ) • Soil roughness ( ) • Surface length ( ) • Surface slope ( / ) When the soil is covered with dense vegetation of height h, everything takes place as though the ground level had been raised to a height of D < h, i.e. the so-called Effect of high vegetation on the velocity profile. displacement height. When a windbreak is installed, its height minus D should be taken into account rather than its total height.14 Fighting wind erosion—one aspect of the combat against desertification
  17. 17. > FOCUS | Soil drought—the result of wind erosion In the 1950s, natural vegetation cover, especially grasses, fixed all sandy areas in semiarid African regions, which benefited from over 150 mm/year of © F. Dumay and M. Mainguet rainfall. Since then, overgrazing has exposed these sandy surfaces to wind mechanisms during the dry season, as well as loss of the A horizon. Wind export of the A horizon led to outcropping of Southern edge of the Aoukar sandsea. Mauritania, 2001. the B horizon, which in turn was subjected to so- As indicated by the perched clumps of vegetation, wind erosion has exported called ‘sealing’, i.e. polishing via sand grain saltation the top soil layer. Wind erosion combined with water erosion is responsible for the completely infertile sealed soil area. (successive skipping-like grain transport process), thus sealing the soil surface and making it impermeable to rainwater infiltration. This situation makes the soil All of these phenomena are what is called soil drought vulnerable to water erosion: splash, with compaction (occurring within the soil): phenomena, then leading to crusting or runoff with • the vegetation is hampered by both moisture and the formation of rills. This compacted soil behaves oxygen deficits like a rock upon which wind erosion can lead to the • during germination, seeds are unable to penetrate development of corrasion streaks and blowout-type the hardened upper soil layer, so they dry out and aeolian depressions. are exported by the wind.Role of climate crises It should therefore not be claimed that droughts are thein aggravating wind erosion only causes of desertification since they are common events in arid and semiarid ecosystems. SustainablyC l i m at ic v a r i a bi l it y i s ac ute i n d r y t r opic a l cultivated land will recover from droughts upon theecosystems—mean monthly or annual rainfall levels return of normal rainfall conditions, whereas poordo not reflect the chaotic rainfall distributions, despite cultivation practices increase soil degradation.the fact that a dry phase seems to be taking shape inthe last few decades. Although a deficit mean rainfall Excess rainfall participates in destructuring the soilpattern may be recorded, devastating rainstorms couldhave occurred over the same period. Contrasting climatic events in the Sahel are ‘exceptional’. Drought periods can be interlaced with devastatingRecurrent droughts increase the vulnerability rainy climatic events—devastating for both crops andto wind erosion dwellings made of banco (a dried mixture of clay and chopped straw) with flat roofs that become waterloggedFour major droughts have occurred in the Sahel since and collapse (e.g. at the 9 th century Tichit Oasis inthe beginning of the 20th century (1900-1903, 1911- Mauritania, in a hyperarid region, where many houses1920, 1939-1944, 1968-1985), representing a cumulated collapsed during violent rainstorms in 1999). A verytotal of almost 35 years of deficit rainfall conditions. short-lasting violent rainstorm can thus be as fatalHence, it is not surprising that public authorities as a long drought period! On fragile soils that haveput for ward the drought criterion inherent to the been laid bare by drought and excessive cultivation,ecosystem as the main cause of desertification in heavy rains can destroy the soil cohesion, inducingrecent decades. However, dust storms are closely wide loose cracks that can turn into deep gullies,correlated w ith drought periods, suggesting that with downslope deposition of small alluvial cones ofthese droughts are responsible for exportation of loosened material. This soil fraction that was previouslypart of the fine soil fraction, which is the main soil compacted can thus be mobilized by the wind uponfertility and structure constituent. Note also that the the return of dr y periods. Conversely, but just aslast droughts in the 1980s captured the attention of damaging for agriculture, heav y rainfall combineddonors and policymakers because of their high media with aeolian winnowing can lead the formation of acoverage. It is recognized that drought is not unusual surface sealing .in so-called dry ecosystems, and thus should be takeninto account in local food supply management. The It also seems that during years of excess rain localaim is to avoid having to rely on massive food aid farmers tend to give up their parsimonious resourceimports, which may trigger substantial outmigration management in order to benefit maximally from thisof people seeking easy to acquire food resources in ‘climatic godsend’, but the return to normal is justcapital cities. more difficult!Wind erosion onset thresholds and land degradation triggering factors highlighted by remote sensing 15
  18. 18. Wind erosion mechanisms at the Man-Nature interfaceW ind act ions t hat ta ke place at t he soi l- atmosphere interface in a dynamic terrestrial framework are structured in large dynamicunits on synoptic or continental scales—these so-called global wind action systems (GWAS) cover theentire Sahara and Sahel region. Human activitiesalso interact in regional wind action systems (RWAS).El-Ba z (1988) used the expression regiona l w indaction system to describe regional wind circulationphenomena that sweep through Eg ypt from northto south.These synoptic wind units are influenced by mountainobstacles, soil conditions, vegetation and sometimes N’Beika, Mauritania. Seif dunes with a star dune trend.climatic events. Wind dynamics are not limited to sandencroachment, even though this is the most obvious The GWAS theory was formulated in 1972 (Mainguet,and immediately disturbing associated phenomenon. 1972) on the basis of an analysis of an aerial photographWind dynamics should be considered within the GWAS mosaic of northern Chad that had been publishedframework and assessed with respect to all aspects, by the French Institut Géographique National (IGN)e.g. erosion, transport and accumulation, so as to gain along w ith an analysis of data collected in manyinsight into the dynamics of this process overall and field surveys. A single sand-bearing wind currentenhance management of the negative effects. was documented. It was found to be a discontinuous spatiotemporal system spanning several hundreds ofGLOBAL WIND ACTION SYSTEMS—NATURAL UNITS kilometres linking Libya to Chad, then circumventingAND LIVING ENVIRONMENTS the Tibesti Mountains, and extending as far as Niger. These observations were confirmed and supplementedA global wind action system consists of a patchwork by METEOSAT imagery, which is a valuable tool forof small interrelated units that are grouped in three detecting links between Saharo-Sahelian ergs.main units that differ with respect to the sedimentbudget and they are delineated in the environment Sediment budget (SB) in sandy Saharo-Sahelian environmentson the basis of the dunes or wind forms that prevail. SB- Erosion areaThese t hree subunits, which succeed each ot her source area • Longitudinal dunes (sandridges) with a desertaccording to the wind direction, are: a particle export pavement surfacearea w it h a negative sediment budget, a particle • Elevated vegetation clumpstransit area, and a particle accumulation area with • Blowout in sandy hilly landscapesa positive sediment budget. • Parabolic dunes in areas partially secured by vegetationEven smaller local entities may be nested in each Transit area Sand transit areaof these subunits. Depending on the extent of area • Saltation inducing the formation of corrasion ridgesinvolved, human activities can have a range of harmful or smooth surfaces on solid rocksimpacts requiring different management strategies. • Nebkas • Barchan; linear dunesThese three subunits cannot be dissociated whenassessing global wind dynamics. Due to its vastness, Deposition Accumulation area area • Transverse dunessatellite imager y is required to obtain an overview SB + • Star dunes (or pyramidal dunes)of the general system. 16 Fighting wind erosion—one aspect of the combat against desertification
  19. 19. J.J. Lemasson © IRDAeolian particle source area Longitudinal dunes are also found in particle source areas. The a x is of these erosion dunes is para llelT he sou rce a rea is a defl ation a nd w i n now i ng to the prevailing wind. According to Jordan (1964),surface where erosion processes and particle export these are the most common dunes on Earth, andpredominate. The effects of the wind differ according they even cover as much as 72% of all Saharan sandyto the substrate: areas. This confirms the hypothesis that the sand exportation is the main process under way in the• scou r ing corr asion phenomena occu r i n ba re Sahara Desert, probably since the Late Holocene. rocky areas leading to the formation of kaluts and yardangs; On a geolog ic a l t i mesc a le, t h i s Sa ha ra n sa nd• the wind carries away the sandy matrix on stony exportation process has increased the volume of a lluv ia l deposits, leav ing worn dow n dreikanter sandsheets in the actual Sahelian millet-g rowing stones; area (Mainguet, 1985). Longitudinal dunes can reach• wind winnowing occurs in sandy areas and hundreds of kilometres long and 200-1 500 m wide, the ground becomes fixed desert pavement at the separated by 500-3 500 m wide deflation corridors. terminal stage. They are active dunes in arid and hyperarid areas and are only stabilized when the surface develops intoParadoxically, erosion is also involved in building desert pavement consisting of coarse sand particlesindividual or adjoining aeolian dunes such as parabolic (over 400 µm).dunes t hrough a deflation process in hilly sa ndylandscapes fixed by vegetation cover. The wind rushes These dunes are normally fixed by vegetation coverthrough openings in the vegetation cover, etches out in semiarid env ironments (annua l precipitation:sandy hillocks and builds up dunes whose concave 150-600 mm/year). However, destructuring of thewindward slopes consist of shifting sand, whereas surface pavement and/or overgrazing may reactivatevegetation can immobilize the sand on the slipface. and turn them into mobile sand reservoirs. PeopleThese dunes threaten cropping and grazing areas via traditionally settle in the interdune corridors.def lation and the migration of sand particles fromthese dunes.Wind erosion mechanisms at the Man-Nature interface 17
  20. 20. > FOCUS | Wind impacts according to the substrate in the source area • Desert pavement A B © F. Dumay and M. Mainguet Desert pavement of coarse sand grains. Surface state of C longitudinal dune in Akchar Erg (Mauritania, 2005). E • Dreikanter D F © M. Mainguet A dreikanter is a wind-carved stone, generally with three sides, a Impact of layer changes on the size and frequency of corrasion ridges, flat base, two faces under the sharp crest, rounded edges and a matt Sol-Solé sector (18°30’N, 19°20’E), Bembéché, Chad (Aerial photographs surface. Dreikanters are found on the surface of deserts or in geological AE 54-55, NE-34-XIV, n° 254-255; scale: 1/50 000). layers formed under arid climatic conditions with high saltating sand- laden winds. • Corrasion ridges • In the D area, two ridge systems overlap, sometimes with deeper grooves. The best preserved desert varnish Longitudinal variations in the size and frequency of is located right behind the escarpment, which is an area sandstone ridges, in association with layer changes, E where the ridges are likely best preserved. may be noted on these photographs. Four uniform areas succeed each other from NE to SW: The best ridges are found in gentle landscapes and areas with barchan dunes where corrasion is less marked. In • A disorganized area A, where the ridges are not clearly F, a hydroaeolian basin with lacustrine deposits is carved formed. in extensions of the sandstone grooves. With every • A clearcut thin-ridged area B, separated from sector topographical obstacle there is a change in dimension C by an escarpment where the ridges start widening. and frequency since the rock joints (fractured rocks) • The ridges in C are much broader. The best formed continue in the same direction with layer changes, but ridges are found right after the escarpment. The grooves their frequency is altered. become shallower as they get further away from the Source: Mainguet, 1972 escarpment in the windward direction.18 Fighting wind erosion—one aspect of the combat against desertification
  21. 21. Seif dune reactivation in the Trarza sandsea on the eastern outskirts of Nouakchott (Mauritania, 2005). Reactivated seif dunes composed of sand bleached by hydromorphy. They are obliquely oriented NNE 5° to 10° with respect to the direction of the prevailing wind (harmattan, NNE 30°) which is responsible for the paleoclimatic longitudinal dunes. These dunes elongate obliquely within the interdune corridors. © F. Dumay and M. MainguetPresently, the reactivation of longitudinal dunes leads the relief exposed to harmattan winds, and turns into ato sand release. This sand can be locally worked and sandstone escarpment. The Ténéré Sandsea reappearsgive rise to a new generation of seif dunes that, in turn, a few kilometres downwind of an aeolian overdeepeningextend obliquely and cross the interdune corridors depression close to the foot of the escarpment.where they are channelled and reworked into barchandunes or mobile sand veils. In the transit area, along the same current, the wind can bear sand or not. The transporting wind and theSand released from the dunes transits through the material transported should be considered separately.interdune corridors where it can be blocked by human In Saharan winds, material accumulated along thestructures. This increases the risk of sand invasion, desert edges is not necessarily of such remote origin,which is originally due to an erosion phenomenon. i.e. it could have been picked up anywhere along theThis is the situation in Trarza Erg, Mauritania, where wind current. Sand migrates in the form of sand grains,inhabitants are obliged to abandon the interdune saltation veils or dunes.areas—that previously served also as pathways andgrazing areas for herds and as roads for cars—whichare gradually filled with locally reactivated sand. It also > FOCUS | Nebkas—indicators of windseems that this phenomenon is self-sustained by the erosion and short-range transportpopulation. People flee from the interdune depressionsand from the sand transiting through them andmassively settle on the longitudinal dunes. This leadsto destruction of the structure of red soils that wereformed under more humid paleoclimatic conditions,with compaction of the sand and subsequent releaseof even more material that could be carried away bythe wind. © F. Dumay / M. MainguetPredominantly wind transit areasAt the soil-atmosphere interface, sand is transportedby stages for long distances over very long periods(millenia) along hierarchically arranged flow channels,which is why it is tempting to talk about ‘sand rivers’. Nebka and sand veils in theThese wind f lows differ from water f lows. Although Moroccan Sahara (Morocco, 2006).effectively channelled by the topographical features,they have no banks and they blow over relatively Nebkas, or sand arrows, are good indicators of deflationgentle windward slopes. Along these transit areas, the and short distance aeolian transport. They form onsurface roughness can increase to reach such a level the leeward side of small obstacles such as stonethat parts of the area become deposition areas. The blocks or isolated bushes that locally decrease thereverse also holds true, with the most original example wind speed, thus leading to an accumulation of up toconcerning Kaouar escarpment (Niger) in the middle 50 cm high. Here the wind direction is from the rightof Ténéré Desert. The Ténéré sandsea stops upwind of to the left of the photograph.Wind erosion mechanisms at the Man-Nature interface 19
  22. 22. Sand accumulation area their path. When dune movements are slowed down by vegetation cover or rough ground, they can fuseSedimentation processes prevail in sand accumulation together to form barchan dune chains and a system ofareas. Sand can accumulate at a GWAS terminal or transverse dunes that, in turn, form vast depositionlocally ever y time the wind comes up against an sandseas, e.g. Aoukar Erg in Mauritania.obstacle. For instance, a sand-laden wind that meetsa reverse slope that is sufficiently steep to neutralize Seif dunes, or linear dunes, w ith an angular andthe vertical wind component (which it generated itself) sinuous profile, are obliquely oriented with respectwill release its sand load onto a slope if it is gentle to two prevailing winds. They can be several dozensor, if it is steep to vertical, create an echo dune some of kilometres long, a few dozens of metres wide anddistance from the obstacle. As the volume of an echo 20-30 m high. They are formed as a result of thesedune increases, it becomes a climbing dune leaning on two combined winds, by: prolonging barchan dunethe slope, and then surpasses the peak and may extend wings; building up in areas where leeward winds meetfurther to become a linear dune (or seif dune). behind an obstacle; or deriving from the reactivation of longitudinal dunes. The sand that they are made ofSa nd encroach ment concer ns a nd st rateg ies to migrates by surpassing the dune peak and migratescontrol it are critical in such accumulation areas. also parallel parallel to the foot of the dune slope.Here again, the nature of the sediment budget can be The bottoms of the slopes of the seif dunes serve asdetermined on the basis of the types of dunes present. sand transport routes, which is why they are alwaysIn accumulation areas, depending on the type of wind narrow and their dynamics favour elongation. Seifsystem, various dunes may be found, e.g. barchans , dunes on the outskirts of Nouakchott extended 450seif dunes and star dunes . They have several roles: m bet ween 1984 and 1991. They ‘slit her’ (i.e. bysand reservoirs, transport (except star dunes, which sideways undulation) in the direction of the windsare not mobile), and especially accumulation. that shape them. When they are located close enough to each other they can fuse as a result of seasonalA monodirectional wind system is required for the wind changes, giving rise to complex braid-shapedformation of barchan dunes, which are aligned in the dunes or seif clusters. This dune elongation trend isprevailing wind direction. These dunes move in the especially threatening for human infrastructures, e.g.direction of the wind, which blows from the convex to every day during the dry season they block the Routethe concave side of the dune according to the orientation de l’Espoir in Mauritania. Contrary to barchan dunesof their symmetry axis. As barchan dunes are mobile, which just pass over infrastructures, seif dunes do notthey threaten human infrastructures, sometimes move but instead continue lengthening, thus oftenengulfing roads, railway tracks or accumulating on blocking roads and channels running perpendicularlythe windward side of buildings and villages lying on with respect to their elongation direction.> FOCUS | Different types of aeolian dunes 20 Fighting wind erosion—one aspect of the combat against desertification

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