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Fighting wind erosion. one aspect of the combat against desertification

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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.

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Issue3
FIGHTING WIND EROSIONone aspect of the combatagainst desertifi cation
Comité Scientifi que Français de la Désertifi cationFrench Scientifi c Committee on Desertifi cation
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(Institut de recherche pour le développement, France)AuthorsMonique MainguetMember of the Institut universitaire de Franceand Director of the Laboratoire de géographiezonale pour le développement (LGZD), Researcherassociated with theGroupe d’étude sur lesgéomatériaux et environnements naturels etanthropiques(GEGENA²), University of ReimsChampagne-Ardenne, Francemonique.mainguet@univ-reims.frFrédéric DumayResearch Engineer at GEGENA²University of Reims Champagne-Ardenne, Francefrederic.dumay@univ-reims.frScientific editing and iconographyIsabelle AmsallemAgropolis Productionsinfo@agropolis-productions.frDesign and productionOlivier PiauAgropolis Productionsinfo@agropolis-productions.frPhotography creditsDanièle Cavannaand ChristelleMary(INDIGO picture library oftheInstitut de recherche pour ledéveloppement) as well as the authors of thepictures shown in this report.TranslationDavid ManleyEditing, production and distribution ofLes dossiersthématiques du CSFD are fully supported by thisCommittee through the backing of relevant FrenchMinistries as well as the French Development Agency.Les dossiers thématiques du CSFDmay be freelydownloaded from the Committee website:www.csf-desertification.orgPrinted with solvent-free inks on certified chlorine-freebleached paper derived from sustainably managedforests.Printed by:Les Petites Affiches (Montpellier, France)Copyright registration on publicationISSN: 1772-6964  1,500 copies© CSFD / Agropolis International, May 2011For reference:Mainguet M. & Dumay F., 2011. Fightingwind erosion. one aspect of the combat againstdesertification.Les dossiers thématiques du CSFD.N°3.May 2011. CSFD/Agropolis International, Montpellier,France. 44 pp.
French Scientific Committeeon Desertification
The creation in 1997 of the French Scientific Committee onDesertification (CSFD) has met two concerns of the Ministries in chargeof the United Nations Convention to Combat Desertification. First,CSFD materializes the will to involve the French scientific communityversed in desertification, land degradation, and development of arid,semi-arid and sub-humid areas, in generating knowledge as well asguiding and advising policymakers and stakeholders associated inthis combat. Its other aim is to strengthen the position of this Frenchcommunity within the international context. In order to meet suchexpectations, CSFD is meant to be a driving force regarding analysisand assessment, prediction and monitoring, information and promotion.Within French delegations, CSFD also takes part in the various statutorymeetings of organs of the United Nations Convention to CombatDesertification: Conference of the Parties (CoP), Committee on Scienceand Technology (CST), Committee for the Review of the Implementationof the Convention. It also participates in meetings of European andinternational scope. It contributes to advocacy activities concerningdryland development, in connection with civil society and media. Itcooperates with DesertNet International (DNI).CSFD includes a score of members and a President, who are appointedintuitu personaeby the Ministry for Higher Education and Research,and come from various specialties of the main relevant institutions anduniversities. CSFD is managed and hosted by the Agropolis InternationalAssociation that gathers, in the French town of Montpellier andLanguedoc-Roussillon region, a large scientific community specialisedin agriculture, food and environment of tropical and Mediterraneancountries. The Committee acts as an independent advisory organwithout decisionmaking powers or legal status.Its operating budget is financed by subsidies from the French Ministriesof Foreign and European Affairs and of Ecology, Energy, SustainableDevelopment and the Sea in charge of Green Technologies and ClimateChange Negotiations as well as the French Development Agency. CSFDmembers participate voluntarily to its activities, as a contribution fromthe Ministry for Higher Education and Research.More about CSFD:www.csf-desertification.org
Également disponibleen version française originaleMainguet M. et Dumay F., 2006.Combattre l’érosion éolienne : un voletde la lutte contre la désertification.Les dossiers thématiques du CSFD. N°3.Avril 2006. CSFD/Agropolis International,Montpellier, France. 44 pp.
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Une évaluation indispensable de l’état actuel de la dégradation des terres
Foreword
Mankind is now confronted with an issueof worldwide concern, i.e. desertification,a process induced by human activities. Our planetand natural ecosystems have never been so degradedby our presence. Long considered as a local problem,desertification is now a global issue that affects us all,including scientists, decisionmakers, citizens from boththe South and North. Within this setting, it is urgentto boost the awareness of civil society to convince it toget involved. People must first be given the elementsnecessary to better understand the desertificationphenomenon and the concerns. Everyone should haveaccess to relevant scientific knowledge in a readilyunderstandable language and format.Within this scope, the French Scientific Committeeon Desertification has decided to launch a new seriesentitledLes dossiers thématiques du CSFD, which isdesigned to provide sound scientific information ondesertification, its implications and stakes. This seriesis intended for policymakers and advisers from theNorth and South, in addition to the general publicand scientific journalists involved in development andthe environment. It also aims at providing teachers,trainers and trainees with additional informationon various associated fields. Lastly, it endeavours tohelp disseminate knowledge on the combat againstdesertification, land degradation, and poverty tostakeholders such as representatives of professional,nongovernmental, and international solidarityorganisations.
These reports are devoted to different themes suchas global public good, remote sensing, wind erosion,
of current knowledge on these various subjects. Thegoal is also to set out ideological and new conceptdebates, including controversial issues; to expoundwidely used methodologies and results derived froma number of projects; and lastly to supply operationaland intellectual references, addresses and usefulwebsites.These reports are to be broadly circulated, especiallywithin the countries most affected by desertification,by e-mail (upon request), through our website, andin print. Your feedback and suggestions will be muchappreciated! Editing, production and distribution ofLes dossiers thématiques du CSFD are fully supportedby this Committee thanks to the backing of relevantFrench Ministries and the French Development Agency.The opinions expressed in these reports are endorsedby the Committee.Richard EscadafalPresident of CSFDResearch Director at CESBIO (Centre for the Studyof the Biosphere from Space, Toulouse, France) for IRD(Institut de recherche pour le développement, France)
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This readiness to provide decisionmakers and otherstakeholders with access to this pool of expertise should
Preambleesertification and land degradation give riseDto several phenomena (or processes) whoseprevailing conditions: soil fertility loss, reduction inplant and tree cover, loss of soil water retention capacity,runoff, water and wind erosion. The impacts includeincreased environmental fragility, reductions in cropyields and thus in inhabitants’ income, increasedfood insecurity and increased overall vulnerabilityof societies to climatic risks and economic crises, thuspromoting outmigration.ThisDossier thématique du CSFD is of considerableinterest as it is focused on one of the most flagrantsigns of desertification—wind erosion. As pointed outby the authors at the outset, winds and dust stormsare currently worsening, thus fostering wind erosion.This situation is especially worrisome in Mauritaniaand along the Niger Loop.TheDossier highlights wind-driven mechanisms,factors that trigger erosion and land degradation, aswell as monitoring strategies. It also provides a crucialupdate on what is currently known about techniquesfor combating sand invasion and this form of erosion.The need for an integrated approach is also stressed.Many examples are presented which demonstrateboth the advantages and shortcomings of differentcontrol methods, while also providing the cost rangesfor carrying out control operations.
„ In 1993, wind erosion phenomena in Mauritania was thetopic of his postgraduatediplôme d’études approfondiesPhD qualification thesis. In 2007, he became a ResearchEngineer at GEGENA².Frédéric was highly proficient in his field work, underpinnedby his preliminary satellite image analysis skills.His research was carried out in Africa (Morocco, Mauritania,Senegal, Niger, northern Cameroon) and Asia (Israel, China).From 14 to 27 May 2010, he launched a collaboration withHunan University in Changsha (China). Frédéric was alwaysready to invest time and energy in helping others.Vincent Barbin,Professor of Geoscience, Director of GEGENA²Monique Mainguet,Professor associated with GEGENA²
specialized on erosion and sustainable water and soilmanagement (EGCES Network), supported by theAgenceuniversitaire de la francophonie, is in full agreementwith the conclusions of thisDossier and praises theinitiative. This Network, which developed out of aformer network that was very active at ORSTOM, i.e.now theInstitut de recherche pour le développement(IRD), would like to play an active role, while workingalongside communities of francophone researchers,developers and teachers in developed and developingcountries to boost awareness on the serious globalproblem of soil erosion. The EGCES Network alsowelcomes researchers from various scientific fieldswho could fuel the soil erosion debate: methods(spatial biophysical indicators), socioeconomicaspects, conventional and modern erosion controlstrategies, water and water quality management, landrehabilitation and biodiversity.The EGCES Network is close to the activities of CSFD insemiarid regions where vegetation cover degradationgives rise to most problems concerning land erosion(since the wind is highly active), dam silting andaccess to good quality water supplies. It is essentialto overcome these problems in order to be able tofulfil the basic needs of a growing population. Theseissues were stressed during the Network’s scientificseminars that were held within the framework of the lastInternational Soil Conservation Organization (ISCO)Conference, in May 2006, Marrakech, Morocco.Éric RoosePresident of the EGCES NetworkGeorges De NoniScientific Secretary of the EGCES Network
Fighting wind erosionone aspect of the combat against desertification
Frédéric Dumay (1970-2011)Frédéric Dumay passed away on 18 February 2011, he was41 years old. He has been a very fine, excellent, intelligent,devoted and efficient collaborator for the last 20 years. Hisuniversity studies and professional functions were basedat the University of Reims Champagne-Ardenne from 1989until his passing.„ After beginning his university studies in Geography,Frédéric immediately took a passionate interest in tropicaldryland environments. In 1992, he became a TechnicalCollaborator for the French Laboratoire de géographiezonale pour le développement (LGZD).„ In his MSc thesis in 1993, he analysed the negativeenvironmental impacts of the war in Koweit.„ In 1993, he successfully passed the French nationalcompetitive design engineering examination and becameone of the pillars of LGZD.
Table of Contents
Table of Contents
Wind erosion—a complex environmental degradation mechanismWind erosion onset thresholds and land degradation triggeringfactors highlighted by remote sensingWind erosion mechanisms at the Man-Nature interfaceWind erosion control techniquesHope for the future of wind erosion control
List of acronyms and abbreviationsFor further information…Glossary
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Wind erosiona complex environmentaldegradation mechanism
ind erosion is one of the most seriousWenvironmentaldegradationmechanisms,Tmoackoermsbsahtouwlidnbdeearwosairoenofmtohreesepfafceec-ttiivmeley,codencciespitosno-as 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 usingremote 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.to* Terms defined in the glossary (page 44) are highlighted in blue andThese three stages must therefore be analysed getherunderlined 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!
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Fighting wind erosionone aspect of the combat against desertification
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SArrival of a sandstorm in the area ofMare 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 © IRD
Wind erosion—a complex environmental degradation mechanism
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Fighting wind erosionone aspect of the combat against desertification
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DETECTION OF THE FIRST SIGNS OF WIND EROSIONThe wind erosion onset threshold is the point at whichthe bala* between natural resources and humanncepressure, aggravated by severe climatic conditions,is upset. It is essential to determine this thresholdso as to be able to rapidly (on temporal and spatialscales) identify premonitory signs of environmentaldegradation before the degradation stage and ultimatedesertification stage. Improvements are needed withrespect to detecting these thresholds and the differentdegrees of preliminary degradation in order to be ableto rapidly boost awareness on wind erosion onset.Precursory signs detected byremote sensing includeoverdeepening of the surface layer (blowouts), increasesin the density of small aeolian sand deposits ornebkas(sand arrow leewards of a bush or stone), the emergenceof irregular mobile sand deposits, reactivation ofthe crests of sand dunes, changes insoil reflectance.Monitoring tools are available that should be tailoredto different monitoring scales and supplemented byfield surveys and quantified data. These signs mustalso be clearly and quickly interpreted so as to enablerapid and effective interventions.
Ideforestation, overgrazing, excessive irrigation and,consequently, salinization, 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, towndwellers and policymakers—through their activitiesand developments—play a key role in wind erosiononset. This process may also be accelerated by severeclimatic events (drought, flooding, peaks in water andwind erosion).In dryland environments (which in Africa, accordingto our observations, reaches isohyet 600 mm/year inthe southern Sahel), aeolian mechanisms are the mainfactors that determine the vulnerability of farmlandand urban areas located in sandy environments (e.g. inNouakchott, rainfall 100 mm/year), sometimes leadingto desertification.This phenomenon is under way in the groundnutgrowing area in Senegal. The extent of degradationnoted in the northern part of this area around the endof the first decade of the 21st century was similar to thatobserved at the latitude of Nouakchott, Mauritania, inthe early 1970s (Fall, 2001). It would thus be essentialto analyse satellite images and aerial photographs,before and after ground checks, to quickly determinethe wind erosion onset threshold in order to developand implement preventive control initiatives.
All concerned countries should have several permanentteams working on this issue.
* In dryland ecosystems, this refers to a steady state in soils and in thenaturally open vegetation cover.
Wind erosion onsetthresholds and landdegradationtriggering factorshighlightedby remote sensing
SOblique aerial view of the centre of the city of Nouakchott,Mauritania, 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 CONCEPTSatellite images are key tools for environmentalinvestigations. They provide users with access toin-depth detailed and repetitive information. Theobservations may be combined to obtain a multiscaleview, ranging from the synoptic scale with METEOSATand NOAA images, to the regional scale with LANDSATand SPOT images.Based on these satellite images, the global (or regional)wind action system (GWAS [or RWAS]) concept wasdeveloped and the limits were determined (Mainguet,1984). A GWAS (or RWAS) is a series of particle-ladenwind currents marked out by open interlinkedergs(sand seas) that formerg chains like those linking theSahara and the Sahel (Mainguet and Dumay, 1995).Geostationary satellites (having a stationary orbit, i.e.METEOSAT for Africa or circumpolar regions, NOAAfor Eurasia) generally covering an entire GWAS.
Y. Boulvert ©IRD
The smallest scale images (METEOSTAT, NOAA) areused to identify different wind system units: sourcearea, transit areas and deposition areas. Scales mayoverlap within these units depending on the presenceof megaobstacles (mountains and plains, Mainguet,1976) and the roughness of the local landscape (relief,vegetation, buildings). Defined subunits may, forinstance, be an erosion area within a deposition unitor, conversely, a deposition area within a unit in whicherosion prevails, as detected on large-scale satelliteimages (SPOT or LANDSAT satellites).When assessing a regional system, an image mosaic canbe constructed, e.g. the regional wind action system inthe Aral Basin in Central Asia (Mainguet, Dumay andLétolle, 2002). Aerial photographic coverage is essentialwhen highly detailed observations are required forinfrastructure protection (buildings, roads, irrigationcanals, etc.).
Wind erosion onset thresholds and land degradation triggering factors highlighted by remote sensing
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> FOCUS |Regional wind actionsystem (RWAS) on the easternside of the Aral BasinAral Basin (1.8 million km²) accounts for 4% of theglobal temperate dryland area. This RWAS, whichis studied in the field and on Cosmos, NOAA andSPOT satellite images, begins at the southernoutlet of the Tourgai Corridor at the interface ofthe Siberian Plain and Aral Basin. At its centre,there is a deflation area where satellite imagesreveal a dense paleohydrographic network, SirDaria and Amou Daria Basins, the only permanentallogenic streams and, in the south, the temporaryTedjen and Mourgab streams. It ends at thenorthern foot of Khopet Dag (3 349 m) with a375 km by 40 km loess strip.The wind releases its load of sand at the outlet ofthe Tourgai Venturi, which has led to the formationof fourergs north of Aral Lake: Grand Barsouki,Small Barsouki, Barsakoum and Karakoum, whichshould not be mistaken for the Grand Karakoumerg. At their southern edge, shiftingbarchandunesare invading the villages of Kulandy andAkespe (46°5 N, 60°3 E). Grand Barsouki (46° à48°N, 58°5 à 60°E) is 250 km long by 25-50 kmwide. At the point of contact with Aral Lake, nearKoulandy (59°3 N and 46°1 E), it is the only ergwest of the lake within the RWAS. In its downwindpart, 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 blanketthe early Quaternary paleovalleys. Their dunesform part of the shores of the Gulfs of Boutakov andTouchibas. The sand derives from glaciofluvial depositsextending from the Mougodjar hills in the southern Uralregion to the Tourgai Corridor plains (Irgiz, etc.).A 1957 geological map (at 7 500 000 scale) indicatesthat the material is of tertiary paleogenic fluvial originand the result of wind erosion. The RWAS extends alongthe eastern side of Aral Basin.Deflation in the Kazakhsteppe south of the Siberian plate is the source of sandfor the NNW-SSElongitudinal dunes in the northernKarakoum erg. Sir Daria interrupts the continuity of theergs and supplies sand for the Kyzylkoum erg that beginsjust south of it. The general NNW-SSE orientation of thedunes continues south of Sir Daria and then shifts 44°N tobecome NNE-SSW as the dunes encompass Aral Basin,which in turn serves as an obstacle to the wind.Sand deposits thus do not accumulate to a great extentin the sheltered lake basin but rather on the first reverseslopes. The inflexion can be seen on satellite images up to150 km east of the lake, at the northern edge of the AmouDaria paleodelta where the wind direction measured inthe field is NNE-SSW 10°. Between Sir Daria delta and thesoutheastern Aral Basin (former Akpekti Archipelago), theKyzylkoum erg sand particle size decreases to becomesilt in the Sir Daria delta region.
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SThis RWAS clearly highlights the regional aspect of aeolian currents thatdetermine the surface features in sandy regions covering an area of 1 500 kmfrom north to south.1. Aral  2. Sand sheets and ergs  3. Paleo-wadis  4. Western boundary of riverinepaleochannels not blocked by aeolian material  5. Major wind directions in the regionalwind action system (RWAS) in Aral Basin  6. Sheltered areas on the leeward side ofobstacles  7. Erg boundaryZana Daria paleostreams crossed the Kyzylkoum erguntil the late Bronze Age, whereas the Amou and SirDaria deltas converged. The western part consists oflongitudinal dunes that cross the network of paleovalleysvisible on satellite images.Less clearcut dunes are found on the older easternpart. Kyzylkoum sandseas extends to the right banksof Syr Daria river and, simultaneously, to the left banks.The wind streams cross the river carrying the sand thathas emerged during the dry season. This could explainwhy alluvial material blends fine carbonate-rich (20-30%) aeolian material from Kyzylkoum sandsea withalluvial silts. In this sector, the wind currents form adivergence which is responsible for the formation ofGrand Karakoum sandsea, which is followed downwindby the Kyzylkoum sandsea. The Karakoum sandsea isformed north of the loess strips in the southern partof the RWAS.From Mainguet M., Dumay F. and Létolle R., 2002.
Fighting wind erosionone aspect of the combat against desertification
> EXAMPLE |Using aerial photographs to monitor sand dunes in Mauritania
On an aerial photograph taken in 1991, thereis a clear increase in the number of seifdunes and in the extent of sand invasion inthe vicinity of the water tower northwest ofNouakchott.
SN. Nouakchott  W. Water tower S. Seif dunes  F. Fence1. Aerial photograph IGN MAU 1991 12/150 n°58 © IGN 19982. Interpretation sketch drawn by LGZD.
ESSENTIAL GROUND CHECKSThe combined analysis of satellite images and aerialphotographs is an essential support for field surveys.Preliminary overall knowledge of sectors investigatedAERIAL PHOTOGRAPHS AND CLOSE MONITORING OFin the field enhances thein situ understanding ofSAND DUNE TYPES AND THEIR SURFACE STATUSsand encroachment and wind erosion areas and theirdynamics. In the field, geographers’ expertise—anW hen used a longside satellite images, aeria l essential tool—and ability to consider landscapesphotographs provide more detailed information, in terms ofmorphodynamic 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 (Nouakchottseif dunes,longitudinal dunes, etc.) inform geographersin Mauritania, Draa Oasis, or the Kénitra-Larache on the nature of the localaeolian sediment budget: ismotorway in Morocco). the area in a particle loss (erosion, with a negativesediment 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 ofnebkas, increasedsand(concerning the shape and surface status of particlespowdering orsand 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
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