The erosion and transport of fertile topsoil is a serious problem in the U.S., Australia, China and throughout Europe. It results in extensive environmental damage, reduces soil fertility and productivity, and causes significant environmental loss. It is as big a threat to the future sustainability of global populations as climate change, but receives far less attention. With both chemicals (fertilizers, pesticides, herbicides) and biological pathogens (bacteria, viruses) preferentially sorbing to silt and clay sized soil particles, estimating contaminant fluxes in eroded soil also requires predicting the transported soils particle size distribution. The Hairsine-Rose (HR) erosion model is considered in this thesis as it is one of the very few that is specifically designed to incorporate the effect of particle size distribution, and differentiates between non-cohesive previously eroded soil compared with cohesive un-eroded soil. This thesis develops a new extended erosion model that couples the HR approach with the one-dimensional St Venant equations, and an Exner bed evolution equation to allow for feedback effects from changes in the local bed slope on surface hydraulics and erosion rates to be included. The resulting system of 2I +3 (where I = number of particle size classes) nonlinear hyperbolic partial differential equations is then solved numerically using a Liska-Wendroff predictor corrector finite difference scheme. Approximate analytical solutions and series expansions are derived to overcome singularities in the numerical solutions arising from either boundary or initial conditions corresponding to a zero flow depth. Three separate practical applications of the extended HR model are then considered in this thesis, (i) flow through vegetative buffer strips, (ii) modelling discharge hysteresis loops and (iii) the growth of antidunes, transportational cyclic steps and travelling wave solutions. It is shown by comparison against published experimental flume data that predictions from the extended model are able to closely match measurements of deposited sediment distribution both upstream and within the vegetative buffer strip. The experiments were conducted with supercritical inflow to the flume which due to the increased drag from the vegetative strip, resulted in a hydraulic jump just upstream of the vegetation. As suspended sediment deposited at the jump, this resulted in the jump slowly migrating upstream. The numerical solutions were also able to predict the position and hydraulic jump and the flow depth throughout the flume, including within the vegetative strip, very well. In the second application, it is found that the extended HR model is the first one that can produce all known types of measured hysteresis loops in sediment discharge outlet data. Five main loop types occur (a) clockwise, (b) counter-clockwise, (c,d) figure 8 of both flow orientations and (e) single curve. It is clearly shown that complicated temporal rainfall patterns or bed geometry are not required to developed complicated hysteresis loops, but it is the spatial distribution of previously eroded sediment that remains for the start of a new erosion event, which primarily governs the form of the hysteresis loop. The role of the evolution of the sediment distribution in the deposited layer therefore controls loop shape and behavior. Erosion models that are based solely on suspended sediment are therefore unable to reproduce these hysteretic loops without a priori imposing a hysteretic relationship on the parameterisations of the erosion source terms. The rather surprising result that the loop shape is also dominated by the suspended concentration of the smallest particle size is shown and discussed. In the third application, a linear stability analysis shows that instabilities, antidunes, will grow and propagate upstream under supercritical flow conditions. Numerical simulations are carried out that confirm the stability analysis and show the development and movement of antidunes. For various initial parameter configurations a series of travelling antidunes, or transportational cyclic steps, separated by hydraulic jumps are shown to develop and evolve to a steady form and wave speed. Two different forms arise whereby (a) the deposited layer completely shields the underlying original cohesive soil so that the cohesive layer plays no role in the speed or shape of the wave profile or (b) the cohesive soil is exposed along the back of the wave such that both the non-cohesive and cohesive layers affect the wave profile. Under (a) the solutions are obtained up to an additive constant as the actual location of the boundary of the cohesive soil is not required, whereas for (b) this constant must be determined in order to find the location on the antidune from where the cohesive soil becomes accessible. For single size class soils the leading order travelling wave equations are fairly straightforward to obtain for both cases (a) and (b). However for multi-size class soils, this becomes much more demanding as up to 2I + 3 parameters must be found iteratively to define the solution as each size class has its own wave profile in suspension and in the antidune.

Thesis (MScAgric (Soil Science))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Lesotho is a country with an international reputation for the severe degree of soil erosion in its landscape. Despite several national soil conservation projects, soil erosion continues at an astounding rate. One of the reasons for this is possibly that the interactions between soil properties and erosion in Lesotho are not understood. Soil erosion is a site specific, cyclic phenomenon, controlled by geomorphological thresholds. To control soil erosion, the processes and soil properties which influence soil erosion in the specific place must be understood. In this study the soil properties of a highly eroded sub-catchment in Maphutseng, Lesotho was investigated. The gully extent in the sub-catchment, in 1957 and 2004 respectively, was mapped from aerial photos. These maps show where in the landscape gullies developed during this time. The gully maps were superimposed on maps of several soil erosion factors, to correlate the spatial distribution of the erosion factors with that of the gully distribution. A soil map was especially drawn for this. The spatial analysis shows that gully development between 1957 and 2004 was primarily confined to the area where duplex soils occur. The rest of the sub-catchment underwent negligible differences in gully extent during this time. The initiation of the gullies on the duplex soil area is ascribed to tunnel erosion. The high dispersibility of the duplex soil samples, sink holes which occur in this area and previous observations by researchers in this area gave evidence to this hypothesis. In the second part of the study the soil properties of seventeen soil profiles from across the study site were analysed. The difference in gully distribution between the duplex soils area and the rest of the catchment is ascribed to the high dispersibility of the duplex soils. No strong correlations could be found between the dispersion index and other determined soil properties. Segmented quantile regression was used to analyse the data further. Soil samples with moderate levels of total carbon (1.17%), iron oxide (0.9%) and effective cation exchange capacity (13.7 cmolc/kg), have below average dispersibility. When none of these stabilising agents are present in moderate amounts, soils with even low exchangeable sodium percentage values (0.68%) are dispersive. Furthermore, soils which have developed in colluvial material from basaltic origin were found to be less dispersive, presumably because of the amorphous clay minerals present in the volcanic material. The colour and increase in clay content between the A and B horizons of a soil can indicate the tunnel erosion potential of the soil. Dark coloured soils (values less than 4 and chromas less than 3) were found to have low dispersibility and free water can accumulate in the subsoils where the B horizon has much more clay than the A horizon. The accumulation of free water in the subsoil is necessary for tunnel formation. Thus soils with dark colours and/or a low clay accumulation index have low tunnel erosion potential. / AFRIKAANSE OPSOMMING: Lesotho is ‘n land met ‘n internasionale reputasie vir die ernstige graad van gronderosie waaronder die landskap gebuk gaan. Ten spyte van verskeie nasionale grondbewaringsprojekte duur die erosie teen ‘n verstommende tempo voort. Een van die redes hiervoor is heel moontlik dat die interaksies tussen grondeienskappe en erosie in Lesotho nie verstaan word nie. Gronderosie is ‘n plekspesifieke, sikliese verskynsel, wat deur geomorfologiese drempelwaardes beheer word. Om gronderosie te bekamp moet die prosesse en grondeienskappe wat gronderosie in die spesifieke plek beïnvloed, geïdentifiseer en verstaan word. In hierdie studie is die grondeienskappe van ‘n hoogs geërodeerde opvanggebied in Maphutseng, Lesotho ondersoek. Die dongaverspreiding in die opvanggebied, in 1957 en 2004 respektiewelik, is vanaf lugfoto’s gekarteer. Die kaarte wys waar in die landskap dongas gedurende hierdie tyd ontwikkel het. Die dongakaarte is op kaarte van verskeie gronderosie faktore gesuperponeer om die ruimtelike verspreiding van die erosie faktore met die donga verspreiding te korreleer. ‘n Grondkaart is spesiaal vir hierdie doel opgestel. Hierdie analise het gewys dat donga-ontwikkeling tussen 1957 en 2004 hoofsaaklik op die area waar dupleks gronde voorkom plaasgevind het. Die res van die opvanggebied het weinig verskille in donga verspreiding in hierdie tyd ondergaan. Die ontstaan van die dongas in die dupleksgronde word toegeskryf aan tonnelerosie. Die hoë dispergeerbaarheid van die dupleks grondmonsters, sinkgate wat in die area voorkom en vorige waarnemings deur navorsers in die area verleen bewyse aan hierdie hipotese. In die tweede deel van die studie is die grondeienskappe van sewentien grondprofiele van regoor die opvanggebied ontleed. Die verskil in donga verspreiding tussen die dupleksgrond area en die res van die opvanggebied is toeskryfbaar aan die hoë dispergeerbaarheid van die dupleks gronde. Geen sterk korrelasies is tussen die dispersiwiteits indeks en ander bepaalde grondeienskappe gevind nie. Gesegmenteerde kwantiel regressie is gebruik om die data verder te ontleed. Hierdie ontleding het gewys dat grondmonsters met matige vlakke van totale koolstof (1.17%), ysteroksied (0.9%) en effektiewe katioonuitruilkapasiteit (13.7 cmolc/kg), ondergemiddelde dispergeerbaarheid toon. Waar nie een van hierdie stabiliserings agente in matige hoeveelhede voorkom nie, is selfs gronde met baie lae uitruilbare natriumpersentasie waardes (0.68%) dispersief. Daar is ook gevind dat gronde wat vanuit kolluviale basaltiese afsettings ontwikkel het, minder dispersief is. Die kleur en verskil in klei persentasie tussen die A en B horison van ‘n grond kan as aanduiding dien van die grond se potensiaal vir tonnelerosie. Donker grondkleure (waarde laer as 4 en chroma laer as 3) wys op ‘n lae dispersiwiteit terwyl vrywater in die ondergrond van gronde waar die B horison veel meer klei as die E horison bevat kan akkumuleer. Die aansameling van vrywater in die ondergrond is noodsaaklik vir tonnelvorming. Dus het donker gronde en gronde met ‘n lae klei akkumulasie indeks ‘n lae potensiaal vir tonnelerosie.

In developing natural resources, decision makers are seeking to achieve different objectives, which cannot be reduced to a single objective such as economic efficiency, this covers only part of the problem. Tradeoffs between multiple objective of unequal importance is unavoidable in the process of selection or ranking of alternative developmental projects or plans. Multiobjective technique has the ability to deal with qualitative and quantitative objectives, also it enhances the planning process by involving broader segments of the society in the process of decision making. Compromise programming (CP) and utility worth analysis (UWA), two multiobjective methods were applied on Zarqa River Basin Project (ZRBP) in Jordan. Their appropriateness and suitability as decision aiding tools was examined in this study. For the purpose of the study, five criteria were developed to serve as a basis for the evaluation and 61 farmers and 15 technicians, planners and decision makers were interviewed. High consistency was observed among the results of ranking the six alternatives when both methods were applied, at the same time the ranking of the alternatives according to benefit/cost ratio and the internal rates of return as economic efficiency measures showed no agreement with the multiobjective ranking.

Surface mining operations require a comparison of post-mining erosion rates with pre-mining soil loss to ascertain if remedial measures are needed. In this study the Universal Soil-Loss Equation (USLE) was modified to reflect conditions of western rangelands to develop a procedure for estimating pre-mining soil loss rates. The modification used back-calculation for the C-Factor and an adjusted R-Factor based on storm size. Soil loss simulation based on stochastic precipitation patterns is appropriate to the site--the La Plata mine area in northern New Mexico--and increases the flexibility of the USLE as a soil loss predictor for western rangelands.

Soil and terrain changes as a result of increased erosion and accumulation processes in selected locations Abstract Colluvial soils are formed in areas with increased intensity of soil erosion. Colluvial soils originate as a result of their position in terrain, they are usually found in places where has accumulated erosion material. Very often they are to be found at the around of slopes. Their accumulation horizon is deeper than 25 cm with a great amount of organic carbon buried in it. In my diploma thesis I will attempt to take a closer look at three locations, each having different types of dominant soils. The aim of this diploma thesis is to rebuild the original shape of the terrain before the beginning of the soil erosion process. Keywords: colluvial soil, soil erosion, soil accumulation, land use, digital terrain model (DTM)

Trotz des erheblichen Ausmaßes der Bodenerosion in vielen Entwicklungsländern ist bislang weitgehend unklar, welches ihre wesentlichen anthropogenen Ursachen sind, und damit auch, wo Politiken und Maßnahmen für den Erhalt der Nahrungs- und Produktionsressource Boden ansetzen sollten. Jenseits unmittelbarer natürlicher und landnutzerischer Ursachen stehen heute sozioökonomische Faktoren im Mittelpunkt der Diskussion, von denen angenommen wird, daß sie die Anbau- und Bodenschutzentscheidungen der Landnutzer und darüber das Ausmaß an Bodenerosion beeinflussen, insbesondere: (i) verstärkte Armut, (ii) zunehmender Bevölkerungsdruck, (iii) verzerrte Agrarpreise, (iv) unangepaßter technischer Fortschritt sowie (v) unsichere Landbesitzverhältnisse. Der Bedeutung dieser Bestimmungsfaktoren wird vorwiegend im Rahmen produktionsökonomischer Ansätze und der Theorie der Induzierten Innovation nachgegangen. Allerdings wird die Wirkung einzelner Ursachen in der Literatur sehr unterschiedlich eingeschätzt. So wird beispielsweise in eher optimistischen Szenarien davon ausgegangen, daß Armuts- und Bevölkerungsdruck langfristig zur Entwicklung und Verbreitung bodenschonender Innovationen führen. In negativen Szenarien überwiegen hingegen Stimmen, die gerade in diesem Druck bei gleichzeitigem Preisdruck die wesentlichen Ursachen für die kurzsichtige Übernutzung des Bodens sehen. Empirische Studien zur Fundierung der kontrovers diskutierten Hypothesen liegen bislang nur für einen jeweils begrenzten lokalen Kontext vor und sind kaum verallgemeinerbar. Vor diesem Hintergrund bieten die Daten der ersten weltweiten Erhebung zum Stand der Bodenerosion (GLASOD, UNEP/ISRIC, 1991) nunmehr die Möglichkeit, sozioökonomische und landnutzerische Determinanten der Bodenerosion auf überregionaler Ebene empirisch zu untersuchen. Anhand der Aggregation und Analyse der im GLASOD enthaltenen Informationen wird zunächst deutlich, daß Afrika und Südostasien flächenmäßig mit jeweils rd. 4,5 Mio km2 am meisten zur Degradation durch Bodenerosion und Nährstoffverluste[1] in Entwicklungsländern beitragen, während der Anteil erodierter Fläche an der jeweiligen Gesamtfläche des Subkontinents[2] in Südwestasien (37%), Mittelamerika und Südostasien (jeweils rd. 25%) am höchsten ist. Extrem stark erodierte Länder finden sich v.a. in Mittelamerika und Afrika: In El Salvador, Haiti und Costa Rica sind zwischen 60% und 90% der jeweiligen Landesfläche betroffen. In Afrika sind vor allem die nord- und westafrikanischen Sahelländer Tunesien, Mauretanien, Libyen, Niger, Burkina Faso und Mali, im Osten die Hochlandstaaten Burundi und Rwanda sowie schließlich die Kapverdischen Inseln, besonders stark erodiert (40% bis 80% der Landesfläche). Wassererosion hat den größten Anteil an der Erosionsfläche, in Mittelamerika und Südostasien sind sogar mehr als 70% der erodierten Fläche von Wassererosion betroffen. Für die empirische Analyse der Zusammenhänge zwischen Bodenerosion und möglichen Bestimmungsfaktoren wird ein exploratives, ökonometrisches Vorgehen auf Grundlage nationaler Daten gewählt[3]. Die spezifische Aufeinanderfolge verschiedener Korrelations-, Faktoren- und Regressionsanalysen wird der großen Anzahl in Frage kommender Indikatorvariablen für mögliche Erosionsdeterminanten sowie den zu erwartenden Problemen der Multikollinearität und Modellspezifizierung in besonderem Maße gerecht. Letztere ergeben sich einerseits aus anzunehmenden Abhängikeiten unter verschiedenen Erosionsdeterminanten. Andererseits macht der latente Charakter[4], den die aus einem mikroökonomischen Kontext abgeleiteten Erosionsursachen auf aggregierter Ebene haben, es notwendig, für jede der angenommenen Determinanten verschiedene, u.U. korrelierte Indikatorvariablen zu definieren, was zusätzlich Kollinearität bedingt. Für Bodenerosion werden auf der Basis der national aggregierten GLASOD-Daten verschiedene Erosionsindizes definiert, die prinzipiell den von Wasser- und Winderosion sowie durch Nährstoffverluste betroffenen Anteil der nutzbaren Landesfläche wiedergeben. Die Datengrundlage für mögliche Erosionsdeterminanten wird ausgehend von Datensammlungen internationaler Organisationen für den Zeitraum 1961-1990 zusammengestellt. Für eine große Anzahl der in der Literatur diskutierten sozioökonomischen, landnutzerischen und auch natürlichen Rahmenbedingungen können repräsentative Indikatorvariablen definiert werden. Mangels geeigneter Indikatoren und Daten bleiben allerdings die Art und Sicherheit der Landbesitzverhältnisse unberücksichtigt. Insgesamt umfaßt die Datengrundlage rund 150 Variablen. Die Ergebnisse der Einfachkorrelationsanalysen zwischen den Erosionsindizes und möglichen Determinanten dienen einer ersten Einschätzung der Zusammenhänge. Sie zeigen, daß länderübergreifend insbesondere Variablen des Bevölkerungsdrucks sowie der durchschnittliche Waldanteil mit dem Ausmaß Bodenerosion in Zusammenhang stehen. Die Abholzungsraten in den 80er Jahren sind vor allem mit dem Ausmaß der Wassererosion korreliert. Bei Betrachtung der Länder mittleren Klimas[5] können Zusammenhänge mit Variablen nachgewiesen werden, die die Landnutzungsintensität und die Ausdehnung der tatsächlichen Nutzfläche in Relation zur potentiellen Nutzfläche wiedergeben. Weiterhin stehen in der mittleren Klimazone tendenziell sinkende Produzentenpreise für Agrarprodukte in Zusammenhang mit dem Ausmaß der Erosion. Erwartungsgemäß ist die Bedeutung natürlicher Faktoren für einzelne Erosionsformen und Klimazonen charakteristisch. Insgesamt scheinen Variablen, die das Ergebnis einer vermutlich längerfristigen Entwicklung wiedergeben, mehr Bedeutung für das Ausmaß der Erosion zu haben als solche, die Veränderungen im Referenzzeitraum 1961-1990 erfassen. Anhand verschiedener Faktorenanalysen für 62 Variablen und 73 Länder mit annähernd vollständigen Datensätzen können sodann strukturelle Zusammenhänge unter der Vielzahl möglicherweise relevanter Erosionsdeterminanten aufgedeckt und die Variablenanzahl auf Grundlage dieser Zusammenhänge auf eine geringere Anzahl weitgehend voneinander unabhängiger Größen reduziert werden. Es zeigt sich, daß die Struktur der Variablen durch etwa zehn gut interpretierbare Faktoren bei rd. 75% erklärter Gesamtvarianz klar wiedergegeben werden kann, und daß diese Faktoren auch bei Variation der Ausgangsvariablen sowie der Faktorextraktions- und Rotationsmethode stabil bleiben. Bemerkenswert ist, daß viele der Faktoren einen deutlichen Bezug zu den in der Literatur diskutierten Wirkungsketten unter möglichen Erosionsdeterminanten haben. So werden in dem für die Erklärung der Gesamtvarianz wichtigsten Faktor Variablen gebündelt, die die langfristige Intensivierung der Landnutzung im Zusammenhang mit strukturellem Bevölkerungsdruck und begrenzter Verfügbarkeit landwirtschaftlich nutzbarer Flächen erfassen. Weitere wichtige Faktoren beziehen sich auf strukturelle Armut in Verbindung mit erhöhtem ländlichen Bevölkerungswachstum; auf die mit Bevölkerungsdruck einhergehende langfristige wie auch rezente Expansion der landwirtschaftlichen Nutzfläche und Abholzung von Naturwald; auf Entwicklungswege, die eher auf die Produktion hochwertiger Produkte statt auf eine Flächenexpansion abzielen. Für die Preisentwicklung im Referenzzeitraum kann anhand einer Faktorenanalyse mit reduzierter Länderanzahl[6] gezeigt werden, daß ein Zusammenhang zwischen langfristig geringen oder negativen Preiszuwächsen im Agrarsektor und dem Faktor "Rezente Abholzungsraten" besteht. Um die relative Bedeutung dieser Faktoren für Bodenerosion zu quantifizieren, werden schrittweise Regressionsanalysen mit Bodenerosion als abhängiger Variablen und ausgewählten Repräsentantenvariablen für jeden Faktor als angenommenen unabhängigen Variablen durchgeführt[7]. Es lassen sich drei besonders relevante anthropogene Entwicklungen identifizieren, anhand derer das Erosionsausmaß bis zu rund 75% erklärt werden kann: (1) die langfristige, historische Ausdehnung der landwirtschaftlichen Nutzfläche auf Kosten des Waldbestandes in Zusammenhang mit einem Gesamtbevölkerungsdruck, der gegen Ende der 80er Jahre die agrar-ökologische Tragfähigkeit überschreitet; (2) die rezente Abholzung von Naturwald, die in Zusammenhang mit dem Wachstum der Gesamtbevölkerung zu sehen ist. Hier scheinen weniger der Druck der Agrarbevölkerung und die Ausdehnung der landwirtschaftlichen Nutzfläche - also die Produktionsseite - im Vordergrund zu stehen, als vielmehr der Druck der Nachfrageseite, in Kombination mit einem tendenziell sinkenden Agrarpreisniveau, das den Expansionsdruck auf das Land verstärkt hat. (3) Die langfristige, bevölkerungsdruckinduzierte Intensivierung der Agrarproduktion, vor allem durch Umwandlung von Dauergrünland in Ackerland, verkürzte Brachezeiten und erhöhte Viehbesatzdichten. Ein weiteres Ergebnis ist, daß in keinem Fall ein wesentlicher Einfluß von Armut auf das landesweite Ausmaß der Bodenerosion nachgewiesen werden kann - wie bereits die Ergebnisse der Einfachkorrelationsanalysen für immerhin 15 verschiedene Armutsindikatoren vermuten lassen. Es bestehen Unterschiede in den Erklärungsmustern für verschiedene Erosionsformen und Klimazonen. Die rezenten Abholzungsraten haben für Wassererosion, insbesondere in Ländern der extrem humiden Klimazone, herausragende Bedeutung. Zusätzlich zu den Faktoren (1) und (2) ist die Intensität der landwirtschaftlichen Produktion (3) vor allem für Wassererosion und in Ländern der mittleren Klimazone von Bedeutung. Hier ist auch die negative Wirkung einer sinkenden Agrarpreisentwicklung am stärksten. Gleichzeitig gilt hier: je eher der eingeschlagene Entwicklungsweg auf die Produktion hochwertiger Produkte im Gegesatz zur reinen Flächenexpansion abzielt, desto geringer ist das Erosionsausmaß. Für das Ausmaß der Winderosion und der Degradation durch Nährstoffverluste hingegen sind insbesondere die agroklimatischen Bedingungen ausschlaggebend. Die als erosionsrelevant identifizierten anthropogenen Rahmenbedingungen sind mit zentralen theoretischen Hypothesen konsistent. Fraglos gehören sie eher zu den Größen, deren kurzfristige Beeinflussung durch politische Maßnahmen schwierig ist. Dennoch können folgende Ansätze für eine Schwerpunktsetzung bei der Gestaltung von Politikmaßnahmen zur wirksamen Erosionsverminderung abgeleitet werden: Die Reduktion des Bevölkerungsdrucks durch eine an die natürlichen Bedingungen und relativen Faktorknappheiten angepaßte Erhöhung des Produktionspotentials, gerade auch in Regionen mit relativ niedrigem Potential. Eine stärkere Fokussierung auf Forstpolitiken bzw. auf eine Regulierung der kommerziellen Nutzung von Wäldern, vor allem in humiden Klimazonen. .Eine selektive, langfristig angelegte Verbesserung der incentive-Struktur für bodenschonende Produkte und Anbaumethoden über wirtschaftspolitische Eingriffe sowie durch verbesserte institutionelle und rechtliche Rahmenbedingungen. Von Politiken zur Armutsbekämpfung ist hingegen nicht zu erwarten, daß sie maßgebliche Impulse zur Verminderung der Bodenerosion geben können. Es muß jedoch immer präsent bleiben, daß arme Landnutzer sicherlich am stärksten und häufig existentiell von Erosionsschäden betroffen sind. Die Qualität zukünftiger Forschungsbemühungen auf globaler Ebene wird vor allem von der zukünftigen Datenverfügbarkeit und -qualität bestimmt: Für den Stand der Bodenerosion sind Informationen für verschiedene Zeitpunkte erforderlich; für anthropogene Erosionsdeterminanten eröffnen georeferenzierte Daten der Forschung gänzlich neue Perspektiven. Parallel zu überregionalen Analysen sind weitere lokale, sub-nationale Studien unbedingt notwendig, um umfassend zu ergründen, warum und welche Landnutzer die Ressource Boden in einem konkreten sozioökonomischen Kontext degradieren. Fußnoten: [1]Neben der Wasser- und Winderosion wird eine weitere Degradationsform, der Verlust von Nährstoffen und organischer Substanz, mitberücksichtigt und vereinfachend mit "Nährstoffverluste" bezeichnet.[2]Gemeint ist die nutzbare Landesfläche, Ödland ausgenommen. [3]Georeferenzierte Daten liegen derzeit für sozioökonomische Erosionsdeterminanten noch nicht vor.[4]D.h. Größen, von denen a priori nicht bekannt ist, wie sie beobachtet und gemessen werden können. [5]Dies sind Länder, in denen weder extrem aride noch extrem humide Bedingungen vorherrschen. [6]Für die entsprechende Variable liegen nur Daten für 56 Länder vor.[7]Umgekehrte Wirkungen der Erosion auf die als unabhängig angenommenen anthropogenen Variablen sind im Betrachtungszeitraum - bis auf die Armutswirkung starker Erosion - unwahrscheinlich. / By the end of this century, soil erosion has reached an alarming extent in many developing countries. Still, uncertainty prevails regarding the human-induced causes of soil erosion. In consequence, many efforts to design efficient anti-erosion policies and instruments remain erratic. The actual discussion about human-induced causes of soil erosion focusses on socioeconomic factors that assumably influence the land users´ decisions on agricultural production and soil protection, and, hence, the degree of soil erosion. The most frequently discussed factors are: (i) poverty, (ii) population pressure, (iii) biased agricultural prices, (iv) the introduction of inadequate technical innovations and (iv) insecurity of land tenure. They are basically deduced from and discussed on base of production theory and the theory of induced innovation. Nevertheless, the different views on the importance to be assigned to the single factors are quite controverse. For example, in a rather optimistic scenario, it is argued that poverty and population pressure lead to the development of soil-conserving innovations in the long run. On the other side, poverty and population pressure, in combination with falling agricultural prices, are assumed to lead to a short-termist overuse of the soil. Empirical evidence that supports some of the controverse hypotheses on the causes of soil erosion is restricted to local studies based on local data on soil erosion, their results can hardly be generalized. In this context, the spatial data compiled within the global assessment of human-induced soil degradation (GLASOD; UNEP/ISRIC, 1991) for the first time permits a large-scale empirical analysis of socioeconomic and landuse factors relevant to erosion. By aggregating the information of the GLASOD data, countries and regions whith marked soil erosion can be identified. While Africa and Asia most contribute to the extent of soil erosion and the loss of nutrients[8] in absolute terms (4,5 mio sqkm each), it is in Southwest Asia (37%), Central America and Southeast Asia (25% each), where the proportion of of the land area - excluding wastelands - that is affected reaches the highest levels. Looked at on a national level, countries with an extreme extent of soil erosion are to be found in Central America and Africa: In El Salvador, Haiti and Costa Rica, 60 to 90 percent of the land area[9] are affected. In Africa, Sahelian Countries as Tunesia, Mauretania, Libya, Niger, Burkina Faso and Mali, as well as the eastafrican highlands of Burundi and Rwanda, and also Cape Verde show the highest proportions of eroded land area2 (40 to 80 %). Water erosion is the most widespread type of erosion, in Central America and Southeast Asia it even contributes with about 70% to the area affected by erosion and the loss of nutrients1. The methodological approach chosen for the empirical analysis of human-induced causes of soil erosion is an explorative, econometric one, based on national cross-country data[10]. A specific combination of correlation analyses, factor analysis, and regression analysis is designed, that can handle the great number of possible indicators for the assumed causes of erosion, and cope with related problems of multicollinearity and model specification. Those problems result from supposed interrelationships among different human-induced causes of soil erosion. At the same time, many of the causes of erosion have a latent character when considered on a national level[11], since they are deduced from a microeconomic context. This makes it necessary to define various indicator variables for each of them, which, again, implies additional multicollinearity. On the basis of the aggregated GLASOD data, a set of operational variables for soil erosion is defined. They basically indicate the proportion of a country´s degradable land area (i.e. land area minus wastelands) that is eroded through wind, water, or degraded by the loss of nutrients and organic matter by the end of the 80´s. In turn, the database for possible determinants of erosion is compiled departing from standard international data sets for the time span 1961-1990. Representative indicators can be defined for many of the causative factors discussed in literature, as well for socioeconomic ones, as for landuse, and also for natural factors. They are adapted in a way that they not only best fit and capture the hypothesized determinants, but also the ecological and timely dimension of the analysis. One important field that is not covered is land tenure. The resulting database comprises about 150 variables for possible causative factors, with a varying number of country-data available. The results of correlation analyses between the indicator variables for soil erosion and for possible causative factors facilitate a first assesion of relevant relationships. They show, that variables that quantify population pressure and the proportion of forested area are correlated with soil erosion for all countries. Deforestation rates in the 80´s are especially related to water erosion. Considering only countries without extreme climatic conditions[12] correlations are found between soil erosion and variables for the intensity of land use and the degree of expansion of the agricultural frontier. Producer price declines for relevant agricultural products are also found to be correlated with soil erosion in these countries. Corresponding to theoretical assumptions, the importance of different natural factors vary for different types of erosion and climatic zones. Altogether, variables that express structural conditions and can be regarded as the outcome of historical, long-term developments, seem to have stronger correlation with the extent of soil erosion than variables that quantify changes that took place within the time span under consideration, 1961 to 1990. The next methodological step consists in different factor analyses for 62 of the variables that express possible causative factors and for 73 countries with approximatively complete data sets. The principal objectives are to detect structural interrelationships among the multitude of variables and to reduce their number on the basis of these interrelations, in a way to obtain a set of variables that are largely independent of each other. It turns out that the structure of the 62 variables under consideration can clearly be reproduced by about 10 factors, with about 75% of their total variance being explained. These factors prove to be robust with respect to changes in the set of included variables, and in the methods of extraction and rotation. It is noteworthy, that many of the identified factors refer to cause-effect relationships that are discussed in literature. For instance, the factor that explains the greatest part of total variance, combines variables that quantify the long-run intensification of land use with others that stand for structural population pressure and a limited buffer for the expansion of the agricultural area. Other important factors relate to structural poverty, in combination with high rates of rural population growth; to the long-term and recent deforestation and to total population pressure; to development paths that aim at sopisticated animal procuction and permanent culture rather than at a mere expansion of the agricultural area. Other factors stand for the prevailing natural conditions. Based on a factor analysis for a reduced number of countries, it can be shown that declinig aggregate agricultural producer prices[13] are associated with the factor ´recent deforestation rates´. To quantify the relative importance of the identified factors, stepwise regression analyses are then carried out, with soil erosion as the dependent variable and selected representative variables for each of the factors as presumed independent variables[14]. Three human-induced factors, or developments, show to have particular relevance for the extent of soil erosion, that they can explain to up to 75%: (1) the long-run historical expansion of the agricultural frontier at the expense of the forested area, in combination with a population pressure well above the corresponding supporting capacities in the 80´s; (2) recent deforestation rates in conjunction with total population growth. This effect can rather be associated with a growth of demand for agricultural and forestral products and declining agricultural prices than with pressures directly resulting from agricultural population and expansion; (3) the long-run intensification of land use, mainly throug the conversion of permanent pastures to arable land, the shortening of fallow periods, and the increase of animal densities. This type of intensification is associated with and possibly induced by high structural population pressure in agricultural areas. Another important result is that poverty seems to have minor impact on the extent of soil erosion at the aggregate, national level. None of the included variables that represent the factor ´poverty´ shows a significant relative impact, neither in the models for the sum of erosion nor for specific types of erosion or climatic zones. This fact supports the low correlation coefficients for altogether 15 different poverty indicators that were calculated in the context of simple correlation analysis. Specific models for specific types of erosion and climatic zones show that there exist characteristic patterns of explanation for each type and zone. Recent deforestation rates and the associated features (factor (2))are particularily important in the explanation of water erosion, especially in countries with predominant humid climate. The impact of production-intensity in terms of factor (3) is specific for water erosion, and for countries without extreme climatic conditions, together with the factors (1) and (2). This is also where the negative effect of declinig agricultural prices appears to be strongest. At the same time, the development of sopisticated animal procuction and the growth of the area under permanent culture in contrast to a mere expansion of the agricultural area seem to be favourable to the soil in this context. In the explanation of wind erosion and loss of nutrients, natural factors are in the foreground. The identified, human-induced pressures related to long-term population growth, intesification, agricultural price decline and recent deforestation are consistent with important theoretical hypotheses. Those pressures are clearly not of the type that can be overcome over night through political intervention. Nevertheless, they lead to the following areas of intervention that should be given priority in the design of policy measures for the reduction of soil erosion: A reduction of population pressure through an increase in site-specific production potentials, based upon innovations that match the prevailing agro-ecological and economic conditions. Special attention should be given to low potential areas.A stronger focus on forest policy and the regulation of commercial forest use especially in the humid zone.A selective, long-term improvement of economic incentives for the production of soil-conserving crops with soil-conserving methods, by means of economic policy as well as through improved institutional conditions. Policies that aim at the reduction of poverty can not be expected to play a decisive role in the reduction of soil erosion. In spite of that, it is most necessary that policy makers keep in mind that the poor certainly are most affected by and vulnerable to erosion damages. At a global scale, the quality of future research on the topic will largely be determined by data availability and quality: concerning soil erosion, information at different points in time is necessary; for anthropogenic factors, spatial datasets will bring a new dimension into scientific research. Parallel with global analyses, further in depth local studies are necessary for a comprehensive and detailed insight into why and which land users degrade the resource they depend on in a specific socioeconomic context. footnotes: [8]The loss of nutrients and organic matter, independent of soil erosion, is also considered and is abbreviated with the term ´loss of nutrients´ in this text. [9]Again, it is the land area excluding wasteland that is being referred to. [10]Spatial data sets are not avaiable yet for socioeconomic factors related to soil erosion. [11]I.e. it is not known a priori, how these causes can be measured and quantified. [12]I.e. countries without predominant arid, hyper-arid or humid agroclimatioc conditions. [13]The availability of data for the variable in cause is limited to 56 coutries. [14]Reciprocal effects that soil erosion might have on anthropogenic factors are not very likely to occurr within the considered time span, except a possible increase of poverty due to erosion.

A expansão agropecuária governa as mudanças no uso do solo no Brasil devido à alta demanda dos mercados interno e externo por alimento, fibra e energia. Entretanto, os efeitos e os processos decorrentes dessas alterações no ciclo hidrológico e na conservação do solo são pouco estudados de forma experimental em regiões de clima tropical e subtropical. No Estado de São Paulo, o uso do solo acontece de forma intensiva, as áreas de Cerrado nativo estão fragmentadas e pastagens vêm sendo substituídas por plantações de cana-de-açúcar devido à alta demanda por etanol e açúcar. Este trabalho tem como objetivo compreender as relações, trocas, variações e tendências das componentes do balanço hídrico e dos processos erosivos em potenciais mudanças no uso do solo que são encontradas no Sudeste do Brasil: de condições naturais (Cerrado sensu stricto) para pastagem, cana-de-açúcar e solo exposto. Para isso, foram monitorados, nos diferentes usos do solo, as condições meteorológicas, o escoamento superficial, a evapotranspiração, o conteúdo de água no solo, a erosão do solo e a flutuação do nível freático do aquífero. As alterações no uso do solo modificam significativamente o balanço hídrico, com aumento do escoamento superficial (pelo menos 14 mm ano-1) e diminuição da evapotranspiração (pelo menos 529 mm ano-1) quando o Cerrado sensu stricto é substituído por pastagem ou cana-de-açúcar. Entretanto, no Cerrado sensu stricto o volume de água disponível para percolação ao longo da zona não saturada e potencial recarga do aquífero tende a ser menor que em áreas agrícolas. As observações mostram que o solo exposto e a cana-de-açúcar possuem os maiores valores erosão do solo (16,00 ± 5,97 t ha-1 ano-1 e 0,64 ± 0,49 t ha-1 ano-1, respectivamente). Além disso, há semelhanças entre as taxas de perda de solo na pastagem (0,11 ± 0,04 t ha-1 ano-1) e no Cerrado sensu stricto (0,14 ± 0,06 t ha-1 ano-1). Devido às curtas séries de dados de escoamento superficial e erosão do solo, a adoção de modelos de base física como o WEPP (Water Erosion Prediction Project), é alternativa viável para simulações que considerem as variabilidades climáticas de regiões subtropicais. Projeções climáticas revelam que não ocorrerão alterações significativas nas respostas (escoamento superficial e erosão do solo) em relação ao clima base atual apesar do aumento significativo na precipitação nos cenários mais drásticos (entre 5% e 9%). Por fim, a manutenção do ciclo hidrológico e o controle da erosão do solo alcançados pelo Cerrado sensu stricto são benefícios que contrastam com a diminuição da recarga potencial do aquífero em áreas de vegetação densa. A cana-de-açúcar e a pastagem são usos do solo concorrentes e seus efeitos nos padrões hidrológicos e na erosão do solo se equilibram. / The agricultural expansion in Brazil drives land use changes due to the higher demand of internal and external markets for food, fiber and fuel. However, the effects and processes that result from these changes on hydrological cycle and soil conservation are not well explored in an experimental approach under tropical and subtropical climates. The land use is intense in the State of São Paulo, where the undisturbed woodlands in the Cerrado biome are fragmented and pasturelands are transformed in sugarcane plantations due to the higher sugar and ethanol demands. This thesis aims to comprehend the relations, trade-offs and variations of the water balance components and soil erosion processes under potential land use changes that can be found in southeastern Brazil: from natural landscapes (wooded Cerrado) to pastureland, sugarcane and bare soil. They were monitored in these different land uses: meteorological conditions, runoff, evapotranspiration, soil moisture content, soil erosion, and water table fluctuation. The land uses changes significantly influence the water balance, increasing the runoff (at least 14 mm yr-1) and decreasing evapotranspiration (at least 529 mm yr-1) when wooded Cerrado is substituted by pasture or sugarcane. Nevertheless, the soil water content available for deep percolation through the unsaturated zone and potential aquifer recharge in the wooded Cerrado tend to be smaller than in agricultural fields. Soil loss observations reveal that bare soil and sugarcane have the highest rates (16.00 ± 5.97 t ha-1 yr1 and 0.64 ± 0.49 t ha-1 yr-1, respectively). Additionally, there are similarities between the soil loss rates of pastureland (0.11 ± 0.04 t ha-1 yr-1) and wooded Cerrado (0.14 ± 0.06 t ha-1 yr-1). Due to the short-period observations of runoff and soil erosion, the use of a process-based model such as WEPP (Water Erosion Prediction Project) is a feasible alternative for simulations considering climatic variability under subtropical conditions. Projected climates reveal that in spite of significant increased rainfall (between 5% and 9%) in the most drastic scenarios, there are no significant changes on runoff and soil erosion rates in relation to the actual baseline climate. Finally, the hydrological stability and soil erosion control are benefits reached by the wooded Cerrado that contrast with the decrease in potential aquifer recharge in a dense vegetation area. Sugarcane and pasture are concurrent land uses and their effects on hydrological patterns and soil erosion are equivalent.

A conversão de vegetação nativa e o uso inadequado das terras convertidas resultam em prejuízos graves para as funções e processos dos ecossistemas, impactando diretamente a provisão de serviços ambientais e o bem-estar da sociedade. Em meio ao elevado nível de degradação e fragmentação de habitats naturais, pesquisadores têm buscado alternativas para promover a sustentabilidade de paisagens modificadas pelo homem, procurando conciliar ações conservacionistas com as necessidades de produção agropecuária e do uso consciente dos recursos naturais. Para contribuir com esse tema, este trabalho apresentou uma metodologia para a priorização de áreas para restauração florestal em paisagens agrícolas. Foram utilizados indicadores biofísicos (perda de solo, escoamento superficial e conectividade da paisagem) e técnicas de modelagem espacial para analisar o efeito de cenários alternativos de uso do solo sobre a conservação do solo, da água e da biodiversidade. Foram criados dois grupos de cenários, sendo que um grupo apresenta cenários com diferentes quantidades e arranjos espaciais da cobertura florestal na paisagem, e outro grupo se refere à cenários com diferentes tamanhos de faixas marginais florestadas no entorno da rede de drenagem. Os resultados do primeiro grupo de cenários demonstraram que o arranjo espacial da cobertura florestal influenciou significativamente a perda de solo e o escoamento superficial, mas não apresentou efeitos significativos para a conectividade da paisagem, que se mostrou dependente apenas da quantidade de cobertura florestal. As maiores sinergias entre os indicadores analisados foram expressas a partir da aplicação conjunta de boas práticas agrícolas e a restauração florestal de áreas críticas para conservação do solo e da água, demonstrando a complementariedade dessas ações no manejo de paisagens agrícolas. Quanto ao segundo grupo de cenários, os resultados demonstraram a influência positiva do tamanho da faixa florestada na capacidade de retenção de sedimentos das zonas ripárias e na conectividade da paisagem, de modo que os melhores resultados foram obtidos com o total recobrimento das Áreas de Preservação Permanente (APPs) por florestas. Observou-se que a quantidade de cobertura florestal influenciou positivamente a capacidade de retenção de sedimentos das APPs, sendo determinante no comportamento dessas áreas como fonte ou filtro de sedimentos para os canais de drenagem. De forma geral, os resultados apresentados neste trabalho demonstraram o potencial de estratégias conservacionistas e de restauração florestal em paisagens agrícolas, ao mesmo tempo que reforçaram as necessidades de planejamento e da inclusão de áreas de produção agropecuária no manejo de paisagens. Em um cenário atual de implementação da Lei de Proteção da Vegetação Nativa e de exigências mais brandas para recomposição de APPs e Reservas Legais em propriedades privadas, os resultados sugerem que apenas as ações de comando e controle podem não ser suficientes para promover a provisão de serviços ambientais e a conservação da biodiversidade em paisagens agrícolas. Desse modo, recomenda-se que haja uma desvinculação do que está previsto em lei com o conceito de paisagens sustentáveis, pois apenas a adequação legal de propriedades rurais pode não ser capaz de assegurar o fornecimento de bens e serviços à sociedade. / The conversion of native vegetation and the inappropriate use of converted land have resulted in severe damage to the natural ecosystem functions and processes, affecting the provision of environmental services and the welfare of society. Due to the high level of degradation and fragmentation of natural habitats, researchers have sought alternatives to promote the sustainability of human modified landscapes, aiming to reconcile conservation actions with the agricultural production needs and the conscious use of natural resources. To contribute to this theme, this thesis presented a methodology for prioritizing areas for forest restoration on agricultural landscapes. Biophysical indicators (soil loss, runoff and landscape connectivity) and spatial modeling techniques were used to analyze the effects of alternative scenarios of land use on the conservation of soil, water and biodiversity. These effects were analyzed on two groups of scenarios, wherein one group presents scenarios with different amounts and spatial arrangements of forest cover on the landscape, and another group refers to scenarios with different sizes of forested riparian buffer zones. The results of the first group of scenarios showed that the spatial arrangement of the forest cover significantly influenced the soil loss and runoff, but did not significantly affect the landscape connectivity, which proved to be dependent only by the amount of forest cover. The highest synergies between the analyzed indicators were expressed with the joint application of best management practices and forest restoration of critical areas for soil and water conservation, demonstrating the complementarity of these actions in the management of agricultural landscapes. Considering the second group of scenarios, the results demonstrated the positive influence of the size of forested riparian buffers on sediment retention capacity of riparian areas and landscape connectivity, and the best results were obtained with the total coverage of the Areas of Permanent Preservation (APPs) by forests. It was observed that the amount of forest cover positively influenced the sediment retention capacity of APPs and determined the behavior of these areas as sediment filter or source for stream channels. Overall, the results presented in this thesis demonstrated the potential of conservation and forest restoration strategies on agricultural landscapes, while it reinforced the needs for planning and for the inclusion of agricultural production areas in landscape management strategies. On current expectations for the implementation of the new Brazilian Forest Act and softer requirements for restoration of APPs and Legal Reserves on private properties, the results suggest that only the command and control actions may not be sufficient to promote the provision of environmental services and biodiversity conservation on agricultural landscapes. Therefore, we recommend the detachment from what is required by law to the concept of sustainable landscapes, because only the legal compliance of rural properties may not be able to ensure the supply of goods and services to society.

Orientador: Antonio Roberto Saad / Coorientador: Antonio Manoel dos Santos Oliveira / Banca: José Eduardo Zaine / Banca: Eduardo Soares de Macedo / Banca: Claudio José Ferreira / Banca: Marcio Roberto Magalhães de Andrade / Resumo: O município de Guarulhos, segunda maior cidade em população no Estado de São e pertencente à Região Metropolitana de São Paulo - RMSP vem sofrendo uma ocupação acelerada e desordenada ao longo de décadas. Dividido fisiograficamente em dois macrocompartimentos, separados pela Falha do Rio Jaguarí, Guarulhos possui na sua porção sul uma área já consolidada com terrenos mais apropriados à ocupação e com mais facilidade de acesso a serviços e infraestrtura. As áreas com problemas encontram-se na porção norte do município, que além de serem áreas com presença de declividades acentuadas, são ocupadas irregularmente, carentes em infraestrutura e planejamento do Poder Público. Resultado da pressão de ocupação na porção norte do município foram analisados três microbacias pertencentes aos Bairros Invernada, Fortaleza e Água Azul que possuem ocupações com históricos diferentes resultando ao longo do tempo em degradações ambientais principalmente no que se refere à dinâmica superficial. Para elaboração do Mapa de Degradação Ambiental para as três microbacias foram utilizados: o método de sobreposição de informações do IPT (2004) para as Microbacias Taquara do Reino (Bairro Invernada) e Guaraçau (Bairro Água Azul) e o método de Unidades Básicas de Compartimentação (UBCs) de Vedovello (2000) para a microbacia Lavras (Bairro Fortaleza). Os dados obtidos indicam que as três microbacias sofrem com a degradação ambiental, materializada em problemas de dinâmica superficial como escorregamentos nas regiões declivosas e um grande volume de assoreamentos, principalmente nas planícies aluvionares. A falta de planejamento e de infraestrutura nesses bairros corroboram para um aumento dessa degradação necessitando de cartografia geotécnica que possa auxiliar o Poder Público na melhor forma de uso do solo e a gerenciar os problemas existentes / Abstract: The municipality of Guarulhos, second large in population, state of São Paulo belongs to the RMSP - Metropolitan Region of São Paulo, has suffered from a disorderly and accelerated occupation during the decades. Fisiogeographically divided in two macro compartments, separated by Jaguari River Fault, Guarulhos has in its South portion a well done consolidated area more appropriate to land occupation resulting in facilities to access services and infrastructure as well. The biggest problems are in the Northern-part that beyond to be areas with an accentuated declivity, are irregularly occupied with no infrastrueture or governmental assistance planning. Due to the occupation stress in the Northern portion were analyzed three micro basins belonging to the Invernada, Fortaleza and Agua Azul Quarters with different historical occupations resulting in a short term in an environmental degradation especially regarding to dynamie surface. To elaborate the three micro basins Environmental Degradation Map were used two methods: overlaping information for the micro basins applied for Taquara do Reino (Invernada Quarter) and Guaraçau (Água Azul quarter) and the Compartimentation Basie Units methods applied to the Lavras micro basin (Fortaleza Quarter). The obtained datas suggest that the three basins suffer with the environmental degradation revealed in dynamic surface problems such as slide, a great volume of silting, mainly of the alluvial plains. The absence of planning and infrastructures in these places assure to an increase of this degradation, requiring geotechnical cartography in order to help the government to go on in a best way to use the land and manage the existing problems / Doutor

Dans le contexte d'augmentation de la pression démographique et des risques liés aux changements climatiques, la question de l'érosion est de plus en plus cruciale. La gestion actuelle des milieux des montagnes méditerranéennes, particulièrement affectés par ces phénomènes, repose sur une bonne connaissance de leurs sensibilités à l'érosion et de leurs trajectoires à long terme. Une approche rétrospective des trajectoires des environnements a été menée par l'étude de trois archives sédimentaires lacustres prélevées dans les Alpes du Sud (lacs Petit, Vens et Allos). L'étude multidisciplinaire (sédimentologie, géochimie, palynologie) a permis de caractériser les dynamiques érosives et végétales depuis 14000 ans. À Allos et Vens, le début de l'Holocène est marqué par la maturation des écosystèmes, l'altération chimique des sols et la progression altitudinale des forêts. Ces dynamiques environnementales conduisent à la stabilisation progressive des écosystèmes, suivie d'un optimum bio-pédologique, d'une ouverture des paysages et d'une déstabilisation des sols respectivement datées de 12000-8000 cal. BP, 8000-6000 cal. BP, 6000-1900 cal. BP et de 1900 cal.BP à aujourd'hui. Cette dernière période de déstabilisation des sols intervient plus tôt au Lac Petit, à 4200 cal. BP.Les dénominateurs communs au déclenchement de l'érosion sont à chaque fois une période de précipitations fréquentes sur un milieu fragilisé : l'impact des sociétés a provoqué un abaissement du seuil de sensibilité des milieux aux perturbations, particulièrement à l'agressivité des précipitations. / Within the context of increasing demographic pressures and hazards related to climate change, the problems posed by landscape erosion have become particularly crucial. The current management of Mediterranean mountain environments, which are highly exposed to erosion hazards, needs to be supported by a thorough understanding of their susceptibility to these hazards and their long-term trajectories.A retrospective analysis of environmental trajectories has been conducted from the study of three lacustrine sedimentary archives in the Southern Alps (Lakes Petit, Vens and Allos). This multi-disciplinary study, based on sedimentology, geochemistry and palynology, has enabled characterization of the dynamics of erosion and changes in plant cover over the last 14,000 years.In Lakes Allos and Vens, the start of the Holocene is marked by the maturing of ecosystems, by chemical weathering of soils and by forest encroachment at higher altitudes. These environmental changes resulted in progressive ecosystem stabilization, followed by a bio-pedological optimum, and subsequently by more open landscapes, and then soil destabilization. These four successive phases have ben dated, respectively, at 12,000-8000 cal. BP, 8000-6000 cal. BP, 6000-1900 cal. BP, and 1900 cal. BP to Present. The last of these phases occurred earlier, at 4200 cal. BP, in Lake Petit.The triggering of soil erosion has systematically been hinged on periods of heavy precipitation affecting landscapes rendered vulnerable, by human societies, to the effects of such perturbations.