Les bassins versants nord-méditerranéens sont fréquemment soumis à des crues extrêmes liées à des précipitations convectives intenses et aux caractéristiques hydrologiques locales. La région méditerranéenne est considérée comme une des régions les plus affectées par le réchauffement climatique, ce qui laisse présager des changements dans le cycle hydrologique. L’objectif de cette thèse CIFRE est d’évaluer les impacts du changement climatique sur les précipitations extrêmes à travers une méthode dite de « futurisation », dans laquelle une fonction de transfert est construite en comparant la distribution des quantiles de précipitations du climat présent et futur. Les impacts du changement climatique sur les précipitations extrêmes sont évalués à travers les simulations à haute résolution EMCORDEX. L’exercice se focalise sur le bassin versant de l’Orbieu dans le sud de la France. La méthode de futurisation est appliquée à six épisodes majeurs de précipitations ayant généré des crues éclair. Les impacts hydrologiques des équivalents statistiques futurs des épisodes de précipitations sont ensuite évalués à travers un modèle hydrologique évènementiel conceptuel. Une estimation des changements d’humidité du sol liés au changement climatique est réalisée et couplée à la quantification des impacts hydrologiques. Le choix d’une modélisation hydrologique conceptuelle a été motivé par ses futures applications opérationnelles. Les conséquences de ce choix sont évaluées à travers une comparaison avec un modèle hydrologique à base physique. Ce dernier est mis en place grâce à une caractérisation du fonctionnement hydrologique du bassin versant de l’Orbieu appuyée par plusieurs campagnes de terrain. / Northern mediterranean meso-scale river catchments are submitted to extremes floods events linked to intense convective precipitation and local hydrologic features. The Mediterranean region is known to be one of the most affected areas by global warming, and it is likely that changes can be expected in the hydrological cycle. The aim of this CIFRE thesis is to assess the climate change impacts on extreme precipitation events using a so-called “futurization” method, in which a transfer function is built by comparing the quantiles of distribution for both present and future climate precipitation. The climate change impact on extreme precipitation events is assessed over high-resolution EMCORDEX simulations. The focus is on the Orbieu catchment located in southwestern France. The futurization method is applied to six major events of precipitation that trigger flash floods. The hydrological impacts of those future statistical counterpart precipitation events are therefore assessed through a conceptual event-based hydrological model. An assessment of soil moisture changes under climate change is performed and coupled to the hydrological impact quantification. The conceptual hydrological model chosen, have been motivated by its future operational applications. The consequences of that choice are assessed through a comparison to a physically based hydrological model. It has been implemented through the hydrological functioning caracterisation of the Orbieu catchment supported by several field campaigns.

Aujourd'hui une augmentation de l'intensité des extrêmes hydrologiques est attendue avec le réchauffement global. Cependant le manque d'observations des phénomènes torrentiels et de mesures directes des précipitations en altitude sur le temps long ne permet pas d'étayer cette théorie. Les archives naturelles lacustres, par leur capacité à enregistrer l'évolution passée de l'activité torrentielle, offrent l'opportunité de combler ce manque. L'objectif de la thèse est de reconstituer l'évolution de la fréquence et de l'intensité des crues passées, à partir de l'étude de séquences sédimentaires de lacs d'altitude des Alpes Françaises. Afin d'être en mesure d'évaluer le rôle de la température sur l'activité torrentielle, notre étude se focalise sur le dernier millénaire qui a connu des périodes climatiques contrastées ; période chaude de l'Optimum Médiéval (OM), période froide du Petit Age Glaciaire (PAG) et réchauffement global actuel. Les sites d'étude ont été sélectionnés selon un transect nord-sud pour évaluer la variabilité régionale de l'activité torrentielle en réponse aux changements climatiques. Ce travail repose sur des analyses sédimentologiques et géochimiques à haute résolution qui ont permis i) de identifier les dépôts de crue, ii) de les distinguer de dépôts similaires issus de remaniements gravitaires et iii) de déterminer un marqueur fiable de l'intensité des événements. D'autre part plusieurs méthodes de datation ont été combinées dans l'objectif de diminuer les incertitudes des modèles d'âge. La fréquence de crue à l'échelle pluri-séculaire apparaît en augmentation sur l'ensemble des Alpes Françaises au cours de la période froide du PAG. Cependant à l'échelle pluri-décennale la fréquence de crue évolue différemment entre le Nord et le Sud des Alpes. Les maxima de fréquence apparaissent au cours de périodes chaudes dans les Alpes du Nord, alors que les maxima dans les Alpes du Sud semblent correspondre à des phases négatives de l'Oscillation Nord-Atlantique. De plus les événements de crue extrême se produisent au cours des périodes les plus chaudes dans les Alpes du Nord alors qu'ils apparaissent au cours du PAG dans les Alpes du Sud. Ces résultats suggèrent donc une régionalisation des effets du réchauffement global sur les crues extrêmes et les précipitations intenses. / An increase of the intensity of hydrological extremes is expected in the current context of the global warming. However, the lack of observations of torrential events and measurements of precipitation at high elevation areas does not allow supporting this theory. Past flood evolutions can be recorded in lake sediments and allow to explore the evolution of such events and the relationships to past climatic changes. This work aims at reconstructing the past evolution of flood frequency and intensity based on the study of sediment sequences of high elevation lakes of the French Alps. To assess the role of the temperature on this evolution, our study focused over the last millennium which includes thermal-contrasted periods such as the warm Medieval Climate Anomaly and the cold Little Ice Age. Studied sites were selected along a north-south transect to investigate the regional variability of flood evolution in response to the same context of past climatic changes. High-resolution sedimentological and geochemical analyses allowed to identify a large number of flood deposits, distinguish deposits resulting from floods from gravity processes and to assess a reliable flood intensity proxy. Some distinct dating methods were finally used to reduce age uncertainties on the age-depth relationships. An increase of the flood frequency at a multi-secular time-scale appeared over the entire French Alps during the Little Ice Age. However at a multi-decennial time-scale flood frequencies evolved differently between the north and the south of the French Alps. Maxima of flood frequency appear during the warmer periods in the northern Alps while maxima of flood frequency appear during negative phases of the North Atlantic Oscillations. Finally the most extreme events occurred during the warmer periods in the northern Alps while they occur during the Little Ice Age in the southern ones. Those results suggest thus distinct regional effects of the global warming on extreme flood events and intense precipitations and, especially, an increase of flood hazard in the northern French Alps owing to an increase of flood intensity with the warming context.

In this diploma thesis current state of vegetation cover was analyzed after spring flood in 2006 on the lower section of the Sázava river. Immediately after the flood there were 38 sites with alluvial deposits identified in the floodplain. The differences in species composition and species diversity between sites with alluvial deposits and sites without deposits were analyzed. The data were collected by using phytosociological relevés in pairs i.e. - plot with an alluvial deposit and plot without. There were 86 vascular plant species recorded out of which 10 species were non-native but not invasive species and 7 species were non-native invasive species. Results of this thesis revealed that species composition and diversity of invasive species differ between plots with alluvial deposits and plots without alluvial deposits. Moreover, plots without alluvial deposits host more invasive species. Keywords: vegetation succession, river floodplain, extreme floods, species diversity, plant invasion

Thesis (M. Sc. (Statistics)) -- University of Limpopo, 2021 / The effects of ozone depletion on climate change has, in recent years, become a reality, impacting on changes in rainfall patterns and severity of extreme floods or extreme droughts. The majority of people across the entire African continent live in semi-arid and drought-prone areas. Extreme droughts are prevalent in Somalia and eastern Africa, while life-threatening floods are common in Mozambique and some parts of other SADC (Southern African Development Community) countries. Research has cautioned that climate change in South Africa might lead to increased temperatures and reduced amounts of rainfall, thereby altering their timing and putting more pressure on the country’s scarce water resources, with implications for agriculture, employment and food security. The average annual rainfall for South Africa is about 464mm, falling far below the average annual global rainfall of 860mm. The Limpopo Province, which is one of the nine provinces in South Africa, and of interest to this study, is predominantly agrarian, basically relying on availability of water, with rainfall being the major source for water supply. It is, therefore, pertinent that the rainfall pattern in the province be monitored effectively to ascertain the rainy period for farming activities and other uses. Modelling and forecasting rainfall have been studied for a long time worldwide. However, from time to time, researchers are always looking for new models that can predict rainfall more accurately in the midst of climate change and capture the underlying dynamics such as seasonality and the trend, attributed to rainfall. This study employed Exponetial Smoothing (ETS) State Space and Seasonal Autoregressive Integrated Moving Average (SARIMA) models and compared their forecasting ability using root mean square error (RMSE). Both models were used to capture the sporadic behaviour of rainfall. These two models have been widely applied to climatic data by many scholars and adjudged to perform creditably well. In an attempt to find a suitable prediction model for monthly rainfall patterns in Limpopo Province, data ranging from January 1900 to December 2015, for seven weather stations: Macuville Agriculture, Mara Agriculture, Marnits, Groendraal, Letaba, Pietersburg Hospital and Nebo, were analysed. The results showed that the two models were adequate in predicting rainfall patterns for the different stations in the Limpopo Province. / National Research Foundation (NRF)