Précipitations

Résumé

Les précipitations désignent toutes les formes de l'eau à l'état liquide ou solide provenant de l'atmosphère. Ces hydrométéores (cristaux de glace ou gouttelettes d'eau), ayant été soumis à des processus de condensation et d'agrégation à l'intérieur des nuages, sont devenus trop lourds pour demeurer en suspension dans l'atmosphère et tombent au sol ou s'évaporent en virga avant de l'atteindre. Les précipitations se caractérisent par trois principaux paramètres : leur volume, leur intensité et leur fréquence qui varient selon les lieux et les périodes (jours, mois ou années). Par extension, le terme peut également être utilisé pour des phénomènes similaires sur d'autres planètes ou lunes ayant une atmosphère. Précipitations terrestres Types vignette|Type de la précipitation selon la structure thermique (bleu sous zéro degré Celsius et rouge au-dessus) La fréquence et la nature des précipitations dans une région géographique donnée sont des

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Clustering of winter precipitations in Valais

Clémence Marie Anne Le Cornec

This master thesis provides an extended study of winter precipitations in Valais (Switzerland). It was rst tried to relate local conditions in Valais, as measured at gauges, to the Hess-Brezowski classication for Europe. It was not possible to clearly relate this classication to local precipitation events in Winter in this region. Self-organizing maps were used to try to distinguish between sub-regions only based on the precipitation regime in Valais. The obtained results present coherence with the existing literature. ERA-Interim (global atmospheric reanalysis) data were then used to develop a new classication to link European meteorological conditions to local events. Three unsupervised clustering techniques were used for this purpose: k-means, k-medoids, hierachical clustering. The optimal number of cluster was determined using a com- bination four commonly used internal clusters indices (Dunn index, Davies-Bouldin index, silhouette index and Calinski-Harabasz index). Two clusters appeared to be optimal compared to the various parameters of the problem. The outputs of the three classication techniques were compared in term of robustness and results. A physical interpretation was then made. It was found that the two classes were associated respectively with small or large precipitation events. The associated synoptic conditions at the European scale reveal that the small precipitation class is associated with a North-East ux over Switzerland whereas the large precipitation class is linked to a South-West ux linked to a stationary front over Switzerland.

2017

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EFFECT OF WATER CONTACTS SEASONALITY ON SCHISTOSOMIASIS TRANSMISSION IN RURAL SAHELIAN AREA IN BURKINA FASO

Anna Faes

patterns due to economic and sociologic activities in African rural area are an important factor in the comprehension of the transmission of this disease. Through interviews and a participatory workshop, activities related to water-contact were identified, quantified and located for the study site of Tougou, situated in the north of Burkina Faso. Tougou, a field site for the 2iE Institute of Ouagadougou since 2004, has a sahelian climate; with a dry season of eight months and all precipitations occurring during the four other months of the year; and is affected by urinary schistosomiasis. The water-contact rates for the identified activities and the attractiveness of the different water points at the site, point out a seasonal variation in water-contact patterns. Using modelling to explore the effect of the seasonality of water-contacts in schistosomiasis transmission cycle, two different issues are observed depending on the considered system. In a local system, including one point of contact with water and one village, the seasonal variation of water-contacts leads to a decrease in the number of parasites per person. In opposite way, in a connected system of several villages having several water points where contacts occur, the seasonal variation of water-contacts lead to an increase in the number of parasites per person, due to the gathering of most water contacts at fewer number of water points during the dry period of the year. Table of content

2015

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This study aims to assess the snow cover spatial and temporal variation in the Source Region of the Yellow River, to understand the effect of snowmelt on the hydrology and to evaluate the potential impact of increasing temperature on the spring snowmelt runoff. The Snowmelt Runoff Model (SRM) developed by Rango and Martinec was used and improved by adding diurnal temperature computation and snow storage computation to model the runoff for 2013, 2014, 2016 and 2017. The snow cover assessment was done using Tera-MODIS satellite images between 2001 and 2017. Results showed that spring snowmelt produces 38% of direct runoff before July and 15% of annual runoff in average. The snow cover is very sensitive to temperature and to winter precipitation. There is a significant decrease of snow cover in May since 2001 in parallel to a significant increase of temperature. The snow cover duration has decreased by more than 15 days (20%) in the permafrost area in 15 years. Simulation suggests that a temperature increase of 2°C or 4°C will reduce the spring snowmelt by at least 20% and 30% respectively, and spring peak runoff will occur up to 15 days and one month earlier. Future investigations should focus on the interaction between snow, frozen ground and ground water recharge in order to extend the knowledge on snowmelt impact to soil related processes which play a key role on the ecosystem degradation in the region.

2018

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