La neige étant une composante importante du climat terrestre, il convient de l'étudier à l'échelle du globe pour en comprendre son rôle.<br /><br />Ces travaux visent ainsi à étudier le contenu en information sur le manteau neigeux à l'échelle globale des observations satellites réalisées à partir de différents instruments couvrant différents domaines du spectre électromagnétique (visible, infrarouge et micro-ondes). Des paramètres autres que la neige interférant dans la problématique, il est nécessaire de les déterminer afin d'encadrer leurs influences. Ensuite, on cherchera comment limiter la pollution qu'engendre ces paramètres sur l'information accessible du manteau neigeux, on aura alors recours à ce que l'on appelera des statistiques locales.<br />Hélas ces statistiques locales ne sont pas disponibles sur l'ensemble du globe. Il est donc inévitable d'utiliser la modélisation pour permettre l'interpolation. Toutefois, une méthode originale d'ajustement de modèle par analyse en composantes principales sera présentée. Au final, la synergie entre les observations satellites, la modélisation et les mesures in situ devra être utilisée pour extraire de façon optimale l'information contenu dans les données et permettre l'inversion de l'épaisseur de neige à grande échelle.<br />D'autre part, il sera intéressant de caractériser les différents types de neiges existants selon une classification non-supervisée des observations satellites afin d'extraire les similitudes et les spécificités de chacun de ces types.

In order to understand the dynamics of desertification after the 1980s droughts, the trends and changes of photosynthetically active fraction of the vegetation cover of the semi-arid region of West Africa have been analyzed across three ecoclimatic entities. This study focuses on six countries (Senegal, Mauritania, Mali, Burkina-Faso, Niger and Benin) that reflect quite well the human and environmental context of semi-arid regions. The main objective of this thesis is to check in the before-mentioned biophysical and human context, if the state of the vegetation improves because of more favourable rainfall conditions, or if, conversely, the long environmental deterioration during recent decades has a healthy dose of irreversibility for several years. The process of investigation proceeds in three stages defined by geographical scales and a geoclimatic gradient. At the regional level, we compared vegetation productivity data from 1982-1999 time series of NOAA-AVHRR NDVI data to rainfall data. We analyzed data from 315 rain gauges distributed across five countries (Senegal, Mauritania, Mali, Burkina-Faso and Niger) with annual average isohyets ranging from 100 to 900 mm. For trends analysis, we used the ratio of the integrated vegetation index (iNDVI) during the crop-growth period (June to October) to the May to October sum of rainfall (RR). This ratio (iNDVI/RR), a proxy of the Rain Use Efficiency, is widely used by ecologists as an indicator for detecting desertification processes. Overall, our results show a significant increase of the net primary production as a response of post-drought rainfall increase. However, the trends of iNDVI/RR ratio suggest that most of the studied stations (54%) in sahelian and sahelo-sudanese belts were stable and 31.4% showed strong to very strong negative change in iNDVI/RR ratio, while 13.3% of the stations showed a moderate decrease and only 1.3% showed a positive trend. At the country level, similar trends were recorded throughout 128 stations in Burkina Faso located between the 500 mm and 1100 mm isohyets. In fact, more than half the studied stations showed a stability of iNDVI/RR ratio. However, 39.8% of them show a negative trend from low to high, while only 2.4% of them show a slight positive trend. In addition, a comparison with more detailed local case studies confirmed these observed trends. However, the obtained results for wetter stations in the southern part of the country should be taken with precaution, as the relationship between NDVI and rainfall tends to weaken when annual rainfall is higher than 1000 mm. Overall, the negative trends highlight a gradual decline in plant productivity. These results recorded in 44.7% of the analyzed stations may reflect ongoing desertification processes in the sahelian and sahelo-sudanian zones over the past two decades. At the local level, a structural characterization of woody units in three bioclimatic regions of the sudanian zone (900 mm 1200 mm) in the north of Benin was conducted to assess their degradation status. We recorded the structural characteristics of stands (basal area, density), species diversity as well as disturbances type and intensity. Multivariate analysis revealed a gradient of productivity between the three regions: there was a high diversity of woody stands in the south-sudanian sector, while the north-sudanian and sudano-sahelian sectors were dominated by savannas and shrub, which had low productivity. The productivity gradient is influenced by a disturbance gradient suggesting that the decline in productivity is stronger from south-sudanian to the sudano-sahelian region. In addition, the spatial component of the observable changes in vegetation cover has been mapped by remote sensing in a restricted area of the sudanian zone in northern Benin with SPOT-XS data over the period 1986-2005. Over the past two decades, 19.6% of the woody stands have completely disappeared; 12.9% of this extinction of woody stands was due to deforestation, and 13.9 % due to degradation processes. In contrast to these trends, 21.8% of the study area were stable, while less than a third (31.7%) of the area were experiencing woody recovery (reforestation). The analysis also reveals significant disparities in the rates of change of the identified land use class units. These variations are more pronounced for the woody units and agricultural land than in villages. These results suggest that land cover degradation throughout the study area is primarily due to anthropogenic factors (livestock and agricultural expansion, logging, breeding). In fact, this area is a preferred destination for agricultural migrants fleeing the unpredictable climatic conditions of the drier semi-arid areas. Overall, our results highlight the rapid decline of vegetation resources, challenging assumptions that the impact of ongoing desertification processes is mixed, outside of the arid and semi-arid regions of West Africa. The developed framework is easily reproducible and allows the implementation of a reliable diagnosis on the state of the vegetation cover from accessible and inexpensive data. Its implementation should facilitate the development of managerial techniques for better assistance to the poor, especially vulnerable to the gradual degradation of their environment.

The study of solar radiation is a key process for the exploitation of solar energy systems. In most cases, however, the availability of information is insufficient and not up-to-date. In addition, the interpolation of ground-level measurement stations data does not allow the observation of microclimatic aspects of solar radiation (Zelenka at al., 1999). Therefore, a statistical model and NOAA-AVHRR satellite images were selected for the determination of solar radiation maps for the entire Iberian Peninsula. The statistical methodology based on Diabate et al. (1989) and Flores (2002) assures the accuracy of the process and the NOAA-AVHRR images provide a fine high resolution of 1 km2, which is needed because of the complexity of the area of study. The period of study covers five years (1998-2002), which assures the study of the year-on-year variation of solar radiation. The statistical model was calibrated with data from 21 ground-level measurement stations all around the Iberian Peninsula. These data were filtered with the aim of eliminating all erroneous registers, which a crucial subject in the method. The satellite images were geometrically calibrated and corrected. Furthermore, a methodology for the detection of clouds was applied in order to obtain the surface albedo (Laine et al., 1999). The evaluation of results, with 7 independient ground-level measurement stations, leads to a MBE of &#8722;3.8% and a RMSE of 24.4% for the daily data and a MBE of &#8722;1.1% and a RMSE of 15.9% for the hourly data. Maps of hourly and daily solar radiation for the years of study and the average tendency were determined. The Iberian Peninsula receives a mínimum of 4.2 MJ&#61472;m&#8722;2 d&#8722;1 and a maximum of 26 MJ&#61472;m&#8722;2 d&#8722;1 of dailly solar radiation, whith a mean of 15.1 MJ&#61472;m&#8722;2 d&#8722;1. The year-to-year variability of solar radiation ranges between 14.9 MJ&#61472;m&#8722;2 d&#8722;1 for year 2002 to 17.3 MJ&#61472;m&#8722;2 d&#8722;1 for year 2000. The results clearly show the usefulness of this work when obtaining solar radiation maps with a high temporal (hourly and daily maps) and spatial resolution (1 km2). / El estudio de la radiación solar es un proceso clave para el aprovechamiento de la energía solar. En la mayoría de los casos, sin embargo, la disponibilidad de información es insuficiente y desactualizada. Adicionalmente, la interpolación de superficie obtenida a partir de estaciones de medición en superficie no permite la observación de los aspectos microclimáticos de la radiación solar (Zelenka et al., 1999). Por esto, se ha elegido un modelo estadístico e imágenes del satélite NOAA-AVHRR para la determinación de mapas de radiación solar para toda la Península Ibérica. El método estadístico basado en Diabaté et al. (1989) y Flores (2002) asegura la exactitud del proceso y las imágenes NOAA-AVHRR una alta resolución espacial del orden de 1 km2, lo cual es necesario dada la complejidad del área de estudio. El período de estudio es de 5 años (1998-2002), lo que permite estudiar la variación interanual de la radiación solar. El modelo estadístico fue calibrado con 21 estaciones de medición en superficie distribuidas en toda la superficie de España. Los datos de las estaciones de medición son acuciosamente filtrados con el propósito de eliminar todos los registros erróneos, aspecto crucial del método. Las imágenes son calibradas y corregidas geométricamente, además se les realiza un proceso de detección de nubes con el fin de obtener el albedo superficial (Laine et al., 1999). La evaluación de los resultados, con datos de 7 estaciones de medición en superficie independientes, permite obtener un MBE de &#8722;3.8% y un RMSE de 24.2% para los dato diarios y un MBE de &#8722;1.1% y un RMSE de 15.9% para los datos horarios. Se obtienen mapas de radiación solar horarios puntuales y diarios para todos los años del período de estudio, además se determina la tendencia media de los mismos. Los resultados muestran que la Península Ibérica recibe un mínimo de 4.2 MJ&#61472;m&#8722;2 d&#8722;1 y un máximo de 26 MJ&#61472;m&#8722;2 d&#8722;1, con una radiación solar media de 15.1 MJ&#61472;m&#8722;2 d&#8722;1. La variabilidad interanual de la radiación solar queda expresada con valores que van desde 14.9 MJ&#61472;m&#8722;2 d&#8722;1 para el año 2002 hasta 17.3 MJ&#61472;m&#8722;2 d&#8722;1 para el año 2000. Los resultados demuestran claramente la utilidad del trabajo en la obtención de mapas de radiación solar horarios y diarios con una alta resolución espacial (1 km2).

The quantification and description of sea surface temperature (SST) is critically important because it can influence the distribution, migration, and invasion of marine species; furthermore, SSTs are expected to be affected by climate change. Recent research indicates that there has been a warming trend in ocean temperatures over the last 50 years. Hence, we sought to identify and demonstrate how a particularly germane SST dataset can be used within the scope of global climate change research. For this project we assembled a 29-year nearshore time series of mean monthly SSTs along the North Pacific coastline, as well as mean monthly SSTs for ice-free regions of the Arctic, using remotely-sensed satellite data collected with the Advanced Very High Resolution Radiometer (AVHRR) instrument. By providing detailed information concerning both dataset generation and data limitations, we aimed to make these data comprehensible to an expanded audience concentrating on life sciences rather than the traditionally physical science-based community. Furthermore, by making these data freely and publically available in multiple formats, including GIS (geographic information systems) layers, we expand their visibility and the extent of their use. We then used the dataset to describe SST patterns of nearshore (< 20 km offshore) regions of 16 North Pacific ecoregions, and of ice-free regions of 20 Arctic ecoregions, as delineated by the Marine Ecoregions of the World (MEOW) hierarchical schema. Our work creates a better understanding of present temperature regimes in these critically sensitive areas, from which we can draw several basic conclusions. 1) AVHRR SST measurements alone are sufficient to identify temperature patterns pertinent to determining health of ecosystems; 2) Within the nearshore North Pacific, ecoregions along the California Current System are most vulnerable to habitat-altering SST changes; 3) sea ice distribution is a major factor affecting SSTs in Arctic ecoregions, causing concern for the welfare of Arctic species. / Graduation date: 2012