Investigating the Drivers of Total Suspended Sediment Regime in the Senegal River Basin Using Landsat 8 Satellite Images

Because total suspended sediments (TSS) influence the penetration of light into the water column and are likely to carry pollutants and nutrients, their study is essential for understanding the functioning of African river ecosystems. If the estimation of solid transport is important in the context of hydro-agricultural developments, its quantification often poses a problem. In addition, in situ data for these areas are rare and, as a result, the environmental factors responsible for the variability of TSS can hardly be understood. This work aims to evaluate the spatiotemporal variability of TSS in the surface waters of the Senegal River using satellite data over the 2014-2018 period. The spatio-temporal dynamics of TSS is reconstructed using a relationship established on several West African sites between in situ data from TSS and satellite reflectances from Landsat 8. These data allow analyzing the relationship between TSS and factors such as rainfall and discharge. We found that the TSS peaks in Bakel coincide with the arrival of the first rains and are followed by peaks in discharge with a lag of 2 months. A time lag between TSS and discharge peaks is also observed on its tributaries like the River Falémé. Concerning the spatial variability, TSS generally increase from the river upstream to the downstream and decrease in the Senegal delta after the Diama dam. The analysis of the TSS upstream and downstream of the Manantali dam, in the upstream area, confirms the relatively low sediment deposits in the dam lake

Relation between seasonally detrended shortwave infrared reflectance data and land surface moisture in semi-arid Sahel

In the Sudano-Sahelian areas of Africa droughts can have serious impacts on natural resources, and therefore land surface moisture is an important factor. Insufficient conventional sites for monitoring land surface moisture make the use of Earth Observation data for this purpose a key issue. In this study we explored the potential of using reflectance data in the Red, Near Infrared (NIR), and Shortwave Infrared (SWIR) spectral regions for detecting short term variations in land surface moisture in the Sahel, by analyzing data from three test sites and observations from the geostationary Meteosat Second Generation (MSG) satellite. We focused on responses in surface reflectance to soil- and surface moisture for bare soil and early to mid- growing season. A method for implementing detrended time series of the Shortwave Infrared Water Stress Index (SIWSI) is examined for detecting variations in vegetation moisture status, and is compared to detrended time series of the Normalized Difference Vegetation Index (NDVI). It was found that when plant available water is low, the SIWSI anomalies increase over time, while the NDVI anomalies decrease over time, but less systematically. Therefore SIWSI may carry important complementary information to NDVI in terms of vegetation water status, and can provide this information with the unique combination of temporal and spatial resolution from optical geostationary observations over Sahel. However, the relation between SIWSI anomalies and periods of water stress were not found to be sufficiently robust to be used for water stress detection

La influencia del fenómeno ENSO sobre la fenología primaveral en los bosques boreales de Siberia central

Ponencia presentada en: V Congreso Internacional de la Asociación Española de Climatología celebrado en Zaragoza del 18 al 21 de septiembre de 2006.[ES]Se analiza la influencia de El Niño-Southern Oscillation (ENSO) sobre la variabilidad interanual de las fechas de aparición de las hojas e inicio de la actividad vegetal primaveral en los bosques boreales del hemisferio norte entre 1982 y 2004. Los resultados muestran negativas y significativas correlaciones entre el fenómeno ENSO, cuantificado mediante el Southern Oscillation Index (SOI), y la fecha de aparición de las hojas en Siberia central, con un retraso en dicha influencia de hasta 9 meses. La correlación entre la fecha de aparición de las hojas y la temperatura de superficie oceánica a escala global muestran un patrón espacial que asemeja al fenómeno ENSO, con correlaciones positivas y significativas en el Este del Pacífico y negativas en el Oeste. Esta influencia se explica por el papel del fenómeno ENSO sobre la presión en superficie, el geopotencial a 500hPa, la dirección e intensidad de los flujos de viento y la temperatura de superficie durante los meses en los que aparecen las hojas (abrilmayo) en las zonas boreales de Siberia central.[EN]This paper analyses the role of the El Niño-Southern Oscillation (ENSO) on the interannual variability of the leaf appearance dates of boreal forests in the Northern hemisphere (1982- 2004). We find significant negative correlations between the ENSO, quantified by means of the Southern Oscillation Index (SOI), and the leaf appearance dates in central Siberia with up to 9 months lag. The correlations between leaf appearance dates and summer Sea Surface Temperatures (SST) show a pattern that resembles the ENSO phenomena with positive and significant correlations in the East pacific and negative in the West. These findings are explained by the role of SOI on Sea Level Pressures, 500 hPa Geopotential and the wind flow direction and intensity during the months of leaf appearance (April and May) and on average temperatures, which determine noticeably the date of leaf appearance.Este trabajo se ha realizado gracias a la financiación del proyecto Siberia II (5º Programa Marco de la Comisión Europea)

The SIBERIA -II Project as seen by Envisat ASAR

ABSTRACT In the SIBERIA-II project Earth Observation (EO) data are used to derive a set of products, which are then fed into global and regional greenhouse gas accounting approaches. The overall aim of the project is to devise an integrated "EO-modelling" methodology for full carbon accounting at regional scale, and to quantify the accounting for an over 3 Mio km 2 area in Central Siberia. For the derivation of the EO products, SIBERIA-II makes use of most of current orbiting satellites and currently represents the major user of ASAR ENVISAT data. Within the project several issues ranging from data post-processing to derivation of EO-products and development of new techniques for land applications are treated. An experiment on geometric and radiometric validation of Wide Swath data showed the importance of the processor used for preparing the data to be further used in applications. The interferometric coherence from ASAR repeat-pass pairs acquired under stable winter conditions shows contrast between forests and bare soils. Alternating Polarisation data seems to be promising for forest/non-forest mapping. For wetlands, ASAR Wide Swath data is used for the operational monitoring of open water surfaces, reaching high classification accuracy. ASAR Wide Swath is also evaluated for pollution damage identification and fire scar detection. Finally, the long time series of ERS and ASAR Image Mode data allows detecting deforestation activities over almost one decade

Observed long-term land cover vs climate impacts on the West African hydrological cycle: lessons for the future ? [P-3330-65]

West Africa has experienced a long lasting, severe drought as from 1970, which seems to be attenuating since 2000. It has induced major changes in living conditions and resources over the region. In the same period, marked changes of land use and land cover have been observed: land clearing for agriculture, driven by high demographic growth rates, and ecosystem evolutions driven by the rainfall deficit. Depending on the region, the combined effects of these climate and environmental changes have induced contrasted impacts on the hydrological cycle. In the Sahel, runoff and river discharges have increased despite the rainfall reduction (“less rain, more water”, the so-called "Sahelian paradox "). Soil crusting and erosion have increased the runoff capacity of the watersheds so that it outperformed the rainfall deficit. Conversely, in the more humid Guinean and Sudanian regions to the South, the opposite (and expected) “less rain, less water” behavior is observed, but the signature of land cover changes can hardly be detected in the hydrological records. These observations over the past 50 years suggest that the hydrological response to climate change can not be analyzed irrespective of other concurrent changes, and primarily ecosystem dynamics and land cover changes. There is no consensus on future rainfall trend over West Africa in IPCC projections, although a higher occurrence of extreme events (rainstorms, dry spells) is expected. An increase in the need for arable land and water resources is expected as well, driven by economic development and demographic growth. Based on past long-term observations on the AMMA-CATCH observatory, we explore in this work various future combinations of climate vs environmental drivers, and we infer the expected resulting trends on water resources, along the west African eco-climatic gradient. (Texte intégral