Rainfall estimation in the Sahel: the EPSAT-NIGER experiment

Abstract

EPSAT-NIGER (Estimation of Precipitation by SATellite—NIGER experiment) has been designed to improve the understanding of the precipitation systems of Sudano-Sahelian Africa and to develop operational rainfall estimation algorithms for this region. It is based on the combined use of a very dense raingauge network (93 gauges over a study area of 16 000 km₂) and a C-band weather radar system. The experiment is scheduled to last three years, 1990–1992. The network pattern, a regular grid with nodes spaced at 12.5 km and a 16 gauge target area where the distance between stations is decreased to 1 km, has allowed for some preliminary studies on the rainfall distribution at various space and time scales. Whereas the long term average rainfall gradient is uniform, rainfall increasing north to south, a single rainy season can be markedly different. The local variability may be extremely large. That variability is enhanced at smaller sampling time steps and the computation of reference areal rainfall for satellite imagery validation is extremely sensitive to the design of the ground-based validation system. The joint processing of gauge and radar data has led to the identification of a few typical features of the drop size distribution of the African squall lines, which could lead to deriving specific algorithms for radar calibration in this region. The data provided by EPSAT-NIGER will be used in various international projects for the assessment of water input from the atmosphere to the continent over the Sahel.     

The Role of Laser Ranging for Calibrating Jason-1: The Corsica Tracking Campaign

The French Transportable Laser Ranging System (FTLRS), a highly transportable Satellite Laser Ranging (SLR) instrument, was set up in Corsica (from January to September 2002) for participating to the JASON-1 altimeter verification phase. In addition to the tracking of oceanographic satellite missions and in order to perform an accurate positioning, the FTLRS also acquired laser ranging data on geodetic satellites, STARLETTE and STELLA essentially.

The paper describes the analysis strategy mainly based on the use of a short-arc orbit technique to compute accurate 1 cm local orbits, and then the geocentric positioning (2–3 mm relative to GPS). Finally, we established the JASON-1 absolute calibration value, based on 9 SLR short-arcs (between cycles 1 and 26), at 108.2 ± 8.7 mm; the 10-day repeatability is of 26.1 mm showing that a great accuracy has been reached.     

Unique authigenic mineral assemblages reveal different diagenetic histories in two neighbouring cold‐water coral mounds on Pen Duick Escarpment,Gulf of Cadiz

Alpha Mound and Beta Mound are two cold‐water coral mounds, located on the Pen Duick Escarpment in the Gulf of Cadiz amidst the El Arraiche mud volcano field where focused fluid seepage occurs. Despite the proximity of Alpha Mound and Beta Mound, both mounds differ in their assemblage of authigenic minerals. Alpha Mound features dolomite, framboidal pyrite and gypsum, whereas Beta Mound contains a barite layer and predominantly euhedral pyrite. The diagenetic alteration of the sedimentary record of both mounds is strongly influenced by biogeochemical processes occurring at shallow sulphate methane transition zones. The combined sedimentological, petrographic and isotopic analyses of early diagenetic features in gravity cores from Alpha Mound and Beta Mound indicate that the contrast in mineral assemblages between these mounds is caused by differences in fluid and methane fluxes. Alpha Mound appears to be affected by strong fluctuations in the fluid flow, causing shifts in redox boundaries, whereas Beta Mound seems to be a less dynamic system. To a large extent, the diagenetic regimes within cold‐water coral mounds on the Pen Duick Escarpment appear to be controlled by fluid and methane fluxes deriving from layers underlying the mounds and forcings like pressure gradients caused by bottom current. However, it also becomes evident that authigenic mineral assemblages are not only very sensitive recorders of the diagenetic history of specific cold‐water coral mounds, but also affect diagenetic processes in turn. Dissolution of aragonite, lithification by precipitation of authigenic minerals and subsequent brecciation of these lithified layers may also exert a control on the advective and diffusive fluid flow within these mounds, providing a feedback mechanism on subsequent diagenetic processes.     

Spatially varied soil erosion under different climates

Abstract

The spatial variability of the factors of the universal soil loss equation is examined on the mediterranean basin of Conca de Tremp covering 43.1 km² in Spain. The evaluation of the rainfall erosivity R and the soil erodibility K is relatively straightforward and spatially-averaged values of these parameters can be applied to the entire basin. Conversely, the spatial variability of annual soil erosion losses on large basins depends primarily on the factors L, S and C describing topographic, vegetation and land use parameters. A grid size analysis of soil erosion losses from the Conca de Tremp basin under mediterranean climatic conditions in Spain shows excellent agreement with the earlier results on the Chaudière basin in Canada. It is concluded for both basins that unbiassed estimates of soil erosion losses are obtained for grid sizes less than about 0.125 km². The analysis of the Conca de Tremp basin validates the use of the grid size factor proposed by Julien & Frenette (1987). It is also found that the grid size factor primarily depends on the average slope gradient which decreases with increasing grid size or drainage area. On the other hand, the grid size factor does not depend on the spatial variability of the factors R, K, L and C.     

Fluvial and submarine morphodynamics of laminar and near‐laminar flows: a synthesis

The interaction of flow with an erodible bed in alluvial rivers and deep‐sea channels gives rise to a wide range of self‐formed morphologies, including channels, ripples, dunes, antidunes, alternate bars, multiple‐row bars, meandering and braiding. As the flow is invariably turbulent in field manifestations of these morphologies, there has been a tendency to assume that turbulence is necessary for them to form. While turbulence undoubtedly has an important influence when it is present, it is not necessary for any of these features. Indeed, all of these features can be formed by the morphodynamic interaction of purely laminar or nearly laminar flow with an erodible bed. This paper provides a survey and synthesis of a wide range of laminar or near‐laminar flow analogues of morphologies observed in the field. Laminar‐flow analogues of turbulent‐flow morphologies cannot and should not be expected to satisfy dynamic similarity in terms of all relevant dimensionless parameters. What is of more significance is the convergence of the underlying physics. It is illustrated in this paper that many existing theoretical frameworks for the explanation of turbulent‐flow morphodynamics require only relatively minor modification in order to adapt them to laminar flows.     

ANALYSIS OF THE COMMON MIDPOINT GATHER BY DECOMPOSITION INTO ELEMENTARY WAVEFRONTS1

The imaginary wavefront of a common midpoint gather of seismic data is analysed in terms of elementary wavefronts. The wavefronts emitted by the sources are split into a sequence of elementary circular wavefronts. Each such wavefront is independently transmitted through the subsurface and received by a group of receivers associated with the sources. The theory is first demonstrated in the case of a one-layer medium and is then extended to the case of a multilayer medium. We show that pairs of imaginary stationary points F and F' occur in the first medium through which all the rays, corresponding to the group of sources and receivers associated with the elementary wavefront pass. The location of the points is determined by 2n equations with 2n unknowns; (2n - 1) of these equations correspond to n layers of the medium, and an additional equation is given by the geometry of the CMP gather. Applying Fermat's principle between the points F and F', we obtain an algorithm for CMP ray tracing. The wavefront sampling problem is considered but not theoretically solved. Results obtained in the general case (a multilayer medium with lateral velocity variation) are presented and compared with those obtained by other modelling techniques.     

Nature mineralogique et origine des glauconies du plateau continental du Gabon et du Congo

The detailed analysis of pellets deposited on the continental shelf off Congo and Gabon (Gulf of Guinea) reveals the present formation of goethite and berthierine-granules, as well as that of glauconite. The goethite is being formed in the most oxygenized zones, independently of the initial nature of the granular support and, sometimes, takes the form of pseudooolites. The berthierine is being formed in the Ogooue delta zone. To us the deltaic environment, rather than the temperature factor, seems determinant. Principally, this berthierine is found in the form of coprolites, of initial kaolinic composition. Incrustations of calcareous debris and fillings of animalculae tests are present. This genesis is recent (post-Flandrian) and is still continuing. The glauconite exists practically uniquely in the form of coprolites, from the medium-depth zones of the continental shelf (—50 m) down to the upper continental slope (—300 m). The sèdiment in which it was formed, and is probably still being formed, is older (ante-Flandrian). The actual rate of sedimention is null when glauconite is forming. The glauconite formation may be broken down into several stages, which are found at different sampling depths and, sometimes, in the same samples. The pellets are formed first. These original pellets, of kaolinic nature, gradually become smectitic. The evolution continues, becoming a component with diffractometric behaviour identical with that of an interlayered material (illite-vermiculite) with the illite dominant. Correlatively, the K₂O percentage increases from 2–3% in the smectites to 5–6% in the most evolved component. The mineralogical composition of these last pellets is identical with that of certain fossil glauconites, the evolution of which could be followed up to the best crystallized glauconite mineral. Thus, we were able to reconstitute the formation history of the glauconite from any mineralogical support up to the typical glauconite mineral (Table l). Two conditions necessary for this formation are detected here: the time factor, as has often been pointed out, is important. However, it is, above all, the environment factor which has attracted our attention. The forming of pellets favours and determines the geo-chemical evolution through the creation of a semi-confined environment. Reactions take place there in a more concentrated environment than in the open sea, this environment is characterized by a large surface of reaction, since the granule is porous. Sedimentologically, it is possible to distinguish two groups of glauconites: one, monomineral, corresponds to more or less crystallized glauconite mineral; the other, plurimineral, consists of various TOT minerals. Only the first has the significance generally attributed to all glauconites : open epicontinental marine environment, free of sedimentation for a long time. The various cases of evolution encountered here are summarized below (Table 1).     

The lowermost Mississippi River: a mixed bedrock‐alluvial channel

In this study, the distribution of channel‐bed sediment facies in the lowermost Mississippi River is analysed using multibeam data, complemented by sidescan sonar and compressed high‐intensity radar pulse seismic data, as well as grab and core samples of bed material. The channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls. The consolidated substratum is made up of latest Pleistocene and Early Holocene fluvio‐deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends. The exposed substratum commonly displays a suite of erosional features, including flutes that are quantitatively similar in form to those produced under known laboratory conditions. A total of five bed facies are mapped, three of which include modern alluvial deposits and two facies that are associated with the relict substratum. A radius of curvature analysis applied to the Mississippi River centreline demonstrates that the reach‐scale distribution of channel‐bed facies is related to river planform. From a broader perspective, the distribution of channel‐bed facies is related to channel sinuosity — higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment. For example, the ratio of alluvial cover to substratum is ca 1·5:1 for a 45 km segment of the river that has a sinuosity of 1·76 and this ratio increases to ca 3:1 for a 120 km segment of the river that has a sinuosity of 1·21. The exposed substratum is interpreted as bedrock and, given the relative coverage of alluvial sediment in the channel, the lowermost Mississippi River can be classified as a mixed bedrock‐alluvial channel. The analyses demonstrate that a mixed bedrock‐alluvial channel boundary can be associated with low‐gradient and sand‐bed rivers near their marine outlet.     

A new Lyrodiscus (Mollusca, Gastropoda) assemblage from Saint-Acheul (Somme Valley): a reappraisal of MIS 11 malacofaunas from northern France

Recent studies undertaken at Saint-Acheul in the Somme Valley (France) have provided an opportunity to recover malacological assemblages from a tufa deposit located at the top of a Middle Pleistocene fluvial sequence. Molluscan communities are rich and, although dominated by open grassland species, contain up to 23 forest taxa. This high diversity of thermophilous molluscs allows the recognition of fully temperate climatic conditions. Moreover, the occurrence of an extinct zonitid belonging to the genus Retinella (Lyrodiscus), along with several species beyond their modern range (Platyla polita, Ena montana, Ruthenica filograna Clausilia pumila, Clausilia dubia, Macrogastra ventricosa, Perforatella bidentata, Monachoides incarnatus, Belgrandia marginata, Hygromia limbata) allows correlation of the Saint-Acheul assemblage with malacofaunas recovered in other MIS 11 tufa deposits from the Somme and Seine valleys. This age attribution is reinforced at Saint-Acheul by stratigraphy and an ESR date on quartz of 403±73 kyr from the underlying fluvial deposits. In addition, old collections containing R. (Lyrodiscus) specimens have been rediscovered, allowing taxonomic reassessment of the species. This shows that Retinella (Lyrodiscus) skertchlyi Kerney, 1976 is a junior synonym of Retinella (Lyrodiscus) elephantium (Bourguignat, 1869) and that at least two extinct species of R. (Lyrodiscus) occurred in western Europe during the Quaternary. Finally, reappraisal of these French molluscan assemblages shows that they are similar to British malacofaunas of Hoxnian age. These new results strengthen the uniqueness and biostratigraphical value of the 'Lyrodiscus assemblage'.     

LAND-USE IMPACT ON WATERSHED RESPONSE: THE INTEGRATION OF TWO-DIMENSIONAL HYDROLOGICAL MODELLING AND GEOGRAPHICAL INFORMATION SYSTEMS

The integration of a two-dimensional, raster-based rainfall–runoff model, CASC2D, with a raster geographical information system (GIS), GRASS, offers enhanced capabilities for analysing the hydrological impact under a variety of land management scenarios. The spatially varied components of the watershed, such as slope, soil texture, surface roughness and land-use disturbance, were characterized in GRASS at a user-specified grid cell resolution for input into the CASC2D model. CASC2D is a raster-based, single-event rainfall–runoff model that divides the watershed into grid cell elements and simulates the hydrological processes of infiltration, overland flow and channel flow in response to distributed rainfall precipitation. The five-step integration of CASC2D and GRASS demonstrates the potential for analysing spatially and temporally varied hydrological processes within a 50 square mile semi-arid watershed. By defining possible land-use disturbance scenarios for the watershed, a variety of rainfall–runoff events were simulated to determine the changes in watershed response under varying disturbance and rainfall conditions. Additionally, spatially distributed infiltration outputs derived from the simulations were analysed in GRASS to determine the variability of hydrological change within the watershed. Grid cell computational capabilities in GRASS allow the user to combine the scenario simulation outputs with other distributed watershed parameters to develop complex maps depicting potential areas of hydrological sensitivity. This GIS–hydrological model integration provides valuable spatial information to researchers and managers concerned with the study and effects of land-use on hydrological response.