Using standardized indicators to analyse dry/wet conditions and their application for monitoring drought/floods: a study in the Logone catchment,Lake Chad basin

The standardized precipitation index (SPI) and standardized streamflow index (SSI) were used to analyse dry/wet conditions in the Logone catchment over a 50-year period (1951–2000). The SPI analysis at different time scales showed several meteorological drought events ranging from moderate to extreme; and SSI analysis showed that wetter conditions prevailed in the catchment from 1950 to 1970 interspersed with a few hydrological drought events. Overall, the results indicate that both the Sudano and Sahelian zones are equally prone to droughts and floods. However, the Sudano zone is more sensitive to drier conditions, while the Sahelian zone is sensitive to wetter conditions. Correlation analysis between SPI and SSI at multiple time scales revealed that the catchment has a low response to rainfall at short time scales, though this progressively changed as the time scale increased, with strong correlations (≥0.70) observed after 12 months. Analysis using individual monthly series showed that the response time reduced to 3 months in October.     

乍得Bongor盆地储层沉积成岩特征及其主控因素

乍得Bongor盆地储层沉积成岩特征具有明显的独特性。结合盆地区域构造演化,利用地震、钻井、岩心、薄片、测试等资料,系统分析了Bongor盆地主力储层沉积展布特征和成岩演化。研究结果表明:Bongor盆地早白垩世强烈拉张裂陷作用形成了一系列NW-SE向深陡凹陷,主力储层局限分布在凹陷内部,以近岸水下扇沉积为主,毗邻控凹断层分布,具有近源、深水、快速沉积特征,结构成熟度和成分成熟度均较低;受多期构造运动及烃类流体改造,储层压实作用不充分,胶结成岩作用减弱,溶蚀现象普遍,目前尚处于中成岩A期阶段;以原生孔隙为主,属中孔、中高渗储层,有效储层埋深下限明显大于现今油层深度,表明盆地深层仍发育有优质储层,具有良好的勘探潜力。     

Caractérisation de la Variabilité Spatiale et Temporelle des Précipitations Annuelles sur le Bassin du Lac Tchad au Cours du 20ème Siècle/Spatial and Temporal Variability of Annual Rainfall in the Lake Chad Basin During the 20th Century

Abstract

The study of changes in annual rainfall in the Lake Chad basin during the 20th century is based on the analysis of 47 stations, i.e. a total of about 1600 station-years for the time series with more than 25 years of data. As previously observed in western and Sahelian Africa, robust tests of shift in time series identify a significant change in mean from the beginning of the 1960s between the latitudes 11 and 13°N, and a little later in this decade for the northern stations. The analysis of decadal rainfall shows that the 1950s decade was very humid and the next three decades were drier. These dry conditions were more and more severe until the 1980s. Data available after 1990 do not show any inversion in the trend. The annual rainfall decreases from south to north, and the regional gradient has changed from 1.5 mm km^?1 in the 1950s to 1.2 mm km^?1 in the 1980s between the latitudes 10 and 14°N.     

Depositional environments of Late Cretaceous Gongila and Fika formations,Chad (Bornu) Basin,Northeast Nigeria

Organic geochemical and palynofacies studies of 172 ditch cuttings samples of possible source rock shales from the Late Cretaceous Gongila and Fika formations in the Chad Basin of NE Nigeria were carried out to determine their paleoenvironments of deposition. Although dominated by amorphous organic matter, C/S ratios and molecular parameters suggest the mostly organic lean shales (TOC contents typically below 1.5%) were deposited in a normal marine environment. Levels of oxygenation influenced by water depth in the depositional environment appear to control organic richness and quality of the dark grey shales.The organic rich (TOC > 2.0%) upper part of the Fika Formation was deposited under anoxic conditions during the Late Cretaceous and could represent an Oceanic Anoxic Event. Mature intervals where such conditions prevailed would have generated liquid hydrocarbon, although none were sampled here.A trend of increasing organic richness towards the central part of the larger Chad Basin observed in this and other studies supports the development of organic rich marine shales (average TOC contents of 2–3%) of equivalent age in the Termit Basin where water depth would have been deeper and oxygen conditions at levels that permitted preservation of marine organic matter.     

乍得湖盆地典型区域降雨入渗补给地下水试验

针对尼日利亚北部乍得湖盆地降雨入渗补给地下水的问题,选择典型区域开展土壤剖面取样工作,采用氯离子平衡法计算各剖面降雨入渗补给量,并结合野外调查、钻探等工作研究影响降雨入渗补给的主要因素。结果表明：4个取样点（Y₁、Y₂、Y₃、Y₄）土壤剖面年平均降雨入渗补给量分别为4.9、1.5、7.9、26.2 mm/a,平均值为10.1 mm/a,年平均降雨入渗补给率仅为0.72%、0.22%、1.17%、3.87%,平均值为1.49%;研究区降雨入渗补给量很少,降雨对地下水资源的补给有限,地下水的主要补给来源为Hadejia河;研究区蒸散发量大,植物根系发达、吸水能力强,地表入渗水分多在表层土壤中或泥质层与风积砂层交界面上消耗于蒸发蒸腾,最终散失到大气中。在人类活动严重改变Hadejia河水文情势的背景下,研究区这种独特的地下水补给特征导致地下水位快速下降,使得区内正面临较严重的地下水资源枯竭问题。     

Groundwater modelling with limited data sets: the Chari–Logone area (Lake Chad Basin,Chad)

One of the most important issues for water resource management is developing strategies for groundwater modelling that are adaptable to data scarcity. These strategies are particularly important in arid and semi‐arid areas where access to data is poor and data collection is difficult, such as the Lake Chad Basin in Africa. In the present study, we establish a numerical groundwater flow model and evaluate the effects of dry and wet periods on groundwater recharge in the Chari–Logone area (96 000 km²) of the Lake Chad Basin. Boundary conditions, flow direction, sources, and sinks for the Chari–Logone local model were obtained by revising and remodelling the Lake Chad Basin regional hydrogeological model (508 400 km²) developed by the BRGM (Bureau de Recherches Géologiques et Minières) in the 1990s. The simulated aquifer water level showed good agreement with observed levels. Aquifer recharge is primarily determined by river–aquifer interactions and mostly occurs in the southern section of the study area. In wet years, groundwater recharge also occurs in the N'Djamena area. The approach we adopted provided relevant results and was useful as an initial step in more detailed modelling of the area. It also proved to be a useful method for groundwater modelling in large semi‐arid and arid regions where available data are scarce. Copyright © 2013 John Wiley & Sons, Ltd.     

Assessment of CHADFDM satellite-based input dataset for the groundwater recharge estimation in arid and data scarce regions

Aquifer natural recharge estimations are a prerequisite for understanding hydrologic systems and sustainable water resources management. As meteorological data series collection is difficult in arid and semiarid areas, satellite products have recently become an alternative for water resources studies. A daily groundwater recharge estimation in the NW part of the Lake Chad Basin, using a soil–plant-atmosphere model (VisualBALAN), from ground- and satellite-based meteorological input dataset for non-irrigated and irrigated land and for the 2005–2014 period is presented. Average annual values were 284 mm and 30°C for precipitation and temperature in ground-based gauge stations. For the satellite-model-based Lake Chad Basin Flood and Drought Monitor System platform (CHADFDM), average annual precipitation and temperature were 417 mm and 29°C, respectively. Uncertainties derived from satellite data measurement could account for the rainfall difference. The estimated mean annual aquifer recharge was always higher from satellite- than ground-based data, with differences up to 46% for dryland and 23% in irrigated areas. Recharge response to rainfall events was very variable and results were very sensitive to: wilting point, field capacity and curve number for runoff estimation. Obtained results provide plausible recharge values beyond the uncertainty related to data input and modelling approach. This work prevents on the important deviations in recharge estimation from weighted-ensemble satellite-based data, informing in decision making to both stakeholders and policy makers.     

On the occurrence of gold mineralization in the Pala Neoproterozoic formations,South-Western Chad

The Pala region, in southwestern Chad, belongs to the northern part of the Central African Pan-African Fold Belt. It is made up of greenschist-facies schists and is characterized by bimodal, mainly mafic, magmatism. This schist unit named Goueigoudoum Series is intruded by pre- to post-tectonic plutonic rocks dated between 737 and 570 Ma and dykes of quartz. Gold is mined artisanally from alluvial deposits and primary chalcopyrite–pyrite-bearing quartz veins, brecciated and silicified zones and shear zones. The majority of the mineralized shear zones and some quartz veins generally trend N–S to NNE–SSW or NW–SE and are interpreted as extensional shear fractures related to regional NE–SW-trending sinistral strike–slip shear zones. The geological context of the Pala region clearly indicates hydrothermal fluids formed along active continental margins during collisional orogenesis, and subsequent associated fluid migration typically occurred during strike–slip events. Although the origin of fluids may be varied (magmatic, metamorphic or meteoric fluids, Proterozoic seawater, or sedimentary basin formation waters), the distribution of the mineralizations along the granitoid intrusions suggests that magmatism played a major role in the dynamics of the mineralizing fluids.     

New material of Anancus kenyensis (proboscidea, mammalia) from Toros-Menalla (Late Miocene, Chad): Contribution to the systematics of African anancines

New fossil remains of the proboscidean genus Anancus are described. Among them, a complete skull allows us to revisit for the first time the entire Chadian Anancus fossil record. This genus occurred in the Old World from the late Miocene up to the early Pleistocene. The analysis of dental and cranial characters was allowed individual variations from specific characters to be distinguished. In this study we show that Anancus kenyensis and Anancus osiris are very likely synonym taxa which leads us to emend the diagnosis of A. kenyensis. In addition, this study shows that dental characters in anancines lineage are of little significance for biostratigraphical inference, by contrast to previous works. This study brings new data about the phylogenetical and palaeobiogeographical history of the African anancines.     

基于多源遥感数据的非洲乍得湖水面变化监测

乍得湖位于中非撒哈拉沙漠边界, 是非洲重要的淡水湖, 湖面大小对气候变化高度敏感。近40 年, 由于干旱等自然灾害和大规模灌溉等人为因素的影响, 乍得湖面积急剧减少, 受到广泛关注。本文利用多种光学遥感数据提取1973-2012 年乍得湖面积, 分析其变化趋势和驱动机制。结果表明, 1973-2012 年间乍得湖面积总体上在变小, 1973-1975 年间乍得湖面积急剧减少了约71%, 1975-2012 年面积在2000~5000 km²范围内波动。为验证变化趋势的可靠性, 本文利用MODIS影像与同期Landsat 和AVHRR影像的提取结果进行比较, 来检验多源数据的可比性;通过MODIS影像监测乍得湖面积的月变化, 来检验所选数据时相对乍得湖面积变化分析的可能影响。根据气象资料分析, 乍得湖面积与降水量变化存在一致性, 沙里盆地降水量变化是乍得湖面积变化的重要原因;其次, 大量修建水库是导致面积减少的另一重要原因;最后, 将乍得湖分为南北两部分的天然拦截坝通过阻碍水流增加蒸发, 也加剧了乍得湖面积的减少。