Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review

In the West African Sahel, two paradoxical hydrological behaviors have occurred during the last five decades. The first paradox was observed during the 1968–1990s ‘Great Drought’ period, during which runoff significantly increased. The second paradox appeared during the subsequent period of rainfall recovery (i.e., since the 1990s), during which the runoff coefficient continued to increase despite the general re-greening of the Sahel. This paper reviews and synthesizes the literature on the drivers of these paradoxical behaviors, focusing on recent works in the West African Sahelo/Sudanian strip, and upscaling the hydrological processes through an analysis of recent data from two representative areas of this region. This paper helps better determine the respective roles played by Land Use/Land Cover Changes (LULCC), the evolution of rainfall intensity and the occurrence of extreme rainfall events in these hydrological paradoxes. Both the literature review and recent data converge in indicating that the first Sahelian hydrological paradox was mostly driven by LULCC, while the second paradox has been caused by both LULCC and climate evolution, mainly the recent increase in rainfall intensity

Muusaa la Taarikoo aniŋ Suukuwoo: The Story of Moses and Religious Poetry

The entire manuscript is available for download as a PDF file(s). Higher-resolution images may be available upon request. For technical assistance, please contact [email protected]. Fieldwork Team: Dr. Fallou Ngom (Pricipal Investigator; Director, African Studies Center), Ablaye Diakité (Local Project Manager), Mr. Ibrahima Yaffa (General Field Facilitator), and Ibrahima Ngom (photographer). Technical Team: Professor Fallou Ngom (Principle Investigator, Project Director and former Director of the African Studies Center at Boston University), and Eleni Castro (Technical Lead, BU Libraries). This collection of Mandinka Ajami materials is copied as part of the African Studies Center’s African Ajami Library. This is a joint project between BU and the West African Research Center (WARC), funded by the British Library/Arcadia Endangered Archives Programme. Access Condition and Copyright: These materials are subject to copyright and are distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. For use, distribution or reproduction beyond these terms, contact Professor Fallou Ngom ([email protected]). Citation: Materials in this web edition should be cited as: Ngom, Fallou, Castro, Eleni, & Diakité, Ablaye. (2018). African Ajami Library: EAP 1042. Digital Preservation of Mandinka Ajami Materials of Casamance, Senegal. Boston: Boston University Libraries: http://hdl.handle.net/2144/27112. For Inquiries: please contact Professor Fallou Ngom ([email protected]). For technical assistance, please contact [email protected] / Custodial history: The owner inherited them from his father, Mamadou Lamine Diante who was born in Baghere in Casamance and received his Islamic education in Diao-Simaacounda. He later settled in Kontecounda where he served as an Imam and Quranic teacher until his death.Contains two old manuscripts written in classical Arabic. The first one is a prose text with Ajami glosses. The themes discussed in it include the story of Moses and his conversations with God. The second manuscript is a classical Arabic poem that praises God and Prophet Muḥammad written by a Mandinka scholar called Arfang Bakary Faty. The manuscript has extensive Arabic glosses and some Ajami glosses

Assessment of satellite rainfall products for stream flow simulation in Gambia watershed

International audienceSatellite rainfall estimates (SRE) with high spatial and temporal resolution and large areal coverage provide a potential alternative source to force hydrological models within regions where ground-based measurements are not readily available. The Gambia Basin in West Africa provides a good example of a case where the use of satellite precipitation estimates could be beneficial. This study aims to compare three SRE over a 12-year periods (1998-2010), before and after their integration into the GR4J hydrological model over the Gambia Basin. The inter-compared products are Climate Hazards Group Infrared Precipitation with Stations (CHIRPS), Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks-Climate Data Record (PERSIANN-CDR) and TRMM 3B42v7 (Tropical Rainfall Measuring Mission). The calibration and validation of the GR4J model over the Gambia basin using a reference rainfall product (RRP) pointed out a very good performance. The correlation coefficient between simulated and observed daily discharge is higher than 0.8 both for calibration and validation. The inter-comparison of SRE against RRP and using them as forcing data into the calibrated GR4J hydrological model presented some coherence in the product performance. PERSIANN-CDR performs better both when comparing against RRP and when used in GR4J. The low performance of CHIRPS is surprising because it is supposed to be a product that includes ground-base station. This result may also indicate that in areas without ground stations, the CHIRPS is less accurate than other rainfall products that are based only on satellite images. Finally, a bias correction is applied to the SRE using the RRP. The bias correction had significantly improved the product performance. On average, the bias fell from 100 to 1.5% compared to the RRP, but the impact on the error is less significant. When using the corrected SRE in the hydrological model, the impact is very significant both on the bias and error. The overall performance of the different biases that corrected SRE is comparable

Satellite-based rainfall estimates to simulate daily streamflow using a hydrological model over Gambia watershed

ABSTRACTSatellite rainfall products (SRPs) have the potential to overcome the limitations of ground-based rainfall observations and provide an alternative to inadequately or ungauged watersheds. However, due to the relatively poor accuracy and associated uncertainties to SRPs, it is necessary to evaluate their quality and applicability for each investigated watershed. This paper evaluates the usefulness of SRPs as forcing data for hydrological modeling under different scenarios and assesses their applicability for the Kedougou, Mako and Simenti sub-basins of the Gambia River. To achieve this, the “Génie Rural à 4 paramètres Journalier model” (GR4J) hydrological model was employed to simulate the streamflow considering four different scenarios: i) non-calibrated GR4J model run with uncorrected SRPs (Scenario 1); ii) non-calibrated model run with corrected SRPs (Scenario 2); GR4J model was calibrated and validated using uncorrected SRPs, and then they were utilized to drive the model (Scenario 3); GR4J model was calibrated and validated and then run using forcing inputs from corrected SRPs (Scenario 4). Results revealed that under Scenario 1 the SRPs performed poorly over the three sub-basins, while under scenario 2, the simulated daily streamflows showed relative improvement when run using corrected SRPs with 6 or 10 rainfall stations. Under the scenarios 3 and 4, the calibrated model provides significant improvement of the simulated streamflow with both the corrected and non-corrected SRPs. Finally, the SRPs demonstrate potential for use in watersheds where there are no rain gauges. The performance loss from scenario 4 (considered as the reference) to scenario 3 does not exceed 20%. Similarly, the performance loss from scenario 4 to scenario 2 does not exceed 50% when the SRPs are corrected using 3 and 6 rainfall stations (e.g., in the Kedougou sub-basin). Thus, they can be considered acceptable for hydrological simulations when the hydrological model is calibrated with measured streamflow

La paradoxe du Sahel est il toujours paradoxal ?

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Inverse Estuaries in West Africa: Evidence of the Rainfall Recovery?

International audienceIn West Africa, as in many other estuaries, enormous volumes of marine water are entering the continent. Fresh water discharge is very low, and it is commonly strongly linked to rainfall level. Some of these estuaries are inverse estuaries. During the Great Sahelian Drought (1968-1993), their hyperhaline feature was exacerbated. This paper aims to describe the evolution of the two main West African inverse estuaries, those of the Saloum River and the Casamance River, since the end of the drought. Water salinity measurements were carried out over three to five years according to the sites in order to document this evolution and to compare data with the historical ones collected during the long dry period at the end of 20 th century. The results show that in both estuaries, the mean water salinity values have markedly decreased since the end of the drought. However, the Saloum estuary remains a totally inverse estuary, while for the Casamance River, the estuarine turbidity maximum (ETM) is the location of the salinity maximum, and it moves according to the seasons from a location 1-10 km downwards from the upstream estuary entry, during the dry season, to a location 40-70 km downwards from this point, during the rainy season. These observations fit with the functioning of the mangrove, the West African mangrove being among the few in the world that are markedly increasing since the beginning of the 1990s and the end of the dry period, as mangrove growth is favored by the relative salinity reduction. Finally, one of the inverse estuary behavior factors is the low fresh water incoming from the continent. The small area of the Casamance and Saloum basins (20,150 Water 2020, 12, 647 2 of 25 and 26,500 km² respectively) is to be compared with the basins of their two main neighbor basins, the Gambia River and the Senegal River, which provide significant fresh water discharge to their estuary

Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review

In the West African Sahel, two paradoxical hydrological behaviors have occurred during the last five decades. The first paradox was observed during the 1968–1990s ‘Great Drought’ period, during which runoff significantly increased. The second paradox appeared during the subsequent period of rainfall recovery (i.e., since the 1990s), during which the runoff coefficient continued to increase despite the general re-greening of the Sahel. This paper reviews and synthesizes the literature on the drivers of these paradoxical behaviors, focusing on recent works in the West African Sahelo/Sudanian strip, and upscaling the hydrological processes through an analysis of recent data from two representative areas of this region. This paper helps better determine the respective roles played by Land Use/Land Cover Changes (LULCC), the evolution of rainfall intensity and the occurrence of extreme rainfall events in these hydrological paradoxes. Both the literature review and recent data converge in indicating that the first Sahelian hydrological paradox was mostly driven by LULCC, while the second paradox has been caused by both LULCC and climate evolution, mainly the recent increase in rainfall intensity

Are the Fouta Djallon Highlands Still the Water Tower of West Africa?

International audienceA large share of surface water resources in Sahelian countries originates from Guinea's Fouta Djallon highlands, earning the area the name of "the water tower of West Africa". This paper aims to investigate the recent dynamics of the Fouta Djallon's hydrological functioning. The evolution of the runoff and depletion coefficients are analyzed as well as their correlations with the rainfall and vegetation cover. The latter is described at three different space scales and with different methods. Twenty-five years after the end of the 1968-1993 major drought, annual discharges continue to slowly increase, nearly reaching a long-term average, as natural reservoirs which emptied to sustain streamflows during the drought have been replenishing since the 1990s, explaining the slow increase in discharges. However, another important trend has been detected since the beginning of the drought, i.e., the increase in the depletion coefficient of most of the Fouta Djallon upper basins, as a consequence of the reduction in the soil water-holding capacity. After confirming the pertinence and significance of this increase and subsequent decrease in the depletion coefficient, this paper identifies the factors possibly linked with the basins' storage capacity trends. The densely populated areas of the summit plateau are also shown to be the ones where vegetation cover is not threatened and where ecological intensification of rural activities is ancient