Bias correction of CORDEX-Africa regional climate model simulations for trend analysis in northeastern Lake Chad: Comparison of three bias correction methods

In order to better adapt to the consequences of climate change, regional climate models (RCMs) have been set up for simulations. However, these simulations are often subject to biases, making it difficult to use them directly in studies of the impact of climate change. It is therefore necessary to use bias correction methods to reduce discrepancies between observed data and the data simulated by RCMs. The aim of this study is to analyse the results of three bias correction techniques (scaling, EQM and GQM) applied to rainfall data and mean minimum and maximum temperatures from CORDEX-Africa Regional Climate Models (RCMs), specifically in the north-eastern region of Lake Chad. Various statistical measures such as Pbiais, RMSE, R2 and EAM were used to assess the performance of each bias correction method in this study. In addition, the adjusted Mann-Kendall test and the Sen slope estimator were used to examine trends and their magnitude over the recent (1975-2004) and future (2021-2050) periods with a significance level of 5%. Overall, based on the statistical measures evaluating the effectiveness of the bias correction techniques, this study shows that all the methods tested were able to reduce the biases of the RCM outputs satisfactorily. In particular, the linear scaling approach proved to be more effective in correcting biases than the EQM and GQM methods. Therefore, an analysis of future trends in mean annual precipitation and temperature (minimum and maximum) was carried out for the RCP4.5 and RCP8.5 scenarios using the linear scaling method to correct for data biases. An increase in precipitation and temperature was observed in the study area over the recent period. The results of multi-model averaging of regional climate change for the RCP4.5 and RCP8.5 scenarios indicate a significant increase in mean annual temperatures (minimum and maximum) in the future. As far as annual precipitation is concerned, only an increase is forecast under the RCP4.5 scenarios. Under the RCP8.5 scenarios, a trend towards stable precipitation is predominant, with the exception of the south of the zone, where an increase has been observed. In the light of these results, it is clear that the impact of climate change will intensify in the region studied in the future

Bias correction of CORDEX-Africa regional climate model simulations for climate change projections in northeastern Lake Chad: Comparative analysis of three bias correction methods

Climate model simulations are frequently affected by biases, which makes it difficult to incorporate them directly into analyses of the impact of climate change. It is therefore essential to use bias correction methods to minimize discrepancies between real data and that generated by Regional Climate Models (RCMs). This study aims to analyze the results of three bias correction methods (LS, MQE, and MQG) applied to the processing of mean rainfall and temperature data from CORDEX-Africa's Regional Climate Models (RCMs), specifically in the north-eastern region of Lake Chad. Four statistical measures (bias, RMSE, r2, and MEA) were used to assess the effectiveness of each bias correction method. In addition, adjusted Mann-Kendall analysis and the Sen slope estimation method were applied to study trends and their magnitude over the recent (1975-2020) and future (2021-2050) periods, using a 5% significance level. The results highlight the existence of significant biases between the uncorrected RCM outputs and the observed data. After applying the bias correction, significant reductions in bias and comparable performance between the different bias correction methods were observed, with the LS method performing slightly better in correcting biases in monthly mean precipitation and temperature. Consequently, the LS method was selectively applied to correct the biases in the RCM monthly mean precipitation and temperature projections for the 2021-2050 period under the RCP4.5 and RCP8.5 scenarios using the 1975-2004 reference period. The results of multi-model averaging of RCMs under the RCP4.5 and RCP8.5 scenarios indicate a significant increase in mean annual temperatures over the period 2021-2050. As far as annual precipitation is concerned, only an increase is forecast under the RCP4.5 scenario. Under the RCP8.5 scenario, the absence of a precipitation trend is predominant, with the exception of the south of the zone, where an increasing trend has been observed. In light of these results, it is clear that the impact of climate change will intensify in the study area in the future. It is imperative to develop strategies to adapt and reduce the impacts in order to manage the availability of water resources efficiently

Study of the physico-chemical and bacteriological quality of water intended for consumption in the town of Gagal, southwestern Chad

The city of Gagal suffers from insufficient drinking water, and the population turns to surface water, wells and drillings without guarantee of quality. In order to characterize the hydrogeochemical and bacteriological properties of the waters of the aquifer system in the city of Gagal, to contribute to improving its knowledge, field campaigns were undertaken to sample the groundwater.The present study focuses on the hydrogeochemical and bacteriological characterization of drinking water in Gagal, southwestern Chad. The methodology consisted of acquiring existing data, a field campaign, and a chemical and bacteriological analysis of the water in the laboratory. The results of the physicochemical analyses revealed that the values of the parameters such as conductivity, pH (5.23), Ca²⁺ (26.11 mg/L), Mg²⁺(5.14 mg/L), Na⁺(3.54 mg/L), K⁺(1.34 mg/L), HCO3-(81.74 mg/L), Cl-(11.77 mg/L), SO4-(1.94 mg/L), and NO3- (8.70 mg/L) conformed to the WHO potability standards. Piper's diagram showed calcic and magnesian bicarbonate facies represented by 75%; and calcic and magnesian sulfate chloride facies in 25% of the analyzed waters. The bacteriological (Total coliforms (0 to more than 135,200 CFU/100 ml), Escherichia coli (0 and 14,400 CFU/100 ml), faecal enterococci (0 and 4600 CFU/100 ml)) study confirmed that the water from the wells and boreholes showed pollution of bacterial origin. Using these waters may endanger the populations with the risks of hydric diseases

Hydrological, chemical and isotopic functioning of the main tributary of lake Chad : the Chari-Logone system

Les fluctuations spectaculaires d’extension du lac Tchad au cours des dernières décennies ont montré la très grande vulnérabilité de cet écosystème. La surface actuelle du lac a considérablement diminué par rapport à la dernière période humide précédant les années 1980, soulevant la question des rôles respectifs des variations climatiques et des impacts anthropiques sur la réduction de surface du lac Tchad. Cette thèse se propose d'étudier le fonctionnement du fleuve Chari-Logone qui représente jusqu'à 82% des apports en eau au lac à partir de données hydroclimatiques et géochimiques (δ18O, δ2H, éléments majeurs dissous, 87Sr/86Sr). Nos résultats montrent que l’impact des activités anthropiques sur le bassin n’est pas détectable dans les variations considérables des écoulements annuels observés au cours de la période 1950-2015. Ceux-ci répondent essentiellement à la variabilité des précipitations, avec une amplification de ces variations particulièrement prononcées dans les parties les plus arides du bassin versant du Chari-Logone. Les processus d'altération chimique sur le bassin n'ont quant à eux pas changé au cours de cette périodeThe dramatic fluctuations in the extent of lake Chad in recent decades have shown the extreme vulnerability of this ecosystem. The current area of the lake has decreased significantly from the last wet period prior to the 1980s, raising the question of the respective roles of climate variations and anthropogenic impacts in lake Chad surface reduction. This thesis proposes to study the functioning of the Chari-Logone river which represents up to 82% of water input to the lake from hydroclimatic and geochemical data (δ18O, δ2H, dissolved major elements, 87Sr/86Sr). Our results show that the impact of human activities on the basin is not detectable in the considerable variations in annual flows observed during the period 1950-2015. These mainly respond to the variability of rainfall, with an amplification of these variations particularly pronounced in the driest parts of the Chari-Logone watershed. The processes of chemical alteration on the basin have not changed during this perio

Study of the Hydrochemical Characterization of Drinking Water from the Aquifer of the Town of Guitté at Hadjer Lamis

A hydrogeological study was carried out in and around Guitté. This study aims to contribute to the understanding of the hydrochemical parameters of the waters of the Guitté aquifer. with a view to the sound and sustainable management of available water resources. A field mission to measure and sample borehole water was carried out from March 26 to 28. 2023 in and around Guitté. Physicochemical analyses were carried out on 28 water samples from human-driven pumps). The study showed that the groundwater exploited in this area has an average temperature of 29°C. Overall. the pH values measured in the sampled waters are not very heterogeneous and are around neutral. Electrical conductivity values are highly heterogeneous. Two types of chemical facies have been identified in Guitté groundwater: sodium or potassium bicarbonate facies. and calcium and magnesium bicarbonate facies. The overall quality of the groundwater obtained for consumption is 60.71% rated as good. 39.28% as poor. The origin of the mineralization could be linked. on the one hand. to water/rock contact. due to the dissolution of natron. limestone. gypsum and thenardite and on the other hand. to the phenomenon of intense evaporation

DISTRIBUTION OF MEAN SPATIAL RAINFALL IN THE CHARI-LOGONE BASIN FROM 1960-2015

International audienceThe Chari-Logone basin (BCL), which is the focus of this work, is the hydrologically active part of the Lake Chad Basin. It covers an area of 613 000 km², and its banks are surrounded by a series of mountain ranges: Guera, Ouaddaï, Central African Massif and Adamaoua. The climate of the Chari-Logone basin is contrasted, from the tropical climate to the Sahelian climate. We synthesized, analyzed and exploited all rainfall data available on the Chari-Logone basin during the period 1950-2015 in order to characterize spatial contrasts, temporal variations and sensitivities to climatic variations. These data come from Chad's national services and literature. We used the method of Kriging interpolation with the software R for the spatialization of these data. The interpolation is made on eighty-nine rainfall stations, and the mesh chosen is a regular grid with a pitch of 0.1°. Data analysis showed that the distribution of interannual mean rainfall is mainly along a typical south-north gradient of the Sudano-Sahelian region. The highest values are always recorded at the southwestern extremities corresponding to the direction of entry of the monsoon flow in the basin, with probably a slight orographic effect. Thus, Sudan's wetter regions of the southwest (1600 to 800 mm) are progressively transferred to the Sahelian regions, which are sparsely watered in the northeast (between 800 and 200 mm)

Rainfall-discharge relationship and water balance over the past 60 years within the Chari-Logone sub-basins, Lake Chad basin

International audienceStudy RegionChari-Logone River basin, Lake Chad basin.Study FocusThe objective of this study was to better understand the hydrological functioning of the Chari-Logone basin under the effect of the high variability of rainfall which has affected the Sahel during the last sixty years. The study is based on hydro-climatic data for the period between 1960 and 2015 obtained from the national and international institutions. This work based on the following steps: 1) Average hydrological balances were estimated for different geographical areas, to identify the most productive parts of the basin. 2) The rainfall-runoff relation was compared for the wettest and driest decades of the study period, to identify the areas that contribute most to the amplification of the hydrological response to variations in rainfall. 3) The long-term period stability of the rainfall-runoff relation was evaluated for climatic conditions close to the average situation, in order to detect potential evidence of the anthropogenic impact.New Hydrologic Insights for the RegionDuring the period 1960–2015, the average water flow in the Chari-Logone basin (42 mm/year) represented only 5% of precipitation. Between the two climatically most contrasted wet (1960−1971) and dry (1982−1997) periods, the average flow differed by 75 % against a decrease of 15 % of precipitation. Our hydroclimatic data show no clearly detectable evidence of an anthropogenic impact responsible for a decrease in flows or a modification of the hydrological regime in the Chari-Logone basin

Recarga e fluxo de base restringidos pela química das águas superficiais e subterrâneas: estudo de Caso do Rio Chari, bacia do Chade

International audienceThe Chari-Logone watershed is the only hydrologically active part of the Lake Chad Basin (Central Africa). The Chari-Logone River and Lake Chad exchange water with the surrounding unconfined aquifers of sedimentary and crystalline rock types. In this study, the groundwater contribution to stream flow was quantified by taking advantage of a comprehensive hydrological and chemical database from the 1970s for the upper catchment, which has no equivalent to date. The study area was limited to the larger Chari catchment where the exchanges are mainly oriented from the aquifers to the stream. Upon identification of the mixing poles using end-member mixing analysis (EMMA), and a Monte Carlo inversion of a monthly mixing mass-balance model, an annual averaged base flow of 59 ± 10% of the total stream flow was estimated for 1969–1973. Then, a hydrological model accounting for baseflow and surface flow was calibrated using a 47-year-long monthly discharge time series, and the 4-years’ monthly averaged chemical data available. The simulations yielded an average baseflow of 64% of the annual discharge. The time-constant of the reservoir model (τ = 0.2 years) used to simulate the baseflow was interpreted in terms of hydraulic diffusivity, yielding a value in the order of 101 m2 s−1. This large value corresponds to a highly productive bedrock and thick sedimentary aquifers. The main contribution to the Chari stream flows is restricted to a 140,000-km2 subcatchment located in the southern tropical zone, where the recharge rate reaches 72 ± 6 mm year−1.La cuenca hidrográfica de Chari-Logone es la única parte hidrológicamente activa de la cuenca del lago Chad (África central). El río Chari-Logone y el lago Chad intercambian agua con los acuíferos no confinados circundantes de rocas sedimentarias y cristalinas. En este estudio, la influencia de las aguas subterráneas en el caudal de los cursos de agua se cuantificó aprovechando una extensa base de datos hidrológicos y químicos de la década de 1970 para la cuenca superior, que no tiene equivalente hasta la fecha. El área de estudio se limitó a la cuenca superior del Chari, donde los cambios se orientan principalmente desde los acuíferos hacia el arroyo. Tras la identificación de los puntos de intercambio utilizando el análisis de mezcla de los miembros finales (EMMA), y una inversión de Monte Carlo de un modelo de balance de masa de mezcla mensual, se estimó un flujo base anual promediado de 59 ± 10% del flujo total del arroyo para 1969–1973. Luego, se calibró un modelo hidrológico que tenía en cuenta el flujo base y el flujo de superficial, utilizando una serie temporal de descargas mensuales de 47 años de duración, y los promedios mensuales de los cuatro años de datos químicos disponibles. Las simulaciones arrojaron un flujo de base promedio del 64% de la descarga anual. La constante temporal del modelo del reservorio (τ = 0.2 años) utilizado para simular el flujo base se interpretó en términos de difusividad hidráulica, dando un valor del orden de 101 m2 s−1. Este elevado valor corresponde a un lecho de roca altamente productivo y a acuíferos sedimentarios de alto espesor. La principal contribución a los flujos de la corriente de Chari se restringe a una subcuenca de 140,000-km2 situada en la zona tropical meridional, en la que la tasa de recarga alcanza los 72 ± 6 mm año−1.Le bassin Chari-Logone est la seule partie hydrogéologiquement active du bassin du Lac Tchad (Centrafrique). La Rivière Chari-Logone et le Lac Tchad échangent leur eau avec les aquifères libres environnants de types sédimentaire et cristallin. Dans cette étude, la contribution des eaux souterraines au débit du cours d’eau a été quantifiée en mettant à profit une base de données hydrologiques et chimiques complète des années 1970 concernant le bassin supérieur, qui n’a pas d’équivalent à ce jour. L’aire d’étude a été limitée au bassin versant le plus grand du Chari, dans lequel les échanges sont principalement orientés des aquifères vers le cours d’eau. Lors de l’identification des pôles du mélange à l’aide de l’analyse de ses membres extrêmes et d’une inversion de Monte Carlo du modèle de son bilan massique mensuel, un débit de base annuel moyen de 59 ± 10% de l’écoulement total du cours d’eau a été estimé pour la période 1969–1973. Ensuite, un modèle hydrologique rendant compte du débit de base et de l’écoulement de surface a été calé en recourant à une série temporelle du débit mensuel sur une période de 47 ans et aux données chimiques moyennes mensuelles disponibles sur 4 ans. Les simulations ont donné un débit de base moyen correspondant à 64% du débit annuel. La constante de temps du modèle de réservoir (τ = 0.2 ans) utilisée pour simuler le débit de base a été interprétée en terme de diffusivité hydraulique, donnant une valeur de l’ordre de 101 m2 s−1. Cette valeur importante correspond à un substratum fortement productif et à des aquifères sédimentaires puissants. L’apport principal au débit du Chari est réduit à un sous-bassin versant de 140,000 km2 situé dans la zone tropicale méridionale, où le taux de la recharge atteint 72 ± 6 mm an−1.A bacia hidrográfica Chari-Logone é a única parte hidrologicamente ativa da Bacia do Lago Chade (África Central). O Rio Chari-Logone e o Lago Chade trocam água com os aquíferos não confinados circundantes de tipos de rochas sedimentares e cristalinas. Neste estudo, a contribuição da água subterrânea para o fluxo do rio foi quantificada tirando proveito de um banco de dados hidrológico e químico abrangente da década de 1970 para a bacia hidrográfica superior, que não tem equivalente até o momento. A área de estudo foi limitada à bacia hidrográfica de Chari maior, onde as trocas são principalmente orientadas dos aquíferos para o riacho. Após a identificação dos pólos de mistura usando análise de mistura de membro final (AMMF) e uma inversão de Monte Carlo de um modelo de equilíbrio de massa de mistura mensal, um fluxo de base médio anual de 59 ± 10% do fluxo total da corrente foi estimado para 1969–1973. Em seguida, um modelo hidrológico que considera o fluxo de base e o fluxo de superfície foi calibrado usando uma série de tempo de descarga mensal de 47 anos e os dados químicos médios mensais de quatro anos disponíveis. As simulações produziram um fluxo de base médio de 64% da vazão anual. A constante de tempo do modelo do reservatório (τ = 0.2 anos) usada para simular o escoamento de base foi interpretada em termos de difusividade hidráulica, resultando em um valor da ordem de 101 m2 s−1. Este grande valor corresponde a uma base rochosa altamente produtiva e a aquíferos sedimentares espessos. A principal contribuição para os fluxos do córrego Chari é restrita a uma sub-bacia hidrográfica de 140,000 km2 localizada na zona tropical do sul, onde a taxa de recarga atinge 72 ± 6 mm ano−1