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

Hydrogeochemical study of Chari Baguirmi aquifer system (Republic of Chad)

L'étude du système aquifère du Chari Baguirmi vise à améliorer sa connaissance en vue d'une gestion durable etprudente de la ressource en eaux souterraines. Dans la zone d'étude qui couvre 70000 Km2, des campagnes demesures et d'échantillonnages des eaux et de sédiments ont été réalisés. Les eaux échantillonnées ont fait l'objetd'analyses chimiques et isotopiques (O18 et 2H) afin de comprendre le comportement hydrogéochimique dusystème aquifère. La synthèse des données géologiques existantes et la mise en évidence de l'hétérogénéité litho-stratigraphiquepar l'étude granulométrique ont confirmé la complexité hydrogéologique de la zone d'étude. La minéralogie desargiles a montré que le dépôt des différents sédiments se fait de la périphérie vers le centre de la dépression. Cemode de sédimentation permet d'avancer l'hypothèse de l'existence d'une dépression vers laquelle les sédimentsseraient entrainés par l'agent de transport. Cette hypothèse de “ dépression structurale ” réfute l'hypothèseadmise jusqu'à présent d'une origine hydrogéologique (forte évaporation) de cette dépression piézométrique.L'étude basée sur la piézométrie et les données chimiques et isotopiques (O18 et 2H) des eaux a permis decomprendre les processus hydrogéochimiques qui gouvernent la minéralisation des eaux. En plus des isotopes(O18 et 2H), l'utilisation des ratios Br/Cl, Sr/Ca et l'indice d'échanges de bases a permis de mettre en évidenced'une part, l'origine de la salinité des eaux de la dépression piézométrique et d'autre part, les phénomènesd'échanges de bases liés au long temps de séjour des eaux au contact des roches encaissantes et les zones derecharge potentielles de la nappe.The study of the Chari Baguirmi aquifer system aims to improve the knowledge about this system forsustainable groundwater resource management. In the study area, which covers 70,000 km2, measurementcampaigns and sampling of water and sediments were undertaken. The waters have been sampled for chemicaland isotopic analyzes (18O and 2H) to understand the hydrogeochemical behavior of the aquifer system.The synthesis of existing geological data and highlighting the litho-stratigraphic heterogeneity through thegranulometric study confirmed the hydrogeological complexity of the study area. Clay mineralogy showed thatthe deposition of different sediments occurred from the periphery to the center of the depression. This mode ofsedimentation can hypothesize the existence of a depression into which sediments are trained by the transportagent. This assumption of "structural depression" refutes the hypothesis of a hitherto accepted hydrogeologicalorigin (high evaporation) of the piezometric depression. The study based on the piezometry and chemical and isotopic data (18O and 2H) of water enabled to understand the hydrogeochemical processes that govern the mineralization of the water. In addition to isotopes (2H and 18O), the use of ratios Br/Cl, Sr/Ca and base exchanges index has highlighted on the one hand, the origin of the salinity of waters in the piezometric depression and secondly, the phenomena of base exchanges related to longresidence time of water in contact with rocks and areas of potential groundwater recharge. Numerical modeling of the aquifer was performed in steady state and the results confirm the prevalence of the phenomenon of evaporation in the western

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

Groundwater Dynamics in African Endorheic Basins in Arid to Semi-Arid Transition Zones: The Batha Aquifer System, NE Chad

This study investigates the Batha endorheic basin in Chad, situated east of the Lake Chad basin in the arid to semi-arid Sahelian zone. This region has not yet undergone comprehensive geological and hydrogeological studies. More broadly, the transition zone between semi-arid and arid climates has been minimally explored. This research aims to evaluate the resources and dynamics of this multi-layered system using a combined geology-hydrogeology-hydrochemistry-isotopes approach. The multilayer system includes sedimentary layers (Quaternary, Pliocene, and Eocene) over a crystalline basement. A piezometric investigation of the system shows a general SE–NW groundwater, indicating an interconnection between all layers. Hydrochemical analyses identifies four main facies (calcium-bicarbonate, sodium-bicarbonate, sulphate-sodium, and mixed), primarily controlled by water–rock interaction with secondary influences from base-exchange and evaporation. Saturation indices indicate that these waters are close to equilibrium with the calcite-Mg phases, gaylussite and gypsum. Stable isotopes (oxygen-18 and deuterium) categorize groundwater into three groups: ancient water, recent and older meteoric water mixtures affected by evaporation, and mixtures more heavily impacted by evaporation. Tritium contents reveal three groups: current rainwater, modern water, and sub-modern water. These results indicate that ionic and isotopic differentiations cannot be strictly linked to specific layers, confirming the interconnected nature of the Batha system. The observed heterogeneity is mainly influenced by lithological and climatic variations. This study, though still limited, enhances significantly the understanding of the basin’s functioning and supports the rational exploitation of its vital resources for the Batha area’s development. Future investigations to complete the present study are highlighted

Hydrochemical and Isotopic Characteristics of the Basement Aquifer in the Wadi Fira Area, Eastern Chad

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Study of the Effect of Biochar Incubation on Some Physico-Chemical Properties of a Tropical Leached Soil from South-West Chad

Soils in the Sudanian zone of Chad, and more specifically leached tropical soils, were subject to physical and chemical degradation phenomena that hamper crop production. Solving this problem requires solutions for the rational improvement of fertility. The aim of this study, conducted in a controlled environment, was to assess the fertilising potential of biochar on impoverished soil by monitoring the kinetics of the evolution and release of chemical elements that improve the physico-chemical quality of the soil. Four treatments were carried out: control soil (T0), treatment 1B: 100g soil + 0.225g biochar (i.e. 5 t/ha), treatment 2B: 100g soil + 0.3375g biochar (i.e. 7.5 t/ha) and treatment 3B: 100g soil + 0.450g biochar (i.e. 10 t/ha). Each treatment was repeated four times. Four main parameters were measured during the trial period: pH, assimilable phosphorus, exchangeable bases and their sums (Ca, Mg, K, Na and SBE) and cation exchange capacity. These different parameters showed that the biochar treatments significantly improved soil quality, whatever the dose and incubation time, compared with the T0 control. Treatment 1B (5t/ha biochar application) increased pH by an average of 6.5%, phosphorus by 132.5%, SBE by 406.1% and CEC by 43.4%. Treatment 2B (application of 7.5 t/ha of biochar) improved pH by an average of 8.5%, phosphorus by 155.1%, SBE by 207.6% and CEC by 46.2%. Treatment 3B (application of 10t/ha of biochar) produced a marked improvement, increasing pH by 11.2%, phosphorus by 198.7%, SBE by 457.6% and CEC by 29.9%. These results further confirm the vital importance of biochar in fertilising impoverished tropical soils. Based on pH, sum of bases and phosphorus, treatment 3B gave better results by considerably increasing the initial chemical element content of the control soil than the other biochar treatments