Modeling vadose zone hydrological processes in naturally-occurring piezometric depressions. The Chari-Baguirmi region, southeastern of the Lake Chad Basin, Republic of Chad

The Chari-Baguirmi region, southeastern of the Lake Chad (Africa), has a wide naturally occurring piezometric depression with values deeper than the expected regional groundwater level. To date, the most widely accepted hypotheses to explain its origin and dynamics are based on lack of rainwater infiltration and exfiltration processes. The code HYDRUS-1D is applied to numerically simulate the hydrological flow processes along the unsaturated zone in two soil profiles located in the central part and on the boundary of this piezometric depression under bare and vegetated soil coverage. The simulated time period is 2004–2015 with 715 mm annual rainfall average. The computed recharge with respect to total precipitation accounts for 21% on the boundary and 12% in the central part, which is limited by thick silty low permeability layer on the top surface. Considering modelling uncertainty and limitations under the simulated climatic conditions, the rainfall effect is observed only at upper soil layers, which leads to low aquifer recharge, while the upward water flux causing water table evaporation is very low. Past climate conditions, capable of developing a drying front to reach the water table after thousands of years of drying and geological structural constraints, may explain the current depressed area.Peer ReviewedPostprint (published version

Assessment of groundwater hydrology of the Quaternary aquifer of Lake Chad Basin

(English) This thesis aims to enhance the conceptual model of the Lake Chad Basin's quaternary aquifer by gathering updated hydrological, geological, meteorological, and groundwater data. Objectives include analyzing natural recharge-discharge and surface-groundwater interactions and constructing a hydro-stratigraphic-based conceptual model using a water budget approach. The Chad Aquifer Formation (CAF) is one of the world's largest transboundary aquifers, spanning the entire basin, covering 2,381,000 km² in West-Central Africa. The basin consists of the Quaternary, Lower Pliocene, and Continental Terminal aquifer systems, underlain by Precambrian crystalline basement and formations of Late Cretaceous, Tertiary, and Quaternary sandy or sandstone. The quaternary aquifer, is crucial for the basin's water balance and socio-economic development. The research comprises data gathering, storage of new geo-hydrological information, and analysis of collected data using scientific publications, ground data, remote sensing, and digital files from national and international institutions. Limited data availability in the arid and semi-arid Lake Chad Basin makes this study significant. The methodology involves storing new geo-hydrological information, utilizing 430 lithological well logs to update the basin geometry, and generating a 3D description using the RockWare code. Hydro-stratigraphic units are established based on the collected data at the basin scale. Daily natural groundwater recharge in the Quaternary aquifer for the period of 2005-2014 is quantified using a numerical model (Visual Balan). The study discusses the methods and tools employed for analyzing climatic data, estimating recharge, and modeling the unsaturated zone in the Chari-Baguirmi groundwater depression. Various hydrological and geological data sources, including remote sensing platforms, are utilized. The CAF aquifer in the basin consists of different hydrogeological layers, including the upper phreatic aquifer, Upper Pliocene aquitard, and the deep confined or unconfined aquifer of the Continental Terminal. The hydrogeological system encompasses both deep confined-unconfined aquifers and a Quaternary shallow aquifer with varying hydraulic parameters. Hydraulic connectivity exists between the aquifers in the southern part and with the main rivers. The basin boundaries primarily comprise crystalline rocks. During the 2008-2011 period, the potentiometric surface indicates regional groundwater flow towards the central and northern zones of the basin. Limited data is available for the deep confined aquifer (LPli-CT), restricting the display of groundwater contours in the southern part. The groundwater level generally declines towards Lake Chad and the upper northern basin. Groundwater recharge in the Lake Chad Basin is primarily influenced by precipitation, accounting for 0% to 13% of total precipitation in the southern part and dune systems in the north. Surface water-groundwater interactions occur during flood periods in the Komadougou-Yobe and Chari Logone River systems. Discharge primarily occurs through pumping wells, with the Quaternary aquifer supplying surface water. Lake Chad operates as an in-transit hydrologically open system, removing dissolved salts. However, exchanges between the lake and the Quaternary aquifer are limited due to hydraulic aquifer connection constraints. Unsaturated zone modeling in the Chari Baguirmi depression indicates low aquifer recharge primarily affecting the upper soil layers. The research contributes to an updated conceptual model of the Chad Aquifer Formation, enhancing understanding of hydrogeology in arid and semi-arid regions with limited data. The findings have implications for groundwater management, water balance, and flow systems, benefiting decision-makers, stakeholders, and water resource managers in the Lake Chad Basin to ensure sustainable use and management of groundwater resources.(Español) La Cuenca del Lago Chad, con una superficie de 2,381,000 km² en África occidental central, abarca tres tipos climáticos: desierto cálido, semiárido cálido-húmedo y tropical seco. Esta región árida-semiárida tiene poca disponibilidad de datos hidrológicos y la información existente se limita a estudios realizados entre 2004 y 2008. La Chad Aquifer Formation (CAF), parte del graben chadiano en Chad, es uno de los acuíferos transfronterizos más grandes del mundo, compartido por Chad, Nigeria, Níger, Camerún y República Centroafricana. Compuesta por los sistemas acuíferos Cuaternario, Plioceno Inferior y Continental Terminal, la CAF es de vital importancia para el desarrollo socioeconómico de la región, siendo la mayor fuente de agua dulce en la cuenca. El objetivo de esta tesis es mejorar el modelo conceptual del acuífero Cuaternario de la Cuenca del Lago Chad mediante la incorporación de nueva información hidrogeológica. Se recopiló y evaluó información hidrológica, hidrogeológica, geológica y meteorológica, proveniente de publicaciones científicas, informes y datos locales en formato digital. La CAF está compuesta por diferentes unidades hidroestratigráficas, desde el acuífero freático superior hasta el acuífero profundo del Terminal Continental. Se utilizó información de registros litológicos de pozos para actualizar la geometría tridimensional del modelo conceptual en toda la cuenca. La superficie piezométrica establecida para el periodo 2008-2011 indica un flujo regional hacia la zona central y norte de la cuenca, aunque existe escasa información en esta última área. Los niveles de agua subterránea disminuyen hacia el lago Chad y la parte norte superior de la cuenca. El lago Chad funciona como un sistema hidrológicamente abierto, pero la conexión hidráulica con el acuífero Cuaternario es limitada. La recarga natural del acuífero Cuaternario es controlada principalmente por la precipitación en el sur de la cuenca y en los sistemas de dunas del norte. Durante los periodos de inundación, hay interacción entre el agua superficial y el agua subterránea en los sistemas fluviales. La descarga del acuífero ocurre a través de pozos de bombeo, principalmente en la cuenca fluvial y en la parte norte de la cuenca. La investigación también cuantifica la tasa de recarga natural diaria del acuífero Cuaternario utilizando un modelo numérico distribuido para el período 2005-2014. Se encontró que el flujo ascendente desde el nivel piezométrico del acuífero es limitado bajo las condiciones climáticas actuales. En resumen, esta tesis busca mejorar el modelo conceptual del acuífero Cuaternario de la Cuenca del Lago Chad mediante la incorporación de nueva información hidrogeológica. Los resultados obtenidos son de gran importancia para la gestión y uso sostenible de los recursos hídricos subterráneos en la región.DOCTORAT EN ENGINYERIA CIVIL (Pla 2012

A groundwater model for the Lake Chad basin: integrating data and understanding of water resources at the basin scale

This report is part of the Water Global Practice efforts to support the Lake Chad Basin Commission (LCBC) and its Member States (Cameroon, the Central African Republic, Chad, Niger and Nigeria) fulfill their mandate to better monitor and understand water resources dynamics in the Lake Chad Basin.The initial objectives of the efforts portrayed in this report were to (a) integrate all available groundwater data available in the Lake Chad Basin in an updated database; (b) develop an updated and integrative conceptual groundwater model of the Lake Chad Basin, including all old and new information and reflecting the best current understanding, and (c) develop the equivalent numerical groundwater model of the Lake Chad Basin, with emphasis on two focus areas: the Komadougu-Yobe and Chari-Logone river basins, given their relevance for groundwater recharge and use. Even though the overall conceptual model and the numerical model need to be further improved, the basin-wide perspective presented in this work, integrating multiple sources of available data, provides a foundation to better understand and quantify basin-wide hydrogeological dynamics. This enables future efforts to assess potential impacts of future investments and climate futures.The activity leading to this report was coordinated on the World Bank side by Marie-Laure Lajaunie (Lead Water Resources Management Specialist, SAFW1) and Aleix Serrat Capdevila (Senior Water Resources Management Specialist, SAFW2) under the support and guidance of Yogita Mumssen (Practice Manager, SAFW1). The data gathering and analysis, modeling work and the core of the technical writing of this report was done by L. Candela (IMDEA-UPC), F.J. Elorza (IMDEA-UPM), N. Salehi (IMDEA-UPC), G. Vaquero (IMDEA-UPM), D. García-Martínez (IMDEAUPC). Valuable inputs and reviews during the elaboration of the report were provided by Hubert Machard de Gramont, Yves Barthelemy, Marcus Wijnen and Francois Bertone. We also acknowledge the contributions of internal peer reviewers Nagarajao Harshadep, Lucy Lutton, and Francois Bertone. During the data collection and modeling process, we are grateful for the collaboration of the Lake Chad Basin Commission staff Mohammed Bila, Seidu Abderrahmane, Mana Boubakari, Daïra Djoret and Abderamane Hamit Issa, Sanusi Imran Abdullahi, Rohallati Ndara, as well as of Guinbe Amngar (CDIG) and Mahamat Alifa Moussa (Ministère de l'Elevage et de l'Hydraulique, Chad). Also, this work would not have been possible without the invaluable support from Andreas Haveman and Julian Harbrech (BGR); Alexis Gutierrez (BRGM); Christian Leduc (IRD); Gianluca Guidotti and Ismail Musa Mohamed (Project ResEau); Lucas Cornet, Ivann Milenkovic, Calvin Ndjoh-Messina and Olivier Senegas (UNITARUNOSAT); Moussa Isseini (University of N’Djamena, Chad); Ibrahim Goni (University of Maiduguri, Nigeria), Lee Jejung (University of Missouri-Kansas City, USA); Guillaume Favreau (University of Montpellier, France); Justin Sheffield (University of Southampton, UK) and Hylke Beck, Erik Wood and Colby Fisher (Princeton University, USA), and last but not least, the institutional support of UNESCO’s International Hydrologic Programme Groundwater Section, through Alice Aureli and her team. All contributions are gratefully acknowledged.Preprin

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

Modeling vadose zone hydrological processes in naturally occurring piezometric depressions: the Chari-Baguirmi region, southeastern of the Lake Chad Basin, Republic of Chad

The Chari-Baguirmi region, southeastern of the Lake Chad (Africa), has a wide naturally occurring piezometric depression with values deeper than the expected regional groundwater level. To date, the most widely accepted hypotheses to explain its origin and dynamics are based on lack of rainwater infiltration and exfiltration processes. The code HYDRUS-1D is applied to numerically simulate the hydrological flow processes along the unsaturated zone in two soil profiles located in the central part and on the boundary of this piezometric depression under bare and vegetated soil coverage. The simulated time period is 2004–2015 with 715 mm annual rainfall average. The computed recharge with respect to total precipitation accounts for 21% on the boundary and 12% in the central part, which is limited by thick silty low permeability layer on the top surface. Considering modelling uncertainty and limitations under the simulated climatic conditions, the rainfall effect is observed only at upper soil layers, which leads to low aquifer recharge, while the upward water flux causing water table evaporation is very low. Past climate conditions, capable of developing a drying front to reach the water table after thousands of years of drying and geological structural constraints, may explain the current depressed area.Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. This study was partially funded by the Cooperation in International Waters in Africa (CIWA) Program of the World Bank as part of a broader effort on groundwater resources in the Lake Chad Basin