FINAL REPORT CLIMATE VULNERABILITY AND WATER RESOURCES VARIABILITY IN WEST AFRICA SENEGAL AND GAMBIA RIVER BASIN CASES STUDIES March 2019 TABLE OF CONTENTS TABLE OF CONTENTS ------------------------------------------------------------- 2 TABLE OF ILLUSTRATIONS ------------------------------------------------------- 5 List of figures -------------------------------------------------------------------------------------- 5 List of tables --------------------------------------------------------------------------------------- 6 ACKNOWLEDGMENTS ------------------------------------------------------------- 8 LIST OF ACRONYMS AND ABBREVIATIONS ------------------------------------ 9 ANALYTICAL ABSTRACT -------------------------------------------------------- 10 INTRODUCTION ----------------------------------------------------------------- 13 PRESENTATION OF THE STUDY AREA ----------------------------------------- 15 1. Physical determinants of the basin ------------------------------------------------------- 15 2. Climate, hydrogeology and hydrology ---------------------------------------------------- 19 3. Socio-economic and cultural activities --------------------------------------------------- 21 4. Management and planning (OMVS and OMVG) ------------------------------------------ 23 5. IWRM recent perspectives ----------------------------------------------------------------- 26 DESCRIPTION OF THE METHODOLOGY --------------------------------------- 28 1. Analyse of recent bibliography and state of the art on climate variability, climate change and risks, extreme events and climate modeling in West Africa ------------------- 29 2. Data and tools ------------------------------------------------------------------------------- 32 2.1. Identification of data needs --------------------------------------------------------------------------- 32 2.2. Data acquisition ----------------------------------------------------------------------------------------- 33 2.3. Tools ------------------------------------------------------------------------------------------------------ 34 3. Detailled methodology for climate variability assessment ----------------------------- 36 3.1. Descriptives analyses----------------------------------------------------------------------------------- 36 3.2. Segmentation and homogenization procedures --------------------------------------------------- 36 4. Methodology for change and trends detection ------------------------------------------- 37 CLIMATE VARIABILITY, CLIMATE CHANGE AND TRENDS ANALYSIS AND MODELLING ON SENEGAL AND GAMBIA RIVER BASINS -------------------- 40 1. Recent climate variability, climate change and trends ---------------------------------- 40 2 2. Climate change, extremes hydrologic events and consequences on hydrologic regimes -------------------------------------------------------------------------------------------- 43 2.1. Variations in rainfall ------------------------------------------------------------------------------------ 44 2.1.1. Rainfall distribution ---------------------------------------------------------------------------------- 47 2.1.2. Interannual rainfall ---------------------------------------------------------------------------------- 47 2.1.3. Statistical distribution of annual rainfall --------------------------------------------------------- 50 2.1.4. Decennal rainfall ------------------------------------------------------------------------------------- 52 2.1.5. Monthly rainfall -------------------------------------------------------------------------------------- 53 2.1.6. Daily rainfall ------------------------------------------------------------------------------------------ 56 2.2. Surface runoffs ------------------------------------------------------------------------------------------ 60 2.2.1. The available data and their qualities ------------------------------------------------------------ 60 2.2.2. Flow statistics ---------------------------------------------------------------------------------------- 62 2.2.3 Analysis of annual and monthly modules -------------------------------------------------------- 63 3. Characterization of hydrological drought in the Senegal and Gambia catchment area 64 3.1. Methodological approach ------------------------------------------------------------------------------ 64 3.1.1. Methods for analyzing the effects of drought on surface water ----------------------------- 64 3.1.2. Calculation of the drying coefficient -------------------------------------------------------------- 65 3.1.3. Calculation of the volume of water mobilized by aquifers ------------------------------------ 66 3.1.4. Hydrological drought index method -------------------------------------------------------------- 66 3.1.4.1. Normalized Hydrologic Index (HLI) -------------------------------------------------------- 66 3.1.4.2. Flow Drought Index (DFI) ------------------------------------------------------------------- 67 3.1.4.3. Logarithmic Decimal Index of Hydrological Deviations (LDIHD) ---------------------- 67 3.2. Analysis of the watercourse regime ----------------------------------------------------------------- 68 3.3. Analysis of the rain-flow relationship --------------------------------------------------------------- 69 3.4. Analysis of the impact of drought through short term hydrological drought indices -------- 70 3.5. REFRAN-CV analysis over Senegal ------------------------------------------------------------------- 71 3.5.1. Reminders on the physical context of Senegal ------------------------------------------------- 71 3.5.2. Data and tools of the study ------------------------------------------------------------------------ 73 3.5.3. Recent evolution of climatic parameters in Senegal ------------------------------------------- 74 3.5.4. Results of REFRAN-CV analysis ------------------------------------------------------------------- 76 HIGHLINGTING EVIRONMENTAL VULNERABILITY IN THE SENEGAL AND GAMBIA RIVER BASINS THROUGH HOTSPOTS ------------------------------ 79 1. Fouta Djalon Mountain --------------------------------------------------------------------- 80 1.1. Presentation of The study area ----------------------------------------------------------------------- 80 1.2. Methodology for land use changes analysis -------------------------------------------------------- 81 1.3. The Reforestation of Fouta Djalon Mountain, Myth or reality? ---------------------------------- 82 2. The Senegal River Estuary: breaching of the ‘‘Langue de Barbarie’’ sandy spit, socio- environmental changes and vulnerability of coastal communities in the lower estuary -- 84 2.1. The estuary and the new mouth of Senegal River ------------------------------------------------ 85 2.2. Context of the opening of the breach --------------------------------------------------------------- 86 2.3. Evolution of the breach and morphological changes in th lower estuary from 2003 to 2018 89 1.3.1. Methodological approach for Determining Recent Evolution of The lower estuary of Senegal River -------------------------------------------------------------------------------------------------- 89 5.1.1. méthodology choice and validation ----------------------------------------------------------- 90 1.1.2. Methods and tools ----------------------------------------------------------------------------------- 90 1.3.1. Dynamics of the breach --------------------------------------------------------------------------- 91 1.3.2. Morphological changes in the lower estuary ---------------------------------------------------- 93 1.4. Social, economical and environmental consequences of the breach and adaptation attempts 96 3. The Gambia River Estuary ----------------------------------------------------------------- 99 3 CONCLUSION AND FUTURE PROSPECTS ------------------------------------ 101 REFERENCES ------------------------------------------------------------------- 102 ANNEXES ------------------------------------------------------------------------ 115 Climatic database of Senegal (metadata) ---------------------------------------------------- 116 Rainfall database of Senegal River Basin (Metadata) -------------------------------------- 117 Rainfall database of GAMBIA River Basin (Metadata) -------------------------------------- 122 4 TABLE OF ILLUSTRATIONS LIST OF FIGURES Figure 1: Presentation of studied area: Senegal River Basin and Gambia River Basin (Thiam, 2018) --------------------------------------------------------------------------------------------- 13 Figure 2: Geological Map of Senegal River Basin -------------------------------------------------- 17 Figure 3: Dams on Senegal River Basin (© OMVS) ------------------------------------------------ 27 Figure 4 : Climate domains of the Senegal River Basin (modified by SALL (2006) from SOW (1984))------------------------------------------------------------------------------------------------------ 32 Figure 5: Maps of the rainfall network of the Senegal and Gambia Rivers basins ------------- 34 Figure 6: Map of the subset of the SRB and the variation of the isohyets ------------------------ 38 Figure 7: Average monthly rainfall by climatic zone ----------------------------------------------- 39 Figure 8: Monthly variation of climatic parameters ------------------------------------------------ 42 Figure 9:Wind rose at Labe Station ------------------------------------------------------------------- 43 Figure 10: Percentage of gaps in extreme events ---------------------------------------------------- 44 Figure 11: Changes in