Historical climate impact attribution of changes in river flow and sediment loads at selected gauging stations in the Nile basin

The Nile basin is the second largest basin in Africa and one of the regions experiencing high climatic diversity with variability of precipitation and deteriorating water resources. As climate change is affecting most of the hydroclimatic variables across the world, this study assesses whether historical changes in river flow and sediment loads at selected gauges in the Nile basin can be attributed to climate change. An impact attribution approach is employed by constraining a process-based model with a set of factual and counterfactual climate forcing data for 69 years (1951–2019), from the impact attribution setup of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP3a). To quantify the role of climate change, we use the non-parametric Mann-Kendall test to identify trends and calculate the differences in long-term mean annual river flow and sediment load simulations between a model setup using factual and counterfactual climate forcing data. Results for selected river stations in the Lake Victoria basin show reasonable evidence of a long-term historical increase in river flows (two stations) and sediment load (one station), largely attributed to changes in climate. In contrast, within the Blue Nile and Main Nile basins, there is a slight decrease of river flows at four selected stations under factual climate, which can be attributed to climate change, but no significant changes in sediment load (one station). These findings show spatial differences in the impacts of climate change on river flows and sediment load in the study area for the historical period

Flood frequency analysis in West Africa

Devastating flood events attributed to climate change have recently hit many West African countries. To mitigate the impact of such floods and reduce the vulnerability of populations, comprehensive insights into the frequency of these extreme events are crucial. The lack of reliable large-scale hydrometric datasets has hitherto been a major limitation in flood frequency analysis at the scale of West Africa. Utilizing a recently developed African database, we perform a flood frequency analysis on the annual maximum flow (AMF) time series, covering 246 river basins in West Africa, between 1975 and 2018. The trend analysis, performed with the non-parametric Mann-Kendall test, revealed some significant trends, with no discernible spatial pattern. Generalized extreme value (GEV) and Gumbel probability distributions were fitted to AMF time series using three parameter estimation methods: L-moments, Maximum Likelihood (MLE), and Generalized MLE (GMLE). The Akaike and Bayesian information criteria were used to compare the goodness-of-fit of probability distributions. Based on these evaluation criteria, the GEV distribution fitted with the GMLE method was selected as the best distribution in most stations. Furthermore, there was no significant difference between stationary and non-stationary quantiles of AMF series that exhibited a significant trend. Regional envelope curves, covering the entire West African region with unprecedented data coverage, have been generated for the first time. The correlation analysis between flood quantiles and watershed properties suggested stronger influences of physiographic and geological watershed properties than climate on regional variations in flood quantities. The findings from this study hold potential utility for flood risk assessment, design of hydraulic infrastructures, and water resources management. In addition, the probability distribution identified as the most suitable in this research might be favoured for regional flood frequency analysis in West Africa

Characteristics and trends of hydrological droughts in Sub-Saharan Africa

This study on hydrological droughts in Sub-Saharan Africa offers a comprehensive narrative of their evolution, characteristics, and interconnections with meteorological droughts, crucial for understanding their complex dynamics and implications for water resource management and climate adaptation strategies. Hydrological droughts, characterized by prolonged periods of below-normal water availability, present significant challenges across Africa due to their extensive spatial and temporal impacts. The analysis relies on the African Database of Hydrometric Indices (ADHI), integrating streamflow data from 1466 gauging stations spanning 1951 to 2018. This dataset enables a nuanced exploration of hydrological drought trends, revealing distinct temporal patterns. A notable shift in drought characteristics around the 1980s is identified in the major part of the territory, with varying trends in drought duration and severity between the 1951-1980 and 1981-2014 periods. The findings suggest an increase in drought duration and severity in the earlier period followed by a decrease in the latter, marking a significant temporal transition. Spatially, the study identifies diverse patterns in drought occurrence and intensity across sub-Saharan Africa's hydrographic basins. Southern Africa exhibits higher frequency but shorter and less severe droughts compared to central and eastern regions, where droughts are less frequent but more prolonged and intense. These spatial variations reflect regional disparities in climatic conditions and basin characteristics, influencing how hydrological systems respond to rainfall variability. Furthermore, the study explores the complex relationship between meteorological and hydrological droughts. While meteorological droughts (SPI and SPEI) show varied trends, hydrological droughts generally mirror precipitation patterns, despite local variations due to local factors

Recent trends in vegetation cover and phenology in the southern Mediterranean region: Potential impacts on crop production and its link to changing climate extremes.

The southern Mediterranean region stands as one of the most vulnerable areas to climate change and variability in the world. There is a need for further understanding of the complex interactions between climate, vegetation, and crops to fully understand the combined impacts of extreme climate events on the agriculture sector. Using daily Normalized Difference Vegetation Index (NDVI) data, we evaluate recent trends across 15 vegetation phenology indicators between 1982 and 2019. Subsequently, we analyse potential links between recent trends in vegetation phenology and land-use land-cover. We found significant increasing trends in Maximum Value of NDVI (MaxV), length of growing season (LengthGS), and the time between the occurrence of the onset and maximum value (AreaB), especially within croplands. We also noted these vegetation phenological indicators, and their associated changes in the last 38 years, are significantly correlated with an increase in regional crop production. Then, we conducted a comprehensive seasonal trend analysis of extreme (potentially compound) climatic stresses and discussed how it aligns with recent trends in MaxV and LengthGS. We found that extended LengthGS and increased MaxV are consistent with increased precipitation and cooler temperatures during spring and summer, coupled with warmer autumn and winter trends. These trends collectively mitigate extreme heat, water, and compound heat-water stresses, contributing to improved crop yields over the region. By improving our understanding of the potential impacts of recent climate change on vegetation phenology, crop production and land use, our findings can help providing guidance for more informed decision-making and adaptation strategies in the southern Mediterranean region

Long-term variability in hydrological droughts and floods in Sub-Saharan Africa: New Perspectives from a 65-year daily streamflow dataset

International audienceUnderstanding hydrological variability is of crucial importance for water resource management in sub-Saharan Africa (SSA). While existing studies typically focus on individual river basins, and suffer from incomplete records, this study provides a new perspective of trends and variability in hydrological flood and drought characteristics (frequency, duration, and intensity) across the entire SSA. This is achieved by: i) creating a 65-year long, complete daily streamflow dataset consisting of over 600 gauging stations; ii) quantifying changes in flood and drought characteristics between 1950 and 2014; iii) evaluating how decadal variability influences historical trends. Results of daily streamflow reconstructions using random forests provide satisfactory performance over most of SSA, except for parts of southern Africa. Using change-point and trend analyses, we identify-three periods that characterise historical variations affecting hydrological extremes in western and central Africa, and some parts of southern Africa: i) the 1950s-60s and after the 1980s-90s, when floods (droughts) tend to be more (less) intense, more (less) frequent and more (less) persistent; and ii) the 1970s-80s, when floods (droughts) are less (more) intense, less (more) frequent and less (more) persistent. Finally, we reveal significant decadal variations in all flood and drought characteristics, which explain aperiodic increasing and decreasing trends. This stresses the importance of considering multiple time-periods when analysing recent trends, as previous assessments may have been unrepresentative of long-term changes

Long-term variability in hydrological droughts and floods in sub-Saharan Africa: New perspectives from a 65-year daily streamflow dataset

International audienceUnderstanding hydrological variability is of crucial importance for water resource management in sub-Saharan Africa (SSA). While existing studies typically focus on individual river basins, and suffer from incomplete records, this study provides a new perspective of trends and variability in hydrological flood and drought characteristics (frequency, duration, and intensity) across the entire SSA. This is achieved by: i) creating a 65-year long, complete daily streamflow dataset consisting of over 600 gauging stations; ii) quantifying changes in flood and drought characteristics between 1950 and 2014; iii) evaluating how decadal variability influences historical trends. Results of daily streamflow reconstructions using random forests provide satisfactory performance over most of SSA, except for parts of southern Africa. Using change-point and trend analyses, we identify-three periods that characterise historical variations affecting hydrological extremes in western and central Africa, and some parts of southern Africa: i) the 1950s-60s and after the 1980s-90s, when floods (droughts) tend to be more (less) intense, more (less) frequent and more (less) persistent; and ii) the 1970s-80s, when floods (droughts) are less (more) intense, less (more) frequent and less (more) persistent. Finally, we reveal significant decadal variations in all flood and drought characteristics, which explain aperiodic increasing and decreasing trends. This stresses the importance of considering multiple time-periods when analysing recent trends, as previous assessments may have been unrepresentative of long-term changes

Flood hazard variability in Sub-Saharan Africa: The role of large-scale climate modes of variability and their future impacts

Sub-Saharan Africa (SSA) is strongly affected by flood hazards, endangering humanlives and economic stability. However, the role of internal climate modes of variability in driving fluctuations in SSA flood occurrence remains poorly documented andunderstood. To address this research gap, we quantify the relative and combinedcontribution of large-scale climate drivers to seasonal and regional flood occurrenceusing a new 65-year daily streamflow dataset, sea-surface temperatures derived fromobservations, and 12 Single Model Initial-condition Large Ensembles (SMILEs) fromthe Coupled Model Intercomparison Project Phases 5 and 6. We find significantrelationships between floods and large-scale climate variability across SSA, withclimatic drivers accounting for 30-90% of the variability in floods. Notably, western,central, and southern Africa display stronger teleconnections to large-scale climatevariability in comparison to East Africa and the winter-rain region of South Africa,where regional circulation patterns and human activities may play a more importantrole. In southern and eastern Africa, floods are mainly influenced by teleconnectionswith the Pacific and Indian Oceans, while in western and central Africa, teleconnectionswith the Atlantic Ocean and Mediterranean Sea play a larger role. In the futurescenarios, we find that the number of floods is projected to fluctuate by ±10-50%during the 21st century in response to different sequences of key modes of climatevariability. We also note that the relative contributions of large-scale climate variabilityto future flood risks is generally consistent across all SMILEs. Our findings thus provide valuable information for long-term disaster risk reduction and management