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Map of the Niger basin (top left) and the zoomed in image of the Ansongo–Niamey reach basin (bottom right). The red contours on the bottom right plots delineate the three main tributaries contributing to the red flood (C. Casse and L. Gal, based on SIEREM and NOAA database).
Source publication
Since 1950, the Niger River
basin has gone through three main climatic periods: a wet period
(1950–1960), an extended drought (1970–1980) and since 1990
a recent partial recovery of
annual rainfall. Hydrological changes co-occur with these rainfall
fluctuations. In most of the basin, the rainfall deficit caused an enhanced
discharge deficit, but in...
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Citations
... In September, the highest level of the Niger river since the beginning of observations was recorded in Niamey. What happened in 2020 in Niamey is the ultimate example of a phenomena which has been growing in the last two decades in the middle Niger basin (Massazza et al. 2021;Casse et al. 2016). The Niger river at its middle basin experience two annual peaks: (i) The Sahelian flood-also called red flood-occurs during the rainy season, between August and September mainly from the inflows of the right bank tributaries upstream of Niamey (Gorouol, Dargol and Sirba). ...
... Historically the "black" flood had the highest peak, but progressively over the last 3 decades, the "red" peak began to surpass the black one and has been the cause of a succession (Casse et al. 2016). This evolution in the hydrological behavior of the middle Niger basin is the consequence of long terms changes in the land surface superimposed with the occurrence of intense precipitation over the three Sahelian tributaries basins. ...
This article reviews the state of the art in the use of space-borne observations for analyzing extreme rainfall and flood events in Africa. Floods occur across many space and timescales, from very localized flash flood events to slow propagation of discharge peaks in large rivers. We discuss here how satellite data can help us understand the genesis and impacts of these flood events, monitor their evolution, and better constrain prediction models, thereby improving early warning and population protection. To illustrate these topics, we reanalyze major flood events that occurred in Niger, Mozambique, Central African Republic and Ivory Coast, using satellite information.
... may lead to increased floods, as already observed in the Sahelian part of the Niger basin(Casse et al., 2016). ...
Les cours d’eau intermittents sont des cours d’eau qui cessent de couler à un moment donné dans l’espace et le temps. Cependant, la compréhension de la variabilité spatio-temporelle des cours d’eau intermittents ainsi que leur localisation précise dans le réseau hydrographique reste limitée en raison d’un manque de données (hydrométriques, hydrographiques, etc.…). Ces difficultés sont accentuées dans les régions à données limitées comme l’Afrique et cette thèse vise principalement à développer des approches méthodologiques qui s'appuient sur des modèles de forêts aléatoires (Random Forest) ainsi que des analyses statistiques pour caractériser la distribution spatiale des cours d’eau intermittents et mieux appréhender les mécanismes de contrôle de l’intermittence en Afrique afin de pallier les lacunes d’observations. D’abord, cette thèse s’est focalisée à l’échelle régionale au Burkina Faso où 49 stations de jaugeage des cours d’eau avec au minimum quatre ans de données sur la période 1955-1985 ont été examinées. Le nombre moyen de mois à débit nul par an ((Ndry) ̅) a été utilisé pour définir quatre classes croissantes d'intermittence du débit à savoir : permanent (0-1 mois à débit nul), faiblement intermittent (2-4), fortement intermittent (5-7), éphémère (8-12). Une analyse en composante principale (ACP) réalisée sur les 49 stations de jaugeages a montré que, bien que l’ordre de Strahler et la précipitation moyenne annuelle influencent la répartition géographique des différentes classes d’intermittence au Burkina Faso, la perméabilité moyenne et la surface amont des bassins versants expliquent principalement cette répartition. Cette étude suggère que la saisonnalité de la précipitation au Burkina Faso rend également saisonnier l’écoulement des cours d’eau à moins que les processus hydrogéologiques ne soient impliqués, notamment à travers la contribution des aquifères au débit de base pendant les périodes d’étiage. L’approche de prédiction par Random Forest a permis d’estimer sur le réseau hydrographique LCS que 83% de la longueur totale des cours d’eau au Burkina Faso sont intermittents contre 98% dans le réseau hydrographique nationale de référence (IGB-BNDT). Ensuite, à l’échelle de l’Afrique, 1125 stations de jaugeage avec au moins 4 ans de données sont examinées sur la période 1958-1991. Plusieurs modèles Random Forest ont été entrainés à relier les classes d’intermittence observée au droit des stations de jaugeage aux valeurs caractéristiques de 15 variables environnementales clés identifiées. L’indice d’aridité (P/ETP), la surface amont des bassins versants et l’évapotranspiration potentielle moyenne annuelle ont été identifiés comme les facteurs de contrôle les plus importants de l’intermittence à l’échelle continentale de l’Afrique. Cela se confirme d’autant plus que la majorité des stations classées intermittentes dans l’échantillon analysé ont des valeurs d’évapotranspiration potentielle moyenne annuelle qui sont supérieures à la précipitation moyenne annuelle (c.-à-d. indice d’aridité < 1). Cette étude prédit sur le réseau hydrographique LCS qu’en Afrique 44 % en longueur des cours d’eau sont permanents tandis que 56 % sont intermittents (9 % faiblement intermittents, 31 % fortement intermittents, 16 % éphémères). Les prédictions du modèle capturent en général la distribution spatiale de l’intermittence dans les réseaux hydrographiques nationaux de référence de l’Afrique du Sud, du Bénin, de Madagascar, du Mali et un peu moins au Burkina Faso. Enfin, cette thèse s’est appesantie sur les difficultés des réseaux hydrographiques globaux et continentaux à reproduire la variabilité spatiale de la densité de drainage observée sur les réseaux hydrographiques nationaux de référence des différents pays en Afrique. Une méthode simple pour extraire les cours d’eau des MNT (Modèle Numérique de Terrain) à partir de surface contributive (Amin) variable spatialement a été développée.
... These approaches vary from parameter estimation (Nyabeze 2005;Kapangaziwiri and Hughes 2008), regionalization (e.g., Love et al. 2011), assimilation of new data types into hydrological models (Mekonnen et al. 2009;Milzow et al. 2011) and model uncertainty prediction (e.g., Katambara and Ndiritu 2009;Kapangaziwiri et al. 2012). Models have also been applied at different spatial scales from the relatively small to medium catchments (Hamlat et al. 2013;Gal et al. 2017;Nonki et al. 2021) to large rivers (Tshimanga and Hughes 2014;Casse et al. 2016;Munzimi et al. 2019) and even continental scales (Alemaw and Chaoka 2003;Trambauer et al. 2013). Apart from the general catchment water balance modeling, progress has also been made in the use of models for more specific processes such as flooding (Yawson et al. 2005;Ngongondo et al. 2013;Smithers et al. 2013) and drought assessments (Nyabeze 2004), modeling of floodplains (Birkhead et al. 2007;Unami et al. 2009), sediment yields (Ndomba et al. 2008) groundwater surface water interactions (Ayenew et al. 2008;Tanner and Hughes 2015). ...
... These models have used Earth Observation in the Niger Basin to explore a range of issues, such as the use of MODIS and Landsat for vegetation and surface water in groundwater recharge processes (Leblanc et al. 2007), and satellite rainfall products such as ECMWF Re-Analysis (ERA)-Interim for runoff estimation (Oyerinde et al. 2017) and the Climate Prediction Center morphing method (CMORPH), the near-real-time legacy product of Tropical Rainfall Measuring Mission Multi-satellite Precipitation Analysis (TMPA 3B42RT) and the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN) for flood forecasting (Casse et al. 2015). Casse et al. (2016) have employed the ISBA-TRIP model to investigate the long-term changes along the Red Flood tributaries, which are located in the Sahel region and have been facing intensified flood hazard over the last decades. A recent application for the Upper Niger River basin of the MGB model (MGB is the acronym in Portuguese for "Large Scale Hydrological Model", Fleischmann et al. 2018) presented a two-way coupling of large-scale hydrological and hydraulic models over the basin and highlighted the importance of representing the interactions between floodplains and unsaturated soils during the annual flood wave propagation across the Inner Niger Delta. ...
... The main tributaries of the Niger River contributing to the red flood are located downstream of Ansongo and the hydrographs at this location do not show the red flood discharge peak (Cassé et al., 2016). SPM concentrations retrieved from Sentinel-2 MSI are similar at Ansongo and Niamey during the black flood period, from October to May. ...
Study Region: Middle Niger River Basin (MNRB), Ansongo to Niamey, Sahel, West Africa.
Study Focus: Although MNRB hydrology and the red flood phenomena have been addressed in the past literature, water quality parameters and suspended particulate matter (SPM) dynamics remain poorly known. SPM impacts dam silting, exacerbating flooding, and microbial water quality. This study couples in-situ SPM measurements to radiometric measurements by in-situ and satellite sensors to analyse the temporal and spatial evolution of SPM in the MNRB and assess the contribution of the local flood (red flood) to SPM in Niamey.
New Hydrological Insights for the Region: SPM is composed of very fine kaolinites with a major mode around 200–300 nanometers which results in high reflectance in the visible and infrared bands. Radiometric measurements by both radiometer and Sentinel-2 MSI sensors are well correlated to in-situ SPM, allowing efficient spatio-temporal monitoring of SPM concentration. SPM increases very rapidly at the beginning of the rainy season, reaching a peak, characterized by very high SPM values, about one month before the red flood. Satellite data highlight the significant contribution of the right bank tributaries to SPM in the MNRB during this period. SPM then decreases and remains low despite the second runoff increase (black flood) arriving in Niamey after the end of the rainy season from the upper basin.
... These approaches vary from parameter estimation (Nyabeze 2005;Kapangaziwiri and Hughes 2008), regionalization (e.g., Love et al. 2011), assimilation of new data types into hydrological models (Mekonnen et al. 2009;Milzow et al. 2011) and model uncertainty prediction (e.g., Katambara and Ndiritu 2009;Kapangaziwiri et al. 2012). Models have also been applied at different spatial scales from the relatively small to medium catchments (Hamlat et al. 2013;Gal et al. 2017;Nonki et al. 2021) to large rivers (Tshimanga and Hughes 2014;Casse et al. 2016;Munzimi et al. 2019) and even continental scales (Alemaw and Chaoka 2003;Trambauer et al. 2013). Apart from the general catchment water balance modeling, progress has also been made in the use of models for more specific processes such as flooding (Yawson et al. 2005;Ngongondo et al. 2013;Smithers et al. 2013) and drought assessments (Nyabeze 2004), modeling of floodplains (Birkhead et al. 2007;Unami et al. 2009), sediment yields (Ndomba et al. 2008) groundwater surface water interactions (Ayenew et al. 2008;Tanner and Hughes 2015). ...
... These models have used Earth Observation in the Niger Basin to explore a range of issues, such as the use of MODIS and Landsat for vegetation and surface water in groundwater recharge processes (Leblanc et al. 2007), and satellite rainfall products such as ECMWF Re-Analysis (ERA)-Interim for runoff estimation (Oyerinde et al. 2017) and the Climate Prediction Center morphing method (CMORPH), the near-real-time legacy product of Tropical Rainfall Measuring Mission Multi-satellite Precipitation Analysis (TMPA 3B42RT) and the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN) for flood forecasting (Casse et al. 2015). Casse et al. (2016) have employed the ISBA-TRIP model to investigate the long-term changes along the Red Flood tributaries, which are located in the Sahel region and have been facing intensified flood hazard over the last decades. A recent application for the Upper Niger River basin of the MGB model (MGB is the acronym in Portuguese for "Large Scale Hydrological Model", Fleischmann et al. 2018) presented a two-way coupling of large-scale hydrological and hydraulic models over the basin and highlighted the importance of representing the interactions between floodplains and unsaturated soils during the annual flood wave propagation across the Inner Niger Delta. ...
The African continent hosts some of the largest freshwater systems worldwide, characterized by a large distribution and variability of surface waters that play a key role in the water, energy and carbon cycles and are of major importance to the global climate and
water resources. Freshwater availability in Africa has now become of major concern under
the combined effect of climate change, environmental alterations and anthropogenic pressure. However, the hydrology of the African river basins remains one of the least studied worldwide and a better monitoring and understanding of the hydrological processes across the continent become fundamental. Earth Observation, that offers a cost-effective means for monitoring the terrestrial water cycle, plays a major role in supporting surface hydrology investigations. Remote sensing advances are therefore a game changer to develop comprehensive observing systems to monitor Africa’s land water and manage its water resources. Here, we review the achievements of more than three decades of advances using remote sensing to study surface waters in Africa, highlighting the current benefits and difficulties. We show how the availability of a large number of sensors and observations, coupled with models, offers new possibilities to monitor a continent with scarce gauged stations. In the context of upcoming satellite missions dedicated to surface hydrology, such as the Surface Water and Ocean Topography (SWOT), we discuss future opportunities and how the use of remote sensing could benefit scientific and societal applications, such as water resource management, flood risk prevention and environment monitoring under current global change.
... There is indeed a non-negligible contribution of the base flow from Guinean origin on the AMAX even in July-August-September (JAS) period. The red floods were thus dissociated between local streamflow from Sahelian tributaries and Guinean base flow using a method similar to that described in Cassé et al. 2016 [21]. The NS-GEV model was then applied to the AMAX of these Sahelian tributary flows. ...
Niamey, the capital of Niger, is particularly prone to floods, since it is on the banks of the Niger River, which in its middle basin has two flood peaks: one in summer (the red flood) and one in winter (the black flood). In 2020, the Niger River in Niamey reached its all-time highest levels following an abundant rainy season. On the other hand, the floods in Niamey have been particularly frequent in the last decade, a symptom of a change in hydroclimatic behaviour already observed since the end of the great droughts of the 1970s and 1980s and which is identified with the name of Sahelian Paradox. This study, starting from the analysis of the 2020 flood and from the update of the rating curve of the Niamey hydrometric station, analyses the rainfall-runoff relationship on the Sahelian basins of the Medium Niger River Basin (MNRB) that are at the origin of the local flood. The comparative analysis of runoffs, annual maximum flows (AMAX) and runoff coefficients with various rainfall indices calculated on gridded datasets allowed to hydroclimatically characterise the last decade as a different period from the wet one before the drought, the dry one and the post-drought one. Compared to the last one, the current period is characterised by a sustained increase in hydrological indicators (AMAX +27%) consistent with the increase in both the accumulation of precipitation (+11%) and the number (+51%) and magnitude (+54%) of extreme events in the MNRB. Furthermore, a greater concentration of rainfall and extremes (+78%) in August contributes to reinforcing the red flood's positive anomalies (+2.23 st.dev in 2020). The study indicates that under these conditions the frequency of extreme hydrological events in Niamey will tend to increase further also because of the concurrence of drivers such as riverbed silting and levee effects. Consequently, the study concludes with the need for a comprehensive flood-risk assessment on the Niamey city that considers both recent hydroclimatic trends and urbanisation dynamics in flood zones hence defining the most appropriate risk-reduction strategies.
... Updated studies claimed that is not totally clear what is the major driver and have suggested that the mutual influence strongly depends on the local climatic and territorial features [24,35,36]. Thus, each basin or sub-basin should be analysed carefully to understand which aspect leads to the intensification of the flow rate regime. ...
Natural hazards are becoming more and more a global issue since the negative
impacts that are causing on the environment and the entire biological sphere.
Nowadays, these phenomena are even more exacerbated by the ongoing climate
changes and environmental degradation that are affecting, although with different
severity, the whole planet. Among others, one of the hardest hit areas is the Sahel,
the region that embeds all arid and semi-arid countries bordering the great Sahara
Desert. In these regions, climate-related extremes are increasingly threatening the
basis for developing modern societies, from securing the livelihood to the economic
expansion. Besides the rising frequency with which natural catastrophes occur, the
relentless growth of the population makes a large share of it progressively more
exposed and vulnerable to these threats. Notwithstanding these facts that are already
taking place among the African communities, future projections are depicting worse
scenarios in which the hydrological framework will be characterized by an
intensification of precipitation, higher temperatures, and longer dry spells. Thus,
impelling is the necessity of developing adaptation and mitigation strategies to face
anthropic and climate-related impacts.
In this context, the main purpose of the dissertation is to provide a
comprehensive analysis of suitable strategies to deal with hydrometeorological
hazards affecting the Sahel, especially against floods and droughts. All addressed
investigations are characterized by the twofold purpose to be: (a) contextualized,
i.e. indigenous knowledge has represented the starting point of each insight and
analysis; (b) advanced, i.e. enhanced numerical models have been implemented to
examine and meet the goal. The goals pursued in this thesis are (i) to provide all the
preliminary analyses accomplished for the implementation of the first Early
Warning System for flood alertness at the service of riverine populations of the
Sirba river; (ii) to assess the capabilities of indigenous rainwater harvesting
techniques (RWHT) to be adopted as an effective strategy for flood mitigation; (iii)
to comprehensively investigate the benefits induced by using RWHT in mitigating
the plant water stress induced by rainwater shortage.
Firstly, watershed characteristics and the temporal evolution of river flow are
essential data to understand the river behaviour and set up mitigation and prevention
strategies against flooding. This thesis provides the recalibration of the rating
curves and the updating of the discharge dataset of the Sirba River. The analysis of
the updated flow time series underlines the rising trend in flood frequency and
intensity. Moreover, a new changepoint into the dataset is detected in 2008, which
identify the beginning of an epoch characterized by severe floods. The behavioural
alterations of the river caused by land cover and climate changes are depicted
through the calculation of its flow duration curves. Then, the flood hazard
assessment of the Nigerien reach of the Sirba river is presented. The flood-prone
areas are delineated and classified in conformity to the four alert classes currently
used in Niger. The definition of the hazard thresholds exploits advanced non-
stationary method able to consider the changes in the basin response to
hydrometeorological extremes over time.
The second part of this dissertation offers a new concept: the use of indigenous
agricultural practice for reducing runoff and mitigating flood hazard. To confirm
such insight, the thesis analyses the hydraulic performances of the most used
RWHT in sub-Saharan regions in terms of runoff reduction and infiltration increase.
HEC-RAS and Iber are the numerical models chose to study processes occurring in
the runoff formation and propagation. The simulations show that half-moons
organized in staggered lines are the best configuration in reducing runoff. The right
design may lead to runoff retention up to 87% and double the infiltration. Thus, the
runoff collected into the field is water that does not immediately reach the river
during rainstorms. The application of these techniques enhances the hydrological
efficiency of the farmland bringing a noticeable reduction of the runoff coefficient.
The third part examines the capacity of RWHT in mitigating soil water stress
prompted by water scarcity. A new methodology is proposed to compute the
evapotranspiration rate into a two-dimensional distributed hydrological model used
to simulate the water balance of typical Sahelian farmlands. This method expands
the application field of such analyses in a context typically marked by data scarcity.
Intercepting and storing overland flow, RWHT increase the water content in the
root zone and the right design can even bring the water stress to zero. Furthermore,
outcomes reveal that RWHT may lengthen the growing period up to 20 days,
contributing to prevent crop failure.
... Recent applications indicate that one key feature for rainfall product relevance for hydrometeorology in West Africa is the reproduction of the early rise of the Niger in July followed by several sharp peaks during the "red-flood" period (Cassé et al. 2015(Cassé et al. , 2016 which creates severe floods in Niamey. Reproducing this feature requires a good representation of the convective systems' rainfall near Niameya skill shown by TAPEER 1.5 . ...
Improved monitoring of the precipitation at global scale and in conventional data sparse regions is needed to enhance our understanding of the functioning of the water and energy cycle and our ability to forecast its evolution at various scales. Decades of precipitation remote sensing research have shown that the merging of the high frequency cloud top information derived from geostationary infrared observations with the column hydrometeors vertical fluxes estimated from much less frequent passive microwave observations could mitigate the short comings of each approach and provide accurate accumulated rainfall estimates. The classical merging technique is introduced in detail and some more recent developments on the estimation of the uncertainty of the daily accumulation are presented. The products from tropical hydrometeorology and climate monitoring implementations are shown. Their performances and sensitivity are discussed, with emphasis on the configuration of the constellation. Finally, an outlook for future developments is provided.
... Tarpanelli et al. (2017), for the Niger-Benue river, suggested forecasting flood discharge from altimetric water levels, MODIS river width, and rating-curve calibration, however with in situ measurements of water levels being available. Others have proposed simulating discharge using fully fledged calibrated and validated land surface modeling (Pedinotti et al., 2012;Casse et al., 2016;Fleischmann et al., 2018;Poméon et al., 2018), assimilating altimetric levels into elaborate hydrodynamic modeling (Munier et al., 2015), or interpolating discharge based on empirical dynamic models trained on gauge discharge ; however such models are not always available and are less straightforward to transfer to new regions. Therefore we propose combining simplified hydrological models with radar altimetry. ...
The Niger River represents a challenging target for deriving discharge from spaceborne radar altimeter measurements, particularly since most terrestrial gauges ceased to provide data during the 2000s. Here, we propose deriving altimetric rating curves by “bridging” gaps between time series from gauge and altimeter measurements using hydrological model simulations. We show that classical pulse-limited altimetry (Jason-1 and Jason-2, Envisat, and SARAL/Altika) subsequently reproduces discharge well and enables continuing the gauge time series, albeit at a lower temporal resolution. Also, synthetic aperture
radar (SAR) altimetry picks up the signal measured by earlier altimeters quite well and allows the building of extended time series of higher quality. However, radar retracking is necessary for pulse-limited altimetry and needs to be further investigated for SAR. Moreover, forcing data for calibrating and running the hydrological models must be chosen carefully. Furthermore, stage–discharge relations must be fitted empirically and may need to allow for break points.
... In this regard, some work is in progress; for example, the United Nations Development Programme is currently funding a project on "Strengthening Climate Information and Early Warning Systems in Burkina Faso", which has a flood component (UNDP, 2017). Studies that leverage the increase in research capacity in the region (Hughes et al., 2015) in combination with emerging sources of data such as satellite and microwave tower data (Casse et al., 2016;De Coning, 2013;Hoedjes et al., 2014) can be used to inform and implement effective flood forecasting even in traditionally data-poor regions. Other possibilities include studies on the potential transferability of educational programs (e.g., Ashley et al., 2012) or of economic incentives to decrease dumping of solid wastes into canal systems. ...
The recent increase of devastating floods in West Africa implies an urgent need for effective flood risk management. A key element of such management is understanding how perceptions affect the implementation of mitigation measures. This paper uses the technique of framework analysis in conjunction with the conceptual framework of protection motivation theory to interpret flood perceptions and mitigation actions of flood victims and public officials in Ouagadougou, Burkina Faso as conveyed through in-depth semi-structured interviews. The results show that, despite the experience of a devastating flood in 2009 and clear understandings of flood causes, mitigation actions in Ouagadougou after the 2009 flood varied widely. This occurred due to adverse perceptions that mitigation actions are costly and that personal ability and responsibility to effect change is limited. These adverse perceptions offset neutral or positive perceptions that mitigation measures, if correctly implemented, are effective, and that the risk of flooding is high. The paper concludes with a comparative meta-analysis of West Africa flood perception and mitigation literature that reveals the need for actionable studies on the implementation of specific measures for flood risk management.
