Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review
Abstract
:1. Introduction
2. Material and Methods
2.1. Data
2.1.1. Hydrological Data
2.1.2. Rainfall Data and Processing Analysis
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- Most of the rainfall data are provided by National Meteorological Offices (Table 1).
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- However, for three areas, we used the Climate Hazards Group InfraRed Precipitation with Station (CHIRPS) data set [24]. This data set, which contains information from 1981 until just before the present, is an initiative of the U.S. Geological Survey (USGS) and the Climate Hazard Group (CHG), and is supported by several U.S. state institutions. It incorporates 0.05° resolution satellite imagery with in situ station data to create gridded rainfall time series for trend analysis. This data set was already used in several works, including in Africa [29,30,31].
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- The long-term rainfall evolution index in Section 6.2 is simply built with the % of annual variation vs. the mean value.
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- Trends and breaks in the annual rainfall and runoff coefficient series were determined through the following processes: determination of breaks and trends by the rank method and the Lee and Heghinian, Pettitt, Buishand principles, and finally the Hubert segmentation; all are processed using the Khronostat® 1.01 software of IRD [32].
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- Analysis of daily rainfall is conducted in two areas: Senegambia and the Middle Niger River Basin (MNRB). In each one of the areas, a dozen raingauges are considered for the 1951–2015 period. They are homogeneously distributed in space; regionalization was processed using Thiessen polygons. Only the Menaka station (destroyed in 2013) presents data gaps.
2.1.3. Land Use/Land Cover Changes (LULCC)
2.2. Methods of Hydrological Processes Upscaling and Synthesis
2.2.1. Trend of Runoff Coefficient
2.2.2. Comparison of Hydrographs
2.2.3. Principal Component Analysis (PCA)
3. “Sahelian Paradoxes” and Flooding Occurrence: Recent Advances in the Understanding of Sahelian and Sudanian Hydro Climatology
3.1. A Marked Increase in Runoff Coefficients and Discharges of Sahelian Rivers
3.2. A Reduced Lag Time, an Earlier Occurrence of Annual Flood
3.3. A More Intuitive Sudanian Functioning
3.3.1. The Sudanian Functioning at the Basin Scale
3.3.2. A Hewlettian Functioning at the Point and the Plot Scales
3.3.3. Lake Chad as an Indicator of Drought
3.3.4. Some Sudanian Rivers
3.3.5. Guinean Rivers Have the Same Behavior
3.3.6. As a Conclusion, the Temporal Evolution of the Sudanian Hydrological Style
3.4. Impacts of Dams and Irrigation
4. Upscaling of Hydrological Behaviors
4.1. Evolution of Runoff Coefficient during the 1968–1990s Drought
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- All the documented basins completely located in the Sahelian strip registered an increase in runoff coefficient during the dry 1968–1990s period. This finding is based on the studies dedicated respectively to the hydrological functioning of Sahelian [2,9,19,26,41,44,45,47,67,68] and Sudano-Guinean [25,27,28,57,59,60,61,62,63,69] basins, as well as comparisons between both series [3,18,20,51,64,66].
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- However, documented basins are far from covering the entire Sahelian strip. First, these basins are mostly located on exorheic areas, since endorheic ones, except the Lake Chad basin [57,58] are generally composed of small catchments [16,41,48,49,50,70] and are, therefore, not well documented. Otherwise, a great number of extended exorheic basins are either not or are no more extensively documented.
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- Large basins (Senegal River at Bakel, Niger River at Niamey and Chari River at N’Djamena) are much more influenced by the decrease in runoff of their upper basins than by the probable increase in runoff of their Sahelian part. The upper basins generate 90 to 97% of their total discharge.
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- Finally, Figure 3 clearly highlights the contrast between:
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- The pure Sahelian basins, where RC (and, for most of them, discharges) increased during the drought (1st hydrological Sahelian paradox); some of the right bank tributaries of the Niger River at Sahelian latitude and the Nakanbé are those where runoff coefficient increased more significantly; and
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- Other basins, where, more intuitively, RC decreased. As shown by Amogu et al. [67], the decrease was more marked in Sudanian basins than in Guinean ones. As observed by Olivry [20] and Mahé et al. [19], in Sudanian regions, a decrease of 20–25% in rainfall provoked a drop of 55 to 70% in discharges.
4.2. Comparison of Annual Runoff Coefficient Evolution from 1950 to 2015
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- The first one is composed of great basins, where Sudanian behavior predominates: they showed a strong decrease in runoff coefficient (RC) during the 1968–1995 drought (Figure 4a).
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- The second type is the pure Sahelian type, characterized by an increase in RC during the drought, and an exacerbated increase observed with the rainfall recovery after the mid-1990s (Figure 4b).
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- The third one is the “Sudano-Guinean type”, where a partial RC recovery is observed after the rainfall recovery (Figure 4c), with two sub types 3-2 and 3-3 both characterized by a high part of missing data; type 3-2 gathers the basins with total recovery of RC, probably linked to the “sahelization” of the basin and the increase in runoff on degraded or crusted topsoils (Figure 4d); type 3-3 is characterized by a very partial recovery of RC after the drought; it is composed of southern tributaries of the Niger River (Figure 4e).
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- The fourth type is the pure Sudanian one, with no or very little recovery of the basins RC; this can be due to the high soil and landscape WHC, which imposes a very long refilling time of all the natural reservoirs (Figure 4f).
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- The fifth and last type is the pure Guinean type, characterized by high RC in the smaller basins in recent decades; a partial recovery of RC is observed. This area is characterized by a long time (several years) elapsed before being impacted by the drought, due to the high volume of natural reservoirs taking years to be emptied. The subtype 5-1 is that of granitic areas with little reservoirs, strong slopes, and thin soils: the RC dropped 4–5 years after rainfall in 1975–1977 (Figure 4g); type 5-2 is that of the upper Niger basin, where low slopes and metamorphic and alluvial outcrops allow higher volumes of stored water; they lasted 7 to 9 years before showing a marked decrease in RC after the beginning of dry period, around 1979–1980 (Figure 4h).
4.3. A Principal Component Analysis (PCA) of Basin Hydrological Behavior
5. Land Use/Land Cover Changes and Their Hydrological Consequences
5.1. Some Concepts in Land Use/Land Cover Change Studies
5.2. Evaluation of Global Vegetation Evolution through Remote Sensing Studies
5.3. Highlighting the Correlation between Land Use/Land Cover and Runoff Yield
5.4. A Common Explanation: The Reduction of both Hydraulic Conductivity and Roughness
5.5. Some Exacerbating Processes
5.5.1. Urbanization
- On the runoff, by decreasing infiltration: urbanization is known to lead to soil compaction and sharp reduction in water infiltration. It is both a cause of flooding due to high runoff coefficients and a cause of aggravated consequences of flooding, due to inundation and the damage to the built environment. Additionally, gutters are commonly clogged when no maintenance is routinely implemented, as it was observed in Bamako in August 2013;
- On cities and peoples’ vulnerability: flooding caused severe damage in unplanned urbanized areas, which have developed rapidly in sub-Saharan cities in recent decades, and are widespread in flooding zones due to both the strong migration pressure and limited control over this urbanization.
5.5.2. Silting Up
5.5.3. Endorheism Rupture
6. An Intensification of Rainfall
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- Determining the interannual evolution of rainfall amount and rainfall indexes, including the evolution of both the number of rainy days and the daily rainfall amount (Section 6.2);
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- Analyzing the interdecadal evolution of number of extreme events (here, daily rainfall amounts exceeding 60, 80 and 100 mm) (Section 6.3);
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- Studying the annual evolution of monsoon onset and the effectiveness of the monsoon according to agronomical criteria (Section 6.4).
6.1. Main Trends Described in Recent Works
- Locally, a rise in inter-annual variability compared with that observed before the dry period;
- All over the Sudano-Sahelian strip, since the minimum of the 1980s, an increase in extreme rainfall events, more pronounced that the increase in annual rainfall.
- Rainfall decadal variability induces land cover changes, such as ligneous vegetation dying, due to severe drought; this vegetation cover reduction could, in time, explain the increasing runoff and discharges;
- Some studies ([2,78,87,95,104] among others) have demonstrated that the re-greening does not prevent the increase in runoff and in basin discharge, due to the severe degradation of small parts of these basins, generally located near the “kori”. The intensification of rainfall began in the middle of the 1980s, but it reached its maximum in the middle of the 2001–2010 decade [4,7]. At that date, at least 90% of the increase in stream flows had already been realized. The rainfall evolution in West Africa contributes to the increasing trend of runoff and rivers discharges. Below, several indicators are used to assess this contribution. More precisely, we analyze the annual rainfall amount, rainfall index, rainfall intensity and monsoon seasonal pattern. In this section, we will analyze the spatio-temporal evolution of daily rainfall for two areas: Senegambia and the MNRB (see location in Figure 9), and for the 1951–2015 period.
6.2. Annual Rainfall Amount and Indexes
6.3. Extreme Events: An Intensification of Rainfall?
- The current intensification contributes to the increase in runoff (accompanied by other factors);
- The intra-seasonal distribution of extreme events has likely changed, and should be analyzed further in future studies;
- An increase in maximal instant rainfall intensity and maximal rainfall at 5 min time step has been noticed in the very last few years [116].
6.4. The Onset of the Monsoon, through Agronomical Criteria
7. Conclusions and Perspectives
- Urbanization throughout the whole Sudano-Sahelian strip, which exacerbates runoff coefficients;
- The net increase in extreme rainfall events, with severe consequences in rural and, more recently, in urban areas too.
- The period of the 1st Sahelian Paradox (declining rainfall, increasing runoff and discharges), which is commonly attributed to LULCC, the decline of vegetation cover being due both to the severe rainfall deficit and associated woody vegetation dying, and to the increase in vegetation exploitation by a growing population (West Africa has had the highest demographic growth rate in the world since the 1980s);
- The period following the “Great Drought”, which was characterized by a general vegetation re-greening (observed by remote sensing) and vegetal net primary productivity increase, as a rise in discharges of Sahelian rivers was occurring. The 2nd Sahelian paradox (increase in runoff and increase in vegetation cover) is explained by both rainfall intensification and a reduction in soil WHC, probably linked to extension in cropping areas, as well as vegetation degradation and soil erosion over shallow soils [20,26,38,39,53,76,77,80].
- To increase the number of field and satellite observations of the basins, at different spatial scales, in order to document LULCC, associate processes and relevant variables;
- To monitor the soil crusting processes and its spatial extension;
- To better understand, analyze and model urban hydrology, as urbanization progression in West Africa is also the highest in the world, mostly due to the low current urbanization rate;
- To improve the knowledge of the role of imperviousness and of drainage system;
- To implement adapted hydrological modelling accounting for these processes and adopting rigorous statistical attribution approaches, as suggested, e.g., by Merz et al. (2012) [119].
Author Contributions
Acknowledgments
Conflicts of Interest
References
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Country | Time Span of Data | Source | ||
Benin, Burkina Faso, Mali, Mauritania, Niger, Togo, Senegal | 1950–2012 | National Met Offices | ||
Chad, Ghana, Guinea, Nigeria, Cameroon | 1950–1979 | SIEREM [23], Nat Met of | ||
Chad, Ghana, Guinea, Nigeria | 1981–2010 | CHIRPS [24] | ||
Basin | Time Span of Data | Missing | Reconstituted | Source |
Alibori, Bani, Benue, Dargol, Diamangou, Goroubi, Gorouol, Mekrou, Niger-Koulikoro, Niger-Niamey, Sota, Tapoa | 1950–2012 | Numerous gaps | No reconstitution | NBA Niger Basin Authority |
Milo, Upper Niger, Niandan | 1950–2012 | 4 years | 10 months | Nat Hydrol. Service Guinea |
Ouémé | 1952–2012 | / | / | Nat Hydrol. Service Benin |
Bafing, Bakoye, Faleme, Senegal | 1950–2012 | / | / | OMVS (Senegal Basin Authority) |
Gambia, Koulountou | 1950–2005 | / | / | OMVG (Gambia basin authority) |
Chari, Logone, Mayo Tsanaga | 1950–2012 | / | / | CBLT (Chad Lake basin Author) |
Nakanbé, Volta | 1965–2008 | 3 years | No reconstitution | VBA (Volta Basin Authority) |
Ankobra, Bra, Pia, Tano | 1951–1991 | / | / | Opoku et al. [25] |
Gorgol el Abiod, Gorgol al Akhdar, Sokoto | 1958–1985 | 5 years | / | Mahé et al. [9,26] |
Komadugu Yobe | 1950–2005 | Numerous gaps | No reconstitution | Genthon (personal communication) |
Mono, Couffo | 1961–2000 | / | / | Amoussou et al. [27] |
Bougouribga, Diaguiri, Goudebo, Niokolo Koba, Mouhoun, Noaho, Tene, Upper Faleme, Upper Gambia, Upper Gorouol | 1970–2006 | Numerous gaps | No reconstitution | Nka et al. [3] |
Upper Bandama | 1950–2000 | / | / | Soro et al. [28] |
Number | Variable |
---|---|
1 | basin area |
2 | Mean annual rainfall (MAR) 1950–2012 |
3 | % of rainfall decrease during drought period: MAR 1968–1993/MAR 1950–1967 |
4 | % of rainfall recovery after drought MAR 1994–2012/MAR 1968–1993 |
5 | Time elapsed between rainfall break (1967 for all the basins) and discharge break (varying from 1967 to 1981 depending on the basin) |
6 | Mean annual discharge (1950–2012) |
7 | % discharge evolution during the drought (1968–1993)/the previous period (1950–1967) |
8 | % discharge evolution after the discharge break/before the break (year variable) |
9 | Time elapsed between the rainfall break (1967) and the RC (runoff coefficient) break |
10 | Mean annual RC before break (1950–1967) |
11 | % RC evolution during drought (RC 1968–1993/RC 1950–1967) |
12 | % RC evolution after drought (RC 1994–2012/RC 1968–1993) |
Soil Surface Feature | Runoff Coefficient (%) | Saturated Hydraulic Conductivity (mm·h−1) |
---|---|---|
Millet on common sandy soil just after weeding | 4.0 ± 1.4 | 172 ± 79 (20) * |
Fallow on common sandy soil | 10 ± 4 | 79 ± 41 (20) |
Old fallow with bioderm | 25 ± 7 | 18 ± 12 (30) |
Millet and fallow erosion (ERO) crusted soils | 60 ± 8 | 10 ± 5 (30) |
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Descroix, L.; Guichard, F.; Grippa, M.; Lambert, L.A.; Panthou, G.; Mahé, G.; Gal, L.; Dardel, C.; Quantin, G.; Kergoat, L.; Bouaïta, Y.; Hiernaux, P.; Vischel, T.; Pellarin, T.; Faty, B.; Wilcox, C.; Malam Abdou, M.; Mamadou, I.; Vandervaere, J.-P.; Diongue-Niang, A.; Ndiaye, O.; Sané, Y.; Dacosta, H.; Gosset, M.; Cassé, C.; Sultan, B.; Barry, A.; Amogu, O.; Nka Nnomo, B.; Barry, A.; Paturel, J.-E. Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review. Water 2018, 10, 748. https://doi.org/10.3390/w10060748
Descroix L, Guichard F, Grippa M, Lambert LA, Panthou G, Mahé G, Gal L, Dardel C, Quantin G, Kergoat L, Bouaïta Y, Hiernaux P, Vischel T, Pellarin T, Faty B, Wilcox C, Malam Abdou M, Mamadou I, Vandervaere J-P, Diongue-Niang A, Ndiaye O, Sané Y, Dacosta H, Gosset M, Cassé C, Sultan B, Barry A, Amogu O, Nka Nnomo B, Barry A, Paturel J-E. Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review. Water. 2018; 10(6):748. https://doi.org/10.3390/w10060748
Chicago/Turabian StyleDescroix, Luc, Françoise Guichard, Manuela Grippa, Laurent A. Lambert, Gérémy Panthou, Gil Mahé, Laetitia Gal, Cécile Dardel, Guillaume Quantin, Laurent Kergoat, Yasmin Bouaïta, Pierre Hiernaux, Théo Vischel, Thierry Pellarin, Bakary Faty, Catherine Wilcox, Moussa Malam Abdou, Ibrahim Mamadou, Jean-Pierre Vandervaere, Aïda Diongue-Niang, Ousmane Ndiaye, Youssouph Sané, Honoré Dacosta, Marielle Gosset, Claire Cassé, Benjamin Sultan, Aliou Barry, Okechukwu Amogu, Bernadette Nka Nnomo, Alseny Barry, and Jean-Emmanuel Paturel. 2018. "Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review" Water 10, no. 6: 748. https://doi.org/10.3390/w10060748