Analysis: Paradoxically increasing river flows in the Sahel despite long-term drought

A Water and Marine Sustainability Month Feature Article


Collines du Sahel algérois - Creative Commons image credit Yellès Chaouche Mohamed Arif

Okechukwu Amog , Luc Descroix (Grenoble Laboratoire d’étude des Transferts en Hydrologie et Environnement) and co-authors listed in the citation issued a paper addressing paradoxical increases in river flows in the West African Sahel, despite ongoing long-term drought.  The abstract to their good article is:

Despite the drought observed since 1968 in most of the West African Sahel, runoff and rivers discharges have been increasing in the same region. This trend is related with land use change rather than climate change. This paper aims to describe the regional extension of such a phenomenon and to demonstrate that the increase in runoff is observed from the point scale up to the regional scale. It highlights the opposition of functioning between a Sahelian zone, where the Sahel’s paradox applies, and the Sudanian and Guinean areas, where runoff has been logically decreasing with the rainfall. The current trend is evidenced using experimental runoff plots and discharge data from the local to the regional scales.

The introduction to their analysis reads:

West Africa has been going through a long-term drought since the end of the 1960s. This trend has been particularly noted in the Sahel, but appears to have been attenuated in the last decade in the eastern and central parts of the Sahel. On the other hand, annual rainfall remains very low in the western part of the Sahel. It is well established that for the period of 1970–2000, the decrease in the mean annual discharge of the region’s largest rivers, namely the Senegal and Niger rivers, was in proportion almost twice as much as the decrease in rainfall. Similar trends have been observed for smaller river systems while, in contrast, other studies have indicated a runoff increase in some Sahelian catchments . The increase in discharge of the Sahelian Rivers, despite the reduction in rainfall, was first noticed by Albergel in Burkina Faso, hence the term ―Sahelian Paradox‖ used in the following to designate this unexpected behavior. Similar observations were made for the Nakambé River, one of the main tributaries of the Upper Volta River in Burkina Faso, and for the right bank tributaries of the Niger River during its Sahel crossing. In these two areas, the increase was mainly observed during the drought years from 1968 to 1995. This is an almost general observation in Sahelian areas, and it led to an increase in the number, size, and duration of ponds in some endorheic areas, and then a rise in groundwater level, ponds being their main recharge area. A great deal of attention was dedicated to rainfall evolution in West Africa; the great drought that affected the region being the strongest climatic signal observed in the World since the beginning of meteorological observations. Le Barbé and Lebel already characterized this phenomenon, and more recently, Ali and Lebel and Lebel and Ali showed that this is ongoing in the western part of West Africa while it is partially attenuated in the central and eastern Sahel; however this attenuation is linked to an increase in the inter-annual variability. Mahé and Paturel divided the Sahel into four different areas, opposing the North-Western Sahel where drought remains severe and the South-Eastern Sahel where it is almost completely attenuated. As rainfall has decreased during this period, the increase in runoff cannot be due to climatic reasons; land cover has effectively changed significantly in the last few decades. At the local scale, the land use changes have been shown to lead to an evolution of soil physical characteristics, mainly bulk density, porosity, hydraulic conductivity, and a reduction in infiltration rates as well as a significant increase in runoff.

At the meso scale and the regional scale, the cumulative effect of the changes in small catchments leads to modification in the behavior (discharges and regime) of larger river basins. Land-cover changes, most often resulting from land use changes, trigger alterations in hydrodynamic soil behavior—for instance, increase in bare soil areas causes erosion and soil crusting—and can impact the local and regional water budget. Land use changes have obviously occurred, and their hydrological effects have been demonstrated; but how much of a reality is the land use change at the regional scale? What is a more accurate indicator of land use/land cover changes (LULC)?

(1) Some previous studies based on satellite image vegetation indices have perhaps led to an overestimation of vegetation cover through the Sahel in recent years (after 1994). At the local scale, these results have been corroborated by some observations. Non-governmental organizations involved in tree planting have recorded some success, particularly in areas such as Keita in central Niger, as well as near Aguié, near Zinder and along the Niger-Nigeria boundary in eastern Niger. Similar re-greening experiments in Burkina Faso and in eastern Senegal (region of Goudiry) have also been successful. Inversely, there has been a significant decrease in biomass in the Sudanian climatic region: this area was barely exploited until the drought and its population was relatively sparse, due to the prevalence of trypanosomiasis as well as the effect of the slave trade on population density, and this relative vacuum is being filled.

(2) Recent studies of Hountondji et al. and Hein and de Ritter highlighted the limitations of vegetation indices (NDVI, EVI, SAVI, NSAVI, etc.) for the determination of land use and land use changes when using remote sensing at low resolution (AVHRR, SPOT/VGT). These authors recommend the use of NDVI/P or NPP/P ratio (net primary productivity/rainfall) instead of any vegetation index. Most of the land cover studies based on aerial photograph analyses show a decrease in vegetation cover throughout the Sahel. This is corroborated by the official statistical data of cropping areas. For the Niger Republic during the 1999–2006 period, Garba observed a decrease in biomass in a study based on NDVI; but he explains this by the fact that the year 1999–2000 was a humid one. Karambiri et al., Diello et al. and Hauchart also noticed a reduction in vegetation cover in two different regions of Burkina Faso (the Nakambe and Mouhoun basins). Liénou et al. made similar observations in Cameroon, which are consistent with the works of Amani and Nguetora, and Mahé et al. on the west bank of the Niger River. Statistical data of the FAO indicated a yearly decrease of 3.7% in the biomass for the Niger Republic, and Hiernaux et al. measured an annual reduction of 2.7% of this biomass in the Fakara (Western Niger).

(3) Mapping studies of land use and land cover carried out within the AMMA (African Monsoon Multidisciplinary analysis) project indicated that it is necessary to apply different methods depending on the considered area, Sudanian or Sahelian. Glenn et al. achieved better accuracy using the EVI than the NDVI, and obtained rather good correlations with fields data (flux tower particularly). The objectives of this paper are the following:

  • To synthesize the runoff measurements realized in the framework of the AMMA program at the point and the local scales, and compare them with some previous results obtained in the same or similar basins;
  • To analyze the runoff evolution at the meso and the regional scales
  • To determine at which scales runoff evolution can be related with land use changes;
  • To define the spatial and temporal extension of the Sahelian paradox.

Read the full paper in PDF format by clicking here.

Citation: Amogu, O.;Descroix, L.;Yéro, K.S.;Le Breton, E.;Mamadou, I.;Ali, A.;Vischel, T.;Bader, J.-C.;Moussa, I.B.; Gautier, E.;Boubkraoui, S.;Belleudy, P. Increasing River Flows in the Sahel?. Water 2010, 2, 170-199.

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  1. […] of the Niger River during its Sahel crossing. In these two areas, … Here is the original: Analysis: Paradoxically increasing river flows in the Sahel … Share and […]

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