Elsevier

CATENA

Volume 131, August 2015, Pages 130-139
CATENA

Exorheism growth as an explanation of increasing flooding in the Sahel

https://doi.org/10.1016/j.catena.2015.03.017Get rights and content

Highlights

  • An increase in runoff is observed since the beginning of the Sahelian drought.

  • We examine land degradation and soil crusting in the Middle Niger River.

  • An extension of exorheic areas is observed in some parts of the Niger Sahel.

  • This is likely one of the factors of increase in runoff and flood hazard.

  • The flood hazard is threatening new areas in the Sahel.

Abstract

For two decades, the Niamey area, in Niger, has undergone the creation of several new wadis (“koris” in hausa, the most spoken language in West Africa). The significant runoff increase in the Sahelian reach of Niger river makes us interested in the behavior of the basins of the tributary koris of Niger River in the Niamey area, in Niger. These koris generally formed during a single storm event, within depressions previously occupied by ponds; these ponds are overflown creating a new “kori”. This study examines in detail the causes of this new exorheism mechanism. The main explanation of this evolution has been determined as being the strong runoff increase, related to an extension of crusted soils due to agricultural practices, mostly the reduction of fallow duration. The degradation of their structural stability leads to crusting and a strong reduction of their hydraulic conductivity. This is linked to water and sediment balance at the catchment scale.

Introduction

Despite the persistent climatic and agronomical drought in West Africa, the discharge increase of the main Sahelian rivers is going on (Descroix et al., 2013a). Concerning the Niger River, the discharge of its main Sahelian tributaries has been shown as increasing since the beginning of the drought (1970s), an unexpected observation that constitutes the “Sahelian paradox”. Albergel made this observation in 1987 by analyzing data from several experimental (small) catchments in Burkina Faso (Albergel, 1987). Then Mahé et al., 2003, Mahé et al., 2005, Mahé et al., 2011 observed that this increase in runoff applied in larger basins (the Nakambé River at Wayen, 20,000 km2); Mahé (2009), Mahé and Paturel (2009), Amogu et al. (2010), Descroix et al. (2012b) noticed the generalization and regionalization of this paradoxical fact in the whole Sahelian stripe (Fig. 1, Fig. 2). Annual runoff coefficients of the Gorouol, Dargol and Sirba rivers (main tributaries of the Middle Niger River), are three times higher, and discharge 150% higher, at present than those observed fifty years ago (Fig. 3), as a consequence of land use change (Descroix et al., 2012b).

Otherwise, within an endorheic area near Niamey, this runoff increase was shown to cause a groundwater level rise (Leduc et al., 1997), due to the increase in the number, area and duration of ponds, ponds being the main groundwater recharge points.

The Niger River has a double flooding pattern downstream from the Niger Inner Delta (NID) in Mali (Fig. 1) and the confluence with its Sahelian tributaries.

Most of the stream flow comes from the mountains of Guinea (Fig. 1), and causes the main annual flood. It needs a few months to cross the NID. Therefore, the rainy season on the Sahelian area of the Middle Niger Basin triggers the appearance of a previous and secondary flood, before the main one. The most part of the Sahelian catchment of Niger River is located downstream from the NID and upstream from the city of Niamey (capital city of Niger Republic) (Fig. 1). Except the Niger River, all the tributaries have temporary flows, named “koris” (= wadi) in hausa (the most spoken language in West Africa): during the wet season (summer), water discharge rapidly increases after the monsoon onset. Most of the tributary koris of Niger River in its Middle reach are recent, less than 50 years (see Section 4.2) most of them were dug during only one single storm event in depressions previously occupied by ponds.

The main aim of this paper is to determine whether there exist other land factors explaining the changes in the discharge and regime of Sahelian Niger River and its tributaries. For this reason the contribution of the new koris is evaluated. Recent studies (Descroix et al., 2013b) show that there is an increase in extreme rainfall events unless reaching the number observed before the beginning of the dry period in 1968 (see Section 4.1).

Section snippets

Study area

The right bank of the Middle Niger River area, named Liptako Gourma Massif (Fig. 1), is an exorheic area, contrary to the Iullemeden sedimentary basin lying on the left bank; the latter region, located eastward from Niamey, is mostly endorheic and traditionally did not provide water to the main streams. The Liptako Gourma shield is the most water contributing area of the Middle Niger River basin (Fig. 3).

The Liptako–Gourma shield is composed by a granitic basement where the main right bank

Material and methods

In order to determine the river discharge and regime factors of changes, two kinds of analyses are proposed.

  • -

    An analysis of the hydrological data of the Niger river and its tributaries at the following stations: Kandadji and Niamey (Niger river), Alcongui (Gorouol river), Kakassi (Dargol river) and Garbey Korou (Sirba river) (Fig. 2); daily data of each station are available since 1957; they are provided by the Niger Hycos program of the NBA (Niger Basin Authority); rainfall data are supplied by

Hydrological changes

The significant observed runoff increase (see Fig. 3) cannot be attributed to the rainfall evolution, as rainfall has strongly decreased since 1968 (Fig. 5). Fig. 5 shows that the reduction of rainfall began in 1968 while the partial return to the previous rainfall mean is almost observed from 1995.

A set of statistical tests was performed on rainfall and discharge annual data on the three main right bank tributaries of the Niger and the three gathered, in order to detect ruptures and to

Discussion: other hydrological factors of inundation and endorheism bursting

West Africa is undergoing important hydrological and geomorphic changes (Amogu, 2009, Mamadou, 2012), triggered by the drought and human pressure. The specificity of the middle Niger valley is the creation of new tributaries mainly controlled by the increasing runoff on slopes and plateaus. These processes strongly impact local populations, mainly in terms of risks and loss of soil fertility. In recent decades, two main factors have aggravated the flooding intensity and then the inundation

Conclusion

Land use changes have been identified as the main factor of the “Sahelian hydrological paradox”, the increase in runoff observed since the beginning of the drought. In recent years, an increase in extreme rainfall events probably contributes to increase stream flows and to generate inundations.

However, some other factors contribute, in the region of Niamey, to enhance the stream flow and the inundation hazard: i — the increase in the catchment area by endorheism bursting, resulting in an

Acknowledgments

This study was partially funded by the ANR ECLIS (Contribution of livestock to the reduction of rural population vulnerability and to the promotion of their adaptability to climate and society changes in Sub-Saharan Africa) (ANR-08-VULN-003-01/ECLIS) French program, and by the ANR ESCAPE ANR-10-CEPL-005 (Environmental and Social Changes in Africa: Past, present and future) French program. The French Spatial Research Center CNES funded the acquisition of satellite data through the ISIS procedure

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