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Runoff Evolution According to Land Use Change

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Runoff Evolution According to Land Use Change Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Hydrol. Earth Syst. Sci. Discuss., 8, 1569–1607, 2011 Hydrology and www.hydrol-earth-syst-sci-discuss.net/8/1569/2011/ Earth System HESSD doi:10.5194/hessd-8-1569-2011 Sciences 8, 1569–1607, 2011 © Author(s) 2011. CC Attribution 3.0 License. Discussions Runoff evolution This discussion paper is/has been under review for the journal Hydrology and Earth according to land use System Sciences (HESS). Please refer to the corresponding final paper in HESS change if available. Runoff evolution according to land use L. Descroix et al. change in a small Sahelian catchment Title Page Abstract Introduction L. Descroix1, M. Esteves1, K. Souley Yero´ 1, J.-L. Rajot2, M. Malam Abdou3, 1 1 5 6 1 3 S. Boubkraoui , J. M. Lapetite , N. Dessay , I. Zin , O. Amogu , A. Bachir , Conclusions References I. Bouzou Moussa3, E. Le Breton4, and I. Mamadou4 Tables Figures 1IRD/UJF-Grenoble 1/CNRS/G-INP, LTHE UMR 5564, Grenoble, 38041, France 2IRD – BIOEMCO /LISA, Universite´ Paris Est Creteil,´ Creteil,´ France 3Universite´ Abdou Moumouni, Departement de geographie,´ Niamey, Niger J I 4 LGP-CNRS, Meudon Bellevue, France J I 5UMR ESPACE/IRD, Montpellier, France 6LTHE-G-INP, BP 53, 38041 Grenoble cedex 9, France Back Close Received: 14 December 2010 – Accepted: 27 December 2010 – Published: 1 February 2011 Full Screen / Esc Correspondence to: L. Descroix ([email protected]) Printer-friendly Version Published by Copernicus Publications on behalf of the European Geosciences Union. Interactive Discussion 1569 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract HESSD Significant land use changes have been observed in West Africa, particularly in the Sahel region where climatic and demographic factors have led to a rise in cultivated 8, 1569–1607, 2011 areas, in recent decades. These changes caused strong modifications in the water 5 cycle and in river regimes. Runoff evolution By comparing the rainfall-runoff relationships for two periods (1991–1994 and 2004– according to land use 2 2010) in two small neighbouring catchments (approx. 0.1 km each) of the Sahel, this change study highlights the different hydrological consequences of land use change, particu- larly vegetation clearing and the consequent degradation of topsoil. L. Descroix et al. 10 Runoff increased in the upper basin, while it decreased in the lower basin, due to a strong increase in in-channel infiltration. Flood peak durations have become shorter in the downstream part of the catchment due to the huge increase of runoff water Title Page transmission losses within the gullies. Abstract Introduction Further study will consist of equipping one of the catchments with anti-erosion de- 15 vices (mainly “half-moons” and terraces) in order to evaluate the influence of anti- Conclusions References erosion devices on runoff and suspended load. Tables Figures 1 Introduction J I Land use is rapidly changing in the Sahel. Natural vegetation had already almost J I disappeared in extended areas of Western Niger and Eastern Burkina Faso, replaced Back Close 20 with crops and fallows. Furthermore, within the croplands, crop areas are increasing while fallow lands are decreasing Erosion processes entered a new stage where soil Full Screen / Esc crusting linked to the shortening of fallow periods caused a significant increase in soil erosion and runoff within the region, accompanied with the consequent sedimentation Printer-friendly Version downstream (Amogu, 2009; Le Breton, 2011; Mamadou, 2011). This led to the great Interactive Discussion 25 extension of erosion-caused landforms: gullies and alluvial fans strongly increased in extension in the last decades. 1570 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | A significant increase in runoff coefficient is attributable to soil surface features de- veloped in the hillslopes: crusted soils, erosion crusts, algal crusts, etc. (Casenave and HESSD Valentin, 1989). 8, 1569–1607, 2011 Downstream these landforms constitute new infiltration areas (Descroix et al., 2009). 5 The new hillslope behaviour observed combine an increase in runoff at the local scale linked to soil structure degradation and crusting, and a decrease in runoff at the small Runoff evolution catchment scale, attributable to a strong increase in transmission losses within the according to land use sandy deposit in gullies, the alluvial fans and spreading areas. change It was noticed that the Boserup theory (“more people, less erosion”), observed as an L. Descroix et al. 10 existing model in some parts of Sub Saharan Africa, e.g. in Kenya (Tiffen et al., 1994) or in Ivory Coast (Demont and Jouve, 1999), does not apply for the moment in most of the Sahel, while the Hardin “tragedy of the commons” (Hardin, 1968) is partially ob- Title Page servable, soil tenure being always under a traditional system without private property (Descroix et al., 2008). However, some examples of land reclamation successes show Abstract Introduction 15 that the main trend is reversible, as it has been observed in the Central Plateau of Burkina Faso (Reij et al., 2005; Reij et al. 2009) and in Eastern parts of Niger (Lar- Conclusions References wanou et al., 2006; Di Vecchia et al., 2006; CRESA, 2006; Reij et al, 2009). In some Tables Figures cases, a rise in water table is attributable to the new spreading sills (near Keita in Niger, CRESA, 2006) and to land reclamation (Reij et al., 2005, 2009), while it is due to land J I 20 degradation in some endorheic areas of the Sahel (Massuel et al., 2006; Leblanc et al., 2008; Descroix et al., 2009). J I A strong land use change has led to a degradation of soils and vegetation in the Back Close Sahel during the last few decades. Some authors have found a “re-greening” of this region (Rasmussen et al., 2001; Anyamba and Tucker, 2005) others have observed Full Screen / Esc 25 a decrease in albedo (Govaerts et al., 2008). However, Hein and De Ritter (2006) showed that using the RUE concept (rain use efficiency) helps to make satellite infor- Printer-friendly Version mation more consistent with the numerous studies which highlighted a severe decrease in vegetation cover over the Sahel (Ada and Rockstrom,¨ 1993; Loireau, 1998; Chinen, Interactive Discussion 1999; Le Breton, 2011; Leblanc et al., 2008 among others) and corroborates previous 1571 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | observations of Hountondji et al. (2004) in Niger. Hiernaux et al. (2009) determined that in spite of land clearing, in some places, total biomass was increasing because of HESSD the higher value of millet biomass compared with that of natural vegetation. Numerous 8, 1569–1607, 2011 studies have demonstrated the influence of land cover changes on the hydrological 5 regime, at the point scale (Casenave and Valentin, 1989; Vandervaere et al., 1997, etc.), at the hillslope scale (Peugeot et al., 1997; Esteves and Lapetite, 2003), and at Runoff evolution the basin scale (Albergel, 1987; Amani and Nguetora, 2002; Mahe´ et al., 2003; De- according to land use scroix et al., 2009; Amogu, 2009; Amogu et al., 2010). There is a general agreement change that increasing soil crusting has led to increasing runoff coefficients and a rise in runoff L. Descroix et al. 10 and flood irregularity in some parts of the region in spite of the decrease in rainfall. In some endorheic areas, this has led to a rise in the water table (Leduc et al., 2001; Leblanc et al., 2008; Favreau et al., 2009). Karambiri et al. (2003) showed that the sur- Title Page face features caused runoff and severe water erosion in a small catchment in Northern Burkina Faso. Abstract Introduction 15 The aim of this paper is to compare, in small Sahelian catchments, the land use changes and the water cycle evolution, especially in runoff production, during the last Conclusions References two decades, in order to determine whether land use change has caused hydrological Tables Figures changes at the small basin scale. J I 2 Material and methods J I 20 2.1 Study area Back Close The Tondi Kiboro experimental catchments are located 70 km east of Niamey, Niger, Full Screen / Esc on the western part of the Iullemeden sedimentary basin (Fig. 1). In a landscape of dissected laterite-capped plateaus, the experimental site is located on a catena formed Printer-friendly Version by a plateau with loamy-clayey soils and low slopes, the breakaway at the edge of Interactive Discussion 25 this plateau (slopes 4–8%) and a 2 km-long sandy hillslope. The drainage network is composed of parallel gullies that concentrate the water running off from the plateau. 1572 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | The major surface features identified in the catchment area are the following: tiger bush, bare soils stripes of this tiger bush, edge slope, on the plateau; sandy structured HESSD soils in crops and fallow and crusted soils in degraded crops and fallows on the sandy 8, 1569–1607, 2011 hillslope. The gullies include sandy deposit in increasing extension. 5 2.2 Hydrological measurements Runoff evolution according to land use Three basins near the village of Tondi Kiboro, in south-western Niger, were equipped change and monitored from 1991 to 1994 and from 2004 to 2010. Two of them are nested 2 (“amont” and “aval”); their respective sizes are 46 800 m for the upper basin “amont” L. Descroix et al. and 110 540 m2 for the total one at the “aval” station (including the first one); the third 2 10 one (“bodo”), is a 121 800 m one, where the second observation period is only 4 years long (2007–2010). Stream gauge stations were equipped with “Chloe” (Elsyde, Paris, Title Page France) water level recorders during the first period, and “Thalimedes” (OTT, Kempten, Abstract Introduction Germany) water level recorders during the second period.
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