No abstract

Wind erosion is regarded as a major contributor to the desertification process in the Sahel, yet little quantitative information is available for that region on soil losses by wind erosion under different land management practices. A 3‐yr, on‐farm experiment was, therefore, set up to assess the effect of ridging and either banded or broadcast millet stover mulches (2000 kg ha−1) on soil loss in a millet‐cowpea intercrop. For wind directions approximately perpendicular to the orientation of ridges and residue bands, sediment mass balances were calculated from the change in horizontal sediment mass fluxes measured across the experimental plots with Big Spring Number Eight sand traps. Mass balance calculations for 16 events over 3 yr indicated an average soil loss of 17.5, 15.4, and 18.0 Mg ha−1 on control plots, and deposition of 15.5, 15.3, and 7.4 Mg ha−1 on banded residue plots in 1995, 1996, and 1997, respectively. Broadcast and banded residue mulches were not significantly different (P = 0.05) in terms of their sediment trapping efficiency. During the same time period, ridges reduced soil losses by an average of 57% compared with the control plots, but their efficiency was reduced to less than 15% after 100 mm of cumulative rainfall as ridges collapsed. Linear regression analysis using the incoming sand fluxes as the independent variable was used to estimate potential soil losses for all events with sediment fluxes <25 kg m−1 irrespective of wind direction. The calculations indicated potential soil losses of up to 79 Mg ha−1 on control plots and deposition of 41 Mg ha−1 on broadcast residue plots in a single year. For wind erosion control, broadcast millet stover mulching constituted the most effective control technique because it effectively protected the soil against erosion and its trapping efficiency is expected to be independent of wind direction.

dealing with the spatial and temporal variations in Sahelian soil water content as well as with the infiltration of water through deep soil layers of the vadose zone. The purpose of this chapter is to provide an overview of hydrological behaviour throughout West Africa based on point, local, meso and regional scales observations. 2. Background The paradoxical increase in runoff despite drought conditions in sub-Saharan Africa was first noted in a paper by Albergel [3], analysing decadal series of runoff measurements in experimental sites of Burkina Faso. He noticed that this increase was observed in Sahelian areas, but not in the more humid Sudanian regions. The decrease in rainfall during the 1969-1983 period seems to be largely offset by the evolution of surface features in the functioning of small catchments. These changes favoured the conditions of runoff in the Sahelian basins; there are due to both the human actions and the climatic conditions. The reduction of vegetation cover and the widespread crops areas cause soil surface settling and the appearance of impervious superficial layers, as well as the extension of eroded areas. Some sahelian basins have nowadays [in 1987] the common characteristics of basins located northward, with great areas of bare soils; perennial graminaceae are replaced with annual ones, and combretaceae with prickly bush species" [3]. Albergel [3] attributed the contrasting behaviour of Sudanian (mean annual rainfall > 750 mm) and Sahelian (mean annual rainfall < 750 mm) areas to increasing bare soils and decreasing vegetation cover in Sahelian basins. This hypothesis was confirmed in 1999 by Mahé and Olivry [4] and then in 2002 by Olivry [2], who remarked that the discharge of right bank tributaries of Middle Niger River had been increasing since the beginning of the Drought (1968). Similarly, Amani and Nguetora [5] noted that runoff coefficients were increasing significantly in right bank tributaries and showed that the onset of the annual flood was occurring earlier than in previous decades. Mahé et al. [6] analysed the runoff evolution of eight right bank tributaries of the Middle Niger River and noted that the decrease in rainfall did not lead to a decrease in runoff under the Sahelian climate as commonly observed in other basins in the world. Rather, these tributaries exhibited increasing runoff coefficients and in discharges, while "Sudanian" climate tributaries suffered a decrease in discharge and in runoff coefficient [6]. 3. Material and methods This study is mainly based on two sources of data: • field measurements and observations made during the AMMA (African Monsoon Multidisciplinary Analysis) experiment at the Niger experimental site (Niger River middle stretch and Niamey square degree), and: