The role of surface water redistribution in an area of patterned vegetation in a semi-arid environment, south-west Niger

The surface hydrology of a semi-arid area of patterned vegetation in south-west Niger is described. In this region alternating bands of vegetation and bare ground aligned along the contours of a gently sloping terrain give rise to a phenomenon known as "brousse tigrée" (tiger bush). At the selected study site the vegetation bands are 10-30 m wide, separated by 50-100 m-wide bands of bare ground. Five species of shrub dominate, #Guiera senegalensis$, #Combretum micranthum$, #C. nigricans$, #Acacia ataxacantha$ and #A. macrostachya$. Herbaceous vegetation is generally limited to the upslope edges of vegetation bands. A comprehensive field programme was undertaken to investigate the hydrology. Topographic, vegetation and surface feature surveys were carried out in conjunction with the measurement of rainfall, surface and subsurface hydraulic conductivity, particle size and soil moisture content. Four types of vegetation class are recognised, each tending to occupy a constant position relative to the others and to the regional slope. In a downslope direction the classes are : bare ground, grassy open bush, closed bush, bare open bush, bare ground etc. The nature of the ground surface is closely linked to the vegetation class. Over the bare, bare open and grassy open classes various types of surface crust are present with each type of crust tending to occupy a constant position on the regional slope relative to the vegetation class and other crust types. Below closed bush crusts are generally absent. The typical downslope sequence from the downslope boundary of a vegetation band is : structural (sieving) crust -> erosion crust -> (gravel crust) -> sedimentation crust -> microphytic sedimentation crust -> no crust -> sieving crust, etc. It is also shown that these crust types are dynamic and evolve from one to the other as hydrological conditions change... (D'après résumé d'auteur