THE ROLE OF SURFACE-WATER REDISTRIBUTION IN AN AREA OF PATTERNED VEGETATION IN A SEMIARID ENVIRONMENT, SOUTH-WEST NIGER

The surface hydrology of a semi-arid area of patterned vegetation in s outh-west Niger is described. In this region alternating bands of vege tation and bare ground aligned along the contours of a gently sloping terrain give rise to a phenomenon known as 'brousse tigree' (tiger bus h). 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, Ac acia ataxacantha and A. macrostachya. Herbaceous vegetation is general ly limited to the upslope edges of vegetation bands. A comprehensive f ield programme was undertaken to investigate the hydrology, Topographi c, vegetation and surface feature surveys were carried out in conjunct ion 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 pos ition 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 ea ch type of crust tending to occupy a constant position on the regional slope relative to the vegetation class and other crust types. Below c losed bush crusts are generally absent. The typical downslope sequence from the downslope boundary of a vegetation band is: structural (siev ing) crust -->, erosion crust -->, (gravel crust) --> sedimentation cr ust --> 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. Hydraulic con ductivities of surface crusts are low, typically falling within the ra nge 10(-6)-10(-7) m s(-1). The presence of large expanses of crust ove r bare regions tends to generate run-off, which moves down the regiona l slope to be intercepted and pond within and just upslope of vegetate d areas. Such run-off concentrates rainfall by a factor of up to 3.7 b elow vegetated areas. This concentration combined with an absence of c rust development in closed bush areas promotes rapid infiltration belo w and just upslope of vegetation bands. In this way the hydrology of t he area operates to ensure that the bulk of the rain which falls is di rected as quickly as possible to the areas where it is most needed to support the existing vegetation.