Observations on Vegetation Arcs in the Northern Region, Somali Republic

Water Flows and the Dynamics of Desert Vegetation Stripes

“…The occurrence and formation of these patterns seem to originate under a combination of climatic, geomorphological, and edaphic condition: (1) an arid or semiarid climate with few, but high intensity, rains; (2) a gentle but regular slope (0.25 to 1.0%); (3) soils with low permeability and a relative abundance of fine particles, producing intense sheet runoff. A slow migration of stripes upslope has been noted by many authors (Worral 1959;Boaler and Hodge 1964;White 1970). It has been suggested that this vegetation pattern originates either from the progressive degradation of a more or less uniform plant cover under a worsening climate or soil degradation (White 1971;Greig-Smith 1979), or from the evolution of previously bare zones (White 1971;Boudet 1972).…”

“…Many studies describe the structure and composition of these stripes and propose hypotheses for their establishment and origin (Worral1959, Slatyer 1961, Litchfield and Mabbutt 1962, Boaler and Hodge 1964, Hemming 1965, White 1970, Wickens and Collier 1971, Boudet 1972, and Leprun 1979. The occurrence and formation of these patterns seem to originate under a combination of climatic, geomorphological, and edaphic condition: (1) an arid or semiarid climate with few, but high intensity, rains; (2) a gentle but regular slope (0.25 to 1.0%); (3) soils with low permeability and a relative abundance of fine particles, producing intense sheet runoff.…”

On the origin of tiger bush

“…The initial condition is chosen to be an unstable uniform steady state density p+ perturbed by some small-amplitude random noise. This noise stands for the numerous more or less random environmental factors which disturb the uniformity of natural mediums, e.g., ant or termite activity, which is both benefic (nests increase water infiltration by soils) and detrimental (eroded material from nests increases runoff) to vegetation (Ouedraogo and Lepage, 1996), fire or grazing, vegetal debris, animal excreta accumulations,... (Boaler and Hodge, 1964;Hemming, 1965;White, 1971).…”

“…In fact, it is a landscape which may appear in most of the arid and semi-arid regions covering approximately one-third of the emerged surfaces of the earth (White, 1971;Schlesinger et al, 1990). The climate of these regions is characterised by few but intense rains of short duration (Boaler and Hodge, 1964). The annual rainfall ranges from 50 to 750 mm (White, 1969(White, , 1970.…”

“…Often, the ground surface slopes gently from 1 m in 500 m up to 1 m in 50 m (Mabbutt and Fanning, 1987). This favours water flowing without drainage channel formation (Greenwood, 1957;Boaler and Hodge, 1964;Montafia, 1992). According to its orientation with respect to (w.r.t.)…”