Laboratory experiments on sequential scour/deposition and their application to the development of banded vegetation

Alternating bands of vegetation and bare soil, reported from many dryland r egions, have been identified as indicators of rangeland deterioration trigg ered by overgrazing, cattle trampling or climatic change. Banded vegetation occurs at a range of scales and although it has been reported from a numbe r of different environments, it is not characteristic of all degraded range lands. It does appear particularly frequently on low angle, smooth slopes o ver soils of high erodibility but low permeability, where scant rainfall is sporadic or highly seasonal. It has been attributed both to wind and to wa ter erosion, but few data on the processes active or their specific respons e to limiting environmental variables are available. Small-scale banded veg etation associated with small scour steps occurs on low angle alluvio-lacus trine flats surrounding Lake Baringo in semi-arid northern Kenya. In this a rea of strong moisture deficit, the dominant factor controlling the inciden ce of ground vegetation is variation in near-surface moisture storage. The regular spacing of the small-scale vegetation bands reflects preferential m oisture storage in regularly-spaced sediment deposits. Field and laboratory runon and rainfall simulation experiments, previously reported, provided s ome data on the processes and conditions involved in formation of these dep osits. This paper describes more closely-controlled laboratory rainfall sim ulation and runon experiments, carried out to identify the sequential scour and deposition processes involved, which are ultimately responsible for th e regular variations in moisture storage capacity. These experiments, carri ed out in an 8.5 m long flume, show critical stream power (Omega) condition s in sheetwash and rain-impacted sheetflow required for sequential scour/de position as 0.020-0.025 W m(-2) and between 0.043 and 0.055 W m(-2), respec tively. Experimental results indicate that vegetation bands at the scale ob served at Baringo are consistent with development by sequential scour and d eposition, caused by some combination of sheetwash, rainsplash and rainflow . The vegetation bands are composed primarily of unpalatable low herbaceous plants, dominated by Trianthema triquetra. These plants colonize or surviv e better on deposition zones because of better soil moisture status. The ve getation bands can therefore indicate an initial stage in vegetation recove ry rather than continuing rangeland deterioration, but at Baringo further d evelopment of vegetation appears to be restricted by high grazing intensiti es or by allelopathy. (C) 1999 Elsevier Science B.V. All rights reserved.