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

Abstract

Alternating bands of vegetation and bare soil, reported from many dryland regions, have been identified as indicators of rangeland deterioration triggered by overgrazing, cattle trampling or climatic change. Banded vegetation occurs at a range of scales and although it has been reported from a number of different environments, it is not characteristic of all degraded rangelands. It does appear particularly frequently on low angle, smooth slopes over soils of high erodibility but low permeability, where scant rainfall is sporadic or highly seasonal. It has been attributed both to wind and to water erosion, but few data on the processes active or their specific response to limiting environmental variables are available. Small-scale banded vegetation associated with small scour steps occurs on low angle alluvio-lacustrine flats surrounding Lake Baringo in semi-arid northern Kenya. In this area of strong moisture deficit, the dominant factor controlling the incidence of ground vegetation is variation in near-surface moisture storage. The regular spacing of the small-scale vegetation bands reflects preferential moisture storage in regularly-spaced sediment deposits. Field and laboratory runon and rainfall simulation experiments, previously reported, provided some data on the processes and conditions involved in formation of these deposits. This paper describes more closely-controlled laboratory rainfall simulation and runon experiments, carried out to identify the sequential scour and deposition processes involved, which are ultimately responsible for the regular variations in moisture storage capacity. These experiments, carried out in an 8.5 m long flume, show critical stream power ( Ω) conditions in sheetwash and rain-impacted sheetflow required for sequential scour/deposition as 0.020–0.025 W m −2 and between 0.043 and 0.055 W m −2, respectively. Experimental results indicate that vegetation bands at the scale observed at Baringo are consistent with development by sequential scour and deposition, 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 survive better on deposition zones because of better soil moisture status. The vegetation bands can therefore indicate an initial stage in vegetation recovery rather than continuing rangeland deterioration, but at Baringo further development of vegetation appears to be restricted by high grazing intensities or by allelopathy.