干旱区生态环境知识资源中心

The history of sediment yield from a 57 km(2) catchment in the semi-arid Karoo of South Africa was reconstructed from the sediment stored in a reservoir at Cranemere dating back to 1843. The volume of sediment accumulating in the reservoir between different dates was estimated firstly by developing a core chronology based on Cs-137 and Pb-210 using gamma spectrometry and secondly by estimating volumetric changes from the depth of 17 cores located across the dry reservoir surface. The variation in sediment yield was interpreted in relation to historical changes in livestock management, rainfall patterns and catchment connectivity. Daily rainfall data were available from 1891. The results indicated low sediment yields until the 1950s, after which they increased rapidly up to the 1970s before declining gradually to the present day. There is historical evidence that accelerated erosion was initiated before the turn of the 19th Century due to high stock numbers and poor stock management until soil conservation practices were put in place during the 1950s. However, there is little evidence of this increase in erosion in the sedimentary record indicating that there were considerable lags between the onset of erosion and the sediment reaching the reservoir. The rapid increase in sedimentation after the 1950s is explained as a result of increased connectivity between the local sediment sinks and the reservoir following the construction of new culverts under a major road less than a kilometre upstream of the reservoir in the 1950s. Peak sediment yields of the 1970s coincide with high daily rainfalls and widespread flooding. The rainfall record indicates that daily rainfall maxima have increased since 1950, leading to potentially more erodible conditions. Our explanation of the increased sediment yields suggests that we need to examine more closely the links between catchment sources, local sinks within drainage lines and the final sink, the reservoir.