Abstract
In many parts of Africa, soil erosion is an important problem, which is evident from high sediment yields in tropical montane streams. Previous studies in Kenya pointed to a large contribution from catchments cultivated by smallholder farmers. This led to the hypothesis that unpaved tracks and gullies are the main sediment sources in smallholder agriculture catchments of the highlands of Kenya. The aim of this study was to investigate the sediment sources with sediment fingerprinting to generate the knowledge base to improve land management and to reduce sediment yields. Four main sediment sources (agricultural land, unpaved tracks, gullies and channel banks) and suspended sediments were analysed for biogeochemical elements as potential tracers. To apportion the catchments target sediment to different sources, we applied the MixSIAR un-mixing modelling under a Bayesian framework. Surprisingly, the fingerprinting analysis showed that agricultural land accounted for 75% (95% confidence interval 63–86%) of the total sediment. Channel banks contributed 21% (8–32%), while the smallest contributions to sediment were generated by the unpaved tracks and gullies with 3% (0–12%) and 1% (0–4%), respectively. Erosion management strategies should target agricultural lands with an emphasis on disconnecting unpaved tracks form hillslope source areas to reduce sediment yields to Lake Victoria.
Highlights
In many parts of Africa, soil erosion is an important problem, which is evident from high sediment yields in tropical montane streams
The overall aim of the study was to apportion the relative contributions of four potential sediment sources: agricultural land, gullies, unpaved tracks and channel banks to suspended sediment yields within a smallholder agriculture catchment in the headwater of the Sondu River Basin
The fingerprinting analysis shows that agricultural land accounted for 75% of the target sediment at the outlet of the smallholder agriculture catchment while the channel banks contributed with 21%
Summary
In many parts of Africa, soil erosion is an important problem, which is evident from high sediment yields in tropical montane streams. Four main sediment sources (agricultural land, unpaved tracks, gullies and channel banks) and suspended sediments were analysed for biogeochemical elements as potential tracers. Element concentrations are generally enriched in the fine particle fraction (particle size < 63 μm)[8,11,12], which are transported through water e rosion[7] In addition to these on-site effects, soil erosion degrades waterways as suspended sediments reduce the physical, biological and chemical water quality of s treams[13,14]. Agricultural intensification together with poor land management practices accelerate soil erosion[10] and increase the number of source areas that contribute sediment to the stream network. The prior knowledge of an uncertain quantity is described by the probability models[20,40]
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