Assessing the dynamics of soil erosion and sediment transport under increasing land use pressures in East African Rift Catchments

Parts of this thesis are published in peer-reviewed scientific journals: - Chapters 5 and 6 : Wynants, M., Solomon, H., Ndakidemi, P. & Blake, W. H. 2018. Pinpointing areas of increased soil erosion risk following land cover change in the Lake Manyara catchment, Tanzania. International journal of Applied Earth Observation and Geoinformation 71: 1-8. https://doi.org/10.1016/j.jag.2018.05.008 - Chapter 3: Wynants, M., Kelly, C., Mtei, K.,…, & Ndakidemi, P. 2019. Drivers of increased soil erosion in East Africa’s agro-pastoral systems: changing interactions between the social, economic and natural domains. Regional Environmental Change 19: 1909-1921. https://doi.org/10.1007/s10113-019-01520-9 - Chapter 7: Wynants, M., Millward, G., Patrick A., …, & Blake, W.H. 2019. Determining tributary sources of increased sedimentation in East African Rift Valley Lakes. Science of the Total Environment 717: 137266. https://doi.org/10.1016/j.scitotenv.2020.137266State-of-the-art environmental diagnostic tools were applied to further the understanding of the complex spatial and temporal dynamics in land use and land cover change, soil erosion and sediment transport in East African Rift Catchments. This contribution forms a blueprint for future studies using sediment tracing, radionuclide dating, Bayesian Mixing Models and soil erosion mapping, and their reciprocal integration in this challenging environment. An integrated quantitative assessment of soil erosion and sediment dynamics in the Lake Manyara catchment, northern Tanzania, revealed drastic changes in land cover, a tenfold increase in upstream sediment yield, and a fivefold increase in downstream lake sedimentation over the past 120 years. Integrated spatial analysis identified two tributaries as the main sources of accelerating sedimentation in Lake Manyara. The sediment in the most problematic tributary currently mainly originates from hillslope erosion on the open rangelands and maize croplands in the middle catchment zone. However, detailed historical analysis of upstream sediment deposits revealed distinct changes in source zones, land use types and erosion processes over recent decades. Deforestation, continued cropland expansion and increasing grazing pressures resulted into accelerating rates of sheet erosion. Progressive soil degradation and convergence of surface flows eventually led to a regime shift into a highly incised landscape, where high amounts of eroded soils from all over the catchment are rapidly transported downstream by strongly connected ephemeral drainage networks. Increasing land use pressures are the major driver for the upstream exponential increase in sediment yield. However, on the basin scale, rainfall dynamics and sediment connectivity are important factors for explaining observed changes in downstream sediment delivery. This is illustrated by the dominant contribution of one specific sub-tributary, which has experienced similar exponential increased in sediment yield, but is mainly characterised by a higher sediment connectivity compared to other sub-tributaries, to the total downstream sediment transport. By integrating complementary spatial and temporal evidence bases, this study demonstrated links between land use change, increased soil erosion and downstream sedimentation. Such evidence can guide stakeholders and policy makers in targeted management interventions to safeguard soil health and water quality. To be successful, these management plans need to be tailored to the specific local socio-ecological context, while at the same time being integrated in regional and national governance structures