Understanding sediment dynamics and hydrology to manage water resrouces in a tropical montane forest of Kenya

Stenfert Kroese, Jaqueline and Rufino, Mariana and Quinton, John (2020) Understanding sediment dynamics and hydrology to manage water resrouces in a tropical montane forest of Kenya. PhD thesis, Lancaster University.

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Abstract

Montane forests are unique ecosystems in the tropics and they regulate soil and water functions at the landscape scale. Their conservation is important because forests contribute with their abundant above and belowground biomass to increased soil stability and reduced soil erosion. However, tropical forests are the hotspots of land use change mainly due to the fertile soils and the mild climatic conditions where they grow. In the East African highlands, the demand for agricultural land by an increasing human population and the cultivation on steep hillslopes put pressure on forest and water resources. Streams in these montane catchments are often enriched in suspended sediments, which affect water quality and represent a loss of soil capital. However, these tropical montane ecosystems are understudied, especially in sub-Saharan Africa. Having a scarcity of studies on sediment dynamics and hydrology makes it more difficult planning for future sustainable land use. To contribute towards closing this knowledge gap, this thesis uses three study catchments (27-36 km2) under different land use (e.g. natural forest, tea-tree plantations and smallholder agriculture) as a ‘microcosm’ within which to understand process response to disturbance within the headwaters of the Sondu River Basin (3,470 km2) in the Mau Forest Complex of Kenya. A four-year high-resolution sedimentological time series recorded the highest sediment concentrations in the smallholder agriculture catchment, followed by the tea-tree plantation and the natural forest catchment, caused by increased surface runoff. Rainfall-runoff modelling showed that soils of the natural forest catchment had high permeability reflected in the deep water flow pathways, in contrast to the compacted soils in the smallholder agriculture catchment with a dominance of shallow sub-surface flow to surface runoff. Sediment response to rainfall (up to 3.5 hr) was delayed in the smallholder agriculture catchment compared to the nearly instantaneous response (<1.5 hr) in the forested catchment due to sediment supply from near-by sources. Sediment fingerprinting conducted at the smallholder agriculture catchment unravelled the relative contribution of four different sediment sources. Agricultural land accounted for the largest contribution (75% with 95%-confidence interval 63-86%) of the total sediment, while channel banks, gullies and unpaved tracks were shown to be local sediment hotspot sources. Suspended sediment collected with time-integrated, manual- and automatic-event based sediment samplers at the outlet of the three catchments over a period of up to four months, demonstrated that particulate carbon and nutrient concentrations were up to three times higher in the natural forest compared to the smallholder agriculture catchment. The low particulate macronutrient concentrations point to the fast impoverishment of agricultural soils after deforestation. The findings of the study clearly show that land use change has an important impact on sediment dynamics and hydrological pathways, which can affect the water balance of the whole ecosystem and deteriorate downstream water supplies and the water quality of Lake Victoria. The findings of this study further contribute to the wider knowledge of other tropical montane systems facing similar pressures like agricultural expansion and deforestation.