Land use change, landslide occurrence and livelihood strategies on Mount Elgon Slopes, Eastern Uganda

摘要

An investigation of the relationship between the physical, pedological and anthropogenic influences on landslide occurrence on the midslopes within and outside Mt Elgon National Park was carried out. One of the landslides occurred in a protected pristine forest environment within the Park while the other two were located at sites deforested for cultivation within and outside the National Park. Field based surveys, GIS techniques and laboratory tests were used to collect and analyze the data. A household survey was undertaken to establish the main community livelihood strategies, the drivers of land use change and implications for land degradation on the mid slopes of Mt Elgon. Aerial photographs taken in 1960 and orthophoto maps formed the benchmark for the analysis of the respective land use changes between 1995 and 2006, using 30m Landsat TM and 20m SPOT MS images in IDRISI Andes GIS environment. Landslide sites were mapped using a Magellan Professional MobileMapperTMCX and terrain parameters were derived using a 15M Digital Elevation Model. A hybrid supervised/unsupervised classification approach was employed to generate land cover maps from which the areal extent of three land cover classes (agricultural fields, woodlands and forests) was calculated. Particle size distribution and atterberg limits were used to test the hypothesis that soils at the landslide sites are inherently ‘problem soils’ where slope failure can occur even without human intervention. Shear strength parameters (internal of friction and cohesion) were used calculate the slope factor of safety to ascertain slope stability at pristine and disturbed landslide sites. Results from the socio-economic survey revealed that smallholder subsistence agriculture and encroachment on the National Park resources are the main sources of livelihoods for the communities surrounding the Park. The communities also have a strong socio-cultural attachment to the National Park, as it is the source of items used during traditional rituals like circumcision. Encroachment is driven by the high population pressure and the prevalent political climate. Farmers mainly use slash and burn technique to prepare land for cultivation and those close to the National Park are reluctant to adopt appropriate farming and soil conservation practices due to the uncertainties surrounding their future on such plots. Slash and burn iii techniques were observed to accelerate various forms of erosion including rills, gullies and sheet. Soil and water conservation techniques were mainly practiced on privately owned farms. The period 1960 and 1995 was characterized by minimal land use changes and no encroachment into the designated Mount Elgon National Park. Conversely, the period 1995 – 2006 marked a significant loss of woodlands and forest cover particularly on steep concave slopes (36º – 58º) within the National Park. The land use change trends were attributed to the prevalent land politics and exponential population growth in the region. The encroachment onto the critical slopes was noted to have induced a series of shallow and deep landslides in the area. Deforestation on Mt Elgon was reported to have both onsite and offsite climate variability and implications in the form of drought, heat waves, flash floods, economic dislocation, crop failure and associated malnutrition in surrounding low lying areas. The soils on pristine and disturbed slopes contain high amount of clay (>10 percent), are fine textured (>50 percent of the material passing the 0.075mm sieve) and highly plastic. These soil attributes imply low permeability, excessive water retention and high susceptibility to expansion and sliding. The vertic nature of soils at Nametsi was confirmed by the extremely high plasticity indices (averaging 33percent), while, high liquid limits at Buwabwala (53 percent) and Kitati (59 percent) qualified the soils as vertisols which are associated with landslides. The results point to the fact that soils at landslide sites are inherently ‘problem soils’ where slope failure can occur even without human intervention. Therefore, the hypothesis that soils at three landslide sites are inherently ‘problem soils’ where slope failure can occur even without human intervention is accepted. Notwithstanding the fact that the study was focussed on mid-altitude slopes of Mt Elgon, the results are in tandem with investigations carried out earlier on the lower densely populated slopes, thus confirming the widespread nature of problem soils on Mt Elgon. There is an urgent need to control human population growth and restore forest cover on the heavily deforested steep slopes particularly within the National Park, and restrain communities from encroaching on the pristine slopes of Mt Elgon. This will be achieved if the politicians, Park Authorities and local communities jointly participate in the design and implementation of CFMs. Future research could focus on climate change implications of deforestation of Mt Elgon environments and quantification of carbon loss related to deforestation and soil degradation in the mountain environments.