Browsing by Author "Poesen, J."

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    Effectiveness of tropical grass species as sediment filters in the riparian zone of Lake Victoria
    (Soil Use and Management, 2012) Wanyama, J.; Herremans, K.; Maetens, W.; Isabirye, M.; Kahimba, F.; Kimaro, D.; Poesen, J.; Deckers, J.
    The effectiveness of tropical grass species in strips of different length in trapping sediment from cropland was assessed, and the influence of filter length was determined. The assessment was made under natural rainfall which induced sheet and rill erosion in run-off plots and then using simulated run-off which caused concentrated erosion. The evaluated grasses were elephant grass, lemon grass, paspalum and sugarcane. Run-off plots were on a 10% slope in a randomized complete block design replicated three times. Filter lengths were 2.5, 5 and 10 m against a 10-m-long sediment source area planted with maize on a clay loam soil. The results show that sediment trapping effectiveness (TE) increases nonlinearly with increasing filter length for all grasses. Under natural rainfall, more than 70% of sediment was trapped in the first 5 m, and lengthening the strip to 10 m only resulted in a marginal increase in TE. With concentrated run-off, more than 70% of sediment was trapped in the first 5 m and lengthening the strip to 10 m resulted in a significant increase in TE. Paspalum and lemon grass performed significantly better than other grasses (P < 0.05), owing to their spreading growth pattern over the soil surface. Paspalum also has the highest root density in the upper 0.3-m layer of the soil followed by lemon grass, hence offering the greatest resistance to erosion from concentrated flow. The results demonstrate that tropical grass filter strips provide a viable means for reducing the sediment flux from cropland.
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    Estimating the effect of tree uprooting on variation of soil horizon depth by confronting pedogenetic simulations to measurements in a Belgian loess area
    (Journal of Geophysical Research: Earth Surface, 2013) Finke, P. A.; Vanwalleghem, T.; Opolot, E.; Poesen, J.; Deckers, J.
    Spatial patterns of soil often do not reflect those of topographic controls. We attempted to identify possible causes of this by comparing observed and simulated soil horizon depths. Observed depths of E, Bt, BC, C1, and C2 horizons in loess-derived soils in Belgium showed a weak to absent relation to terrain attributes in a sloping area. We applied the soil genesis model SoilGen2.16 onto 108 1 × 1 m2 locations in a 1329 ha area to find possible causes. Two scenarios were simulated.Model 1 simulated soil development under undisturbed conditions, taking slope, aspect, and loess thickness as the only sources of variations. Model 2 additionally included a stochastic submodel to generate tree-uprooting events based on the exposure of trees to the wind. Outputs of both models were converted to depths of transitions between horizons, using an algorithm calibrated to horizon depths observed in the field. Model 1 showed strong correlations between terrain attributes and depths for all horizons, although surprisingly, regression kriging was not able to model all variations. Model 2 showed a weak to absent correlation for the upper horizons but still a strong correlation for the deeper horizons BC, C1, and C2. For the upper horizons the spatial variation strongly resembled that of the measurements. This is a strong indication that bioturbation in the course of soil formation due to treefalls influences spatial patterns of horizon depths.
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    Landslides in a densely populated county at the footslopes of Mount Elgon (Uganda): Characteristics and causal factors
    (Geomorphology, 2006) Knapen, A.; Kitutu, M.G.; Poesen, J.; Breugelmans, W.; Deckers, J.; Muwanga, A.
    Manjiya County on the Ugandan slopes of Mount Elgon is a densely populated mountainous area where landslides have been reported since the beginning of the twentieth century. The numerous fatalities and the damage done during the extreme rainfall events of 1997 to 1999 drew attention to this phenomenon. In order to better understand the causal factors of these landslides, 98 recent landslides in the study area, mostly debris slumps, were mapped and investigated. Together, they displaced 11 millions m3 of slope material. Statistical analysis shows that landslides dominate on steep concave slope segments that are oriented to the dominant rainfall direction (northeast) and at a relatively large distance from the water divide. Based on landslide occurrence and impact, four different zones can be distinguished within the study area. Causal factors as well as landslide characteristics differ greatly between the four zones. Besides the fact that steep slopes, high rainfall and typical soil properties and stratification turn Manjiya into an inherently unstable area, human interference cannot be neglected. Whereas deforestation has reduced the stability of the shallow soils on the eastern slopes of the study area, the excavation of slopes, mainly for house building, is an important destabilizing factor for the western slopes. The growing population density not only increases the risk on damage, but hampers the search for solutions for the landslide problem as well.