Soil Erosion and Conservation in Western Kenya

在肯尼亚西部的维多利亚湖流域，土壤侵蚀正在发展成大的沟壑，同时严重威胁着居民。本研究旨在了解该地区沟道侵蚀的基本过程，并提出流域尺度保护评价的思路。（1）暴雨期间，过度集中的地表水流在表层土和下面的第四纪沉积物上切割出大的沟壑，这些沉积物填充了卡隆多裂谷。（2）年侵蚀速率高达20 m的头切后退。（3）根据岩石地层学证据，将第四纪沉积划分为上Awach段和下Sondu段。Sondu段沉积物的硬度值相对高于Awach段沉积物，这主要是由于胶结碎屑颗粒的氧化铁含量不同和孔隙度不同。（4）沟道侵蚀在侵蚀形态、切头后退速率等方面存在差异。这些特征是侵蚀力复杂组合的结果，如坡面流浓度和悬浮负荷密度，以及岩性。这些因素还受上坡集水区、植被覆盖、土地利用、降水和地质背景的控制。（5）为了三个目的进行了三种类型的图像分析。分析的图像数据是地形图、遥感卫星图像和航空照片。结合这些影像资料的分析结果，可产生上坡集水区的分布图、冲沟型态与资讯。最后进行了叠加分析，以制作侵蚀/贡献区的危险警告图。

In the Lake Victoria Basin in Western Kenya, soil erosion is developing into large gullies while are seriously threatening the inhabitants. This research project aims at understanding the basic process of the gully erosion and proposing ideas of assessment for catchment-scale conservation in the area.(1)Excessive concentration of overland flow during heavy rains has incised large gullies on both topsoil and underlying Quaternary sediments that infill the Kavirondo rift.(2)The annual erosion rate is as high as 20m head cut retreat.(3)Based on the lithostratigraphic evidence, the Quaternary sediments are classified into upper Awach member and lower Sondu member. Hardness values of the Sondu member sediments are relatively higher than the Awach member sediments and this would be due largely to the different content of ferric-oxide that cements the detrital grains and to the different porosity.(4)Gully erosion varies in its physiognomy such as eroding patterns and head cut retreat rate. These characteristics are the result of complex combinations of eroding forces like overland flow concentration and suspended load density, and lithological properties. These factors are also controlled by upslope catchment areas, vegetational coverage, land uses, precipitation and geological backgrounds.(5)Three types of image analysis have been carried out for three purposes. Analyzed image data are topographic maps, remotely sensed satellite imagery and aerial photographs. Results of the analyses of these image data were combined to produce the distribution map, gullying patterns and information of the upslope catchments. Overlay analysis has finally been made to produce hazard warning maps for eroding/contributing zones.