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Wageningen Universiteit (2003)

A hierarchical method for soil erosion assessment and spatial risk modelling : a case study of Kiambu District in Kenya

Okoth, P.F

Titre : A hierarchical method for soil erosion assessment and spatial risk modelling : a case study of Kiambu District in Kenya

Auteur : Okoth, P.F.

Université de soutenance : Wageningen Universiteit

Grade : Doctor of Philosophy (PhD) 2003

Résumé partiel
Though a lot has been done and achieved in erosion research and control in Kenya, most of the erosion research methods have in the past put emphasis more on quantifying soil loss or measuring soil erosion, rather than pinpointing to areas that are likely to suffer soil erosion. In most cases the erosion processes have been assumed to occur in a uniform manner at all levels of the landscape hierarchy, and hence the results of one level observation can be factored to cover other levels for which data was not collected. This has resulted in many people extrapolating site-specific point data to cover wider geographic regions, assuming uniformity of the erosion process over the region. Another interesting aspect of soil erosion is that though most attention is normally put on the negative effects of soil erosion, soil erosion has also some beneficial effects. For example, the deposition of eroded soil material to lower areas has sometimes improved the quality of the soil receiving the sediment, thereby improving agricultural productivity of the depositional areas. There have however been suggestions that the problem of soil erosion has been exaggerated and not proven to actually diminish crop yields against the background of improved crop productivity improvement techniques. All theses schisms makes it necessary to engage in soil erosion research, either to disprove the sceptics or to provide other means of assessing and viewing the problem of soil erosion. The general objective of the thesis was to develop and present a method, which can be used to assess the risk of water erosion for different levels of the landscape system hierarchy using spatial methods. The broad aim was to define relevant levels that form the basis for predicting and managing soil erosion and controlling its risk. The specific objectives were : To conceptualize and define from the landscape continuum hierarchically ordered landscape elements whose internal characteristics and parts influence the occurrence of soil erosion and whose spatial extent and geometry enable their capture and modelling by remote sensing and GIS. To prove that there are spatial features (erosion proxies) which are part-of, and internally contained in the hierarchically defined landscape elements that can aid in soil erosion risk assessment and modelling. To demonstrate that the selected erosion proxies can be related to actual occurrences of soil erosion by statistical methods and similarly be differentiated as either drivers or disrupters of erosion. To demonstrate that prediction models can be derived from field data collected on the erosion proxies and the developed models used for modelling of soil erosion risk spatially in a GIS for each of the defined levels. To test and validate the method in Kiambu. To address these objectives, concepts associated with hierarchy theory, landscape system construction, geographic information systems theories, and soil erosion theories were knitted together to develop a conceptual framework and a practical methodological approach to effect and realise the objectives. Kiambu district was selected for testing the developed methodology due to its intensive utilisation for agriculture and its location in a rugged terrain in the upper footridges and footslopes of the Aberdare Mountains where below-canopy soil erosion is obscured by vegetation vigour and intensive cropping. Soil loss studies through river sediment yields in the district, indicate that there are high amounts of soil lost annually by water erosion. These range from 20 t km -2yr -1in undisturbed forests, to 3000 t km -2yr -1in cultivated to grazing lands. Soil loss studies from runoff plots in Kiambu indicate that cultivated land loses between 20 and 30 t ha -1season -1and bare soil loses more that 70 t ha -1season -1. Other justifications were prompted by the fact that soil conservation in Kenya has been focussed to the ’Catchment Approach’ without necessarily defining what the catchment means.


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