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University of Zurich (2008)

Assessment of rainfall, erosivity, runoff and erosion in the area north and west of Mount Kenya

Meister, Rahel

Titre : Assessment of rainfall, erosivity, runoff and erosion in the area north and west of Mount Kenya

Auteur : Meister, Rahel

Université de soutenance : University of Zurich, Switzerland

Grade : Master 2008

Résumé
This study involves the analysis of rainfall, erosivity, runoff and erosion data of eight research stations around Mount Kenya in the Central Highlands of Kenya. The altitude of these stations ranges from 1’740 (plateau) to 3’048 m a. s. 1. (mountain slopes). The climate in the area varies from semi-arid on the footslopes to sub-humid on the mountain slopes. The climate of the whole region is, among other factors, influenced by the Indian Ocean and experiences clearly higher rainfall amounts during an .El Nino’ event. The soils of the mountainous stations (andosols and acrisols) are .generally deeper, contain more organic matter and have a higher water storage capacity than the ones on the plateau (luvisol, phaeozem, vertisol and lixisol), which are used for (small-scale) agricultural purposes : Small-scale farmers practise rainfed crop production in the study area and pastoralists need sufficient grazing sites for their livestock. Two of the main issues which affect the people in the area is water loss through runoff and erosion on over grazed areas. Intensification in land use (due to an increase of population numbers), cutting of trees for land gain and charcoal production has aggravated the problem of surface runoff and land degradation. This study aims to examine rainfall erosivity, assess the amounts of soil loss and erosion in the study area and to determine the influence of ground cover on runoff and erosion. Data included in this work involved measurements of rainfall amounts and intensity as well as runoff and erosion experiments on various land use treatments and under different cover conditions during several years. This research project was initiated by a collaboration ofthe University of Nairobi and the CDE Berne. Analysis of the number of erosive rainfall events (I30 > 25 mrn/h) showed that the mountainous stations have more rainfall days per year but experience a smaller percentage of erosive storms than the plateau stations. This led to the interesting conclusion that all stations experience a very similar number of erosive events (on average 4 to 7 per year) whereas the total amount of annual rainfall varies greatly among the stations and increases with altitude. In terms of runoff and soil loss amounts, it can be concluded that erosion is not the main problem in the area. In other East African countries such as Ethiopia, erosion rates may exceed 100 t/ha/year and more, while in the area around Mount Kenya the average annual soil loss rates seldom reach 40 t/ha. Runoff on the other hand is seriously affecting agriculture by clearly reducing the annual plant-available water amounts by up to 50 %. Furthermore, high evaporation rates aggravate the water shortage and as a result, crop failures occur. It can be stated that runoff and erosion rates are higher during the long rains (April to June), which is the main rainy season of the year, than during the short rains in November. Generally, a negative relationship between ground cover [%] and runoff [% of rainfall] could be observed, indicating that cover is a runoff determining factor but other parameters such as antecedent soil moisture and rainfall erosivity proved to be important too. However, a multiple linear regression analysis showed that more parameters (such as organic matter content or the degree of crusting) would probably help to better explain the variation in runoff. The results of the test plot experiments can only partially be transferred into reality due to the small scale of the experiments. On the one hand, erosion rates might be overestimated for real catchment soil loss rates since deposition could not be measured with these small-scale experiments but on the other hand, gully erosion may occur on large grass land areas, thus exceeding the measured amounts of soil loss. Finally, common soil and water conservation practises such as mulching or reduced tillage were described and examined as possible approaches to address the degradation

Sujets : Rain ; Stormwater ; Soil erosion ; Mount Kenya ;

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Page publiée le 11 janvier 2017