Investigations on the Morpho Climate and On the Morphology of the Inselberg Region of Machakos, Kenya

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Ahnert F.
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The inselberg region of Machakos is located in the south-eastern part of the highlands of Kenya, on the boundary between the exposed pre-Cambrian basement and its Cenozoic volcanic cover. The morpho climate of this region is controlled by the seasonal fluctuation of the Southeast Trade Winds which are the principal suppliers of moisture from the Indian Ocean. The area has two rainy seasons separated by a short (Jan.-Feb.) and a long (May-Oct.) dry season. Mean annual precipitation varies within the region from more than 1300 mm to less than 600 mm, predominantly as a function of relief and exposure. The year-to-year rainfall variation of individual stations has ratios ranging from about 3: 1 for the more humid stations to about 8: 1 for the dryer ones. As a remarkable rule it was found that the mean precipitation per rainy day is higher at the arid stations than at the humid ones, but that at any individual station it is higher in wet years than in dry ones - i.e. the spatial and temporal patterns do not correspond. Magnitude-frequency analysis for Machakos indicates about 12 days per year with more than 25 mm rainfall (an approximate threshold for overland flow), one day per year with more than 65 mm, and one day in five years with more than 90 mm. Magnitude frequency analyses of shorter-duration high intensity rains corroborate the finding that overland flow events of relatively low magnitude and moderate frequency are cumulatively most effective as agents of denudation.
Bulletin, p. 1-72