An investigation into the causes and dynamics of slope cutting in the Timau area, North Western foot slopes of Mt Kenya
Akal, Akalewold B
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At the North Western foot. slopes of Mount Kenya. bet.ween Nanyuki Town and Kisima Farm. prominent erosion scars with steep, bare and deep edges are widespread on the gently sloping land surface. Unlike the usual gully morphology, it rarely has clearly defined head-cuts and channels. This erosion is associated with land wasting by exposing deep seated subsoil layers. Most of t.h e affected area at present is a small holder crop growing area. The causes time of initiation and changes in the erosion have not been previously documented. In this project, a 122 ha area was used to investigate the above aspects. The nature and conditions of soils, climate, hydrology and land use history of the area were studied. Evidence derived from the land use history indicated that the erosion scars were initiated because of overstocking under communal use of the land by Masai Pastoralists in th e nineteenth century. Overstocking was caused because of the areas transitional location between semi-humid and semi-arid ago-climatic z o n e s , Trampling of s oiI by cattle near watering points or along tracks appears to have exposed sub soil layers which subsequently Could have been deeply scoured by runoff and formed the now prominent erosion sears. The changes in con d i t ion 0 f the erosion were evaluated by comparing air photos taken in 1966 and 1991. The result from the study showed that 2.2 hectares of land were affected by the encroachment of the erosion while. 1.6 hectares of the affected area were reclaimed to good use during the interval. Therefore, the total extent of the affected area in 1991 was found to be 1.1% or 0.4 ha more than what it used to be in 19136. From a seven month monitoring of soil movement from cut edges using silt traps a total of 44 and 24 kg/m2 was eroded from cut edges located in grazing and cropping land use catchments respectively. Those results showed that a substantial quantity of soil is still being washed down to the bed from the cut edges. Soil texture, bulk density, organic matter and exchangeable base content, water inf i1tration, retention and 1iquid 1imi t of layers in several profiles were evaluated as part of .the soil, investigation. Among those parameters, non-uniform variation of bulk density, water intake and water holding capacity in depth were observed and described as factors contributing to the instability of the exposed sub-soiIs. Relatively impermeable layers impeding uniform moisture transmission were found between loose layers. Occasional heavy flooding (eg. 1961) were one of the major factors in the formation and development of the cut edges. Also, occurrence of heavy storms at the end of the dry season when the ground cover is poor appears to have been enhancing instability of the cut edges due to washing and scouring. The erosion scars result from a combination of the above factors. Results from air photo analysis and experience of older settlers confirm that the rate of expansion of the slope cutting is slow. Trends with recent small holder land use show that more eroded area can be changed to good use • The small scale farmers should give emphasis to conserving and reclaiming the eroded bed. A stage by stage treatment of the bed strips with "fanya juu" bunds would trap eroded soil. The eroded soil could be fertilized by organic wastes and later re-seeded with grass after diverting runoff. A guide to this approach is included in the thesis.