| dc.description.abstract | 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. | en |
