dc.description.abstract | Degradation of watershed areas in Kenya’s basins is on the increase and is currently a major concern
for the government. The main causes of watershe
d degradation stem from the abuse and poor
management of forests and
soils, overgrazing, extension of
settlements into watershed areas, and
unsuitable felling of trees for fuel wood. Recent legislative reforms in the water and environmental
sector have been introduced to stem these environmental negative trends. However, extensive
quantitative hydrologic analysis is necessary for the assessment of the water balance of various basins
to form a basis for policy actions
. In this regard, modelling the hydrologic cycle at a local scale still
remains the most important
scientific method of research for the water balance assessment of basins.
The study area chosen in this study is the Nzoia basin in Kenya. This basin is a typical example of a
flood disaster prone basin
experiencing increased flood related disasters due to the increased watershed
degradation in the recent past. The Nzoia
basin is situated between latitudes 1030’N and 0005’S andbetween longitudes 340E and 350
45’E and is the largest basin in
Kenya’s Lake Victoria basin with an
approximate area of 12,709km2
and a length of 334km to its outfall in
to the lake. The Nzoia system has
its sources in the forested highlands (Mt. Elgon,
Cherangani Hills, Nandi Hills and Kakamega forest).
The objective of this study is to simulate streamflow changes as a result of the land use/cover status as
at 1973, 1986 and 20
00. Land use/cover data were
based on Landsat images for these years. The
runoff response as a result of the observed land use/cover change was test
ed by keeping constant all
input datasets in a SWAT model and varying the land use. The results from the model showed that with
the expansion of the area under agriculture, the stream flow increases during the rainy seasons and
reduces during the dry seasons, whereas when the area under forest cover is increased the peak stream
flow reduces, but when the forest cover is reduced to almost zero there is an increased peak and mean
stream flow in the basin.
It is therefore worth noting that a decrease in surface runoff would be desirable, as this would also
decrease the devastating effects of floods; the rapid expansion of urban centres in the lower parts of the
catchment (Mumias, Bungoma, Rwambwa) can be said to be a major contributing factor to the annual
devastating floods. The results also
indicated an increasing trend in rainfall amounts in parts of the
basin between the periods 1970 - 1998. A study
of three rainfall stations (1BD02, 1DA02 and
1DD02A) has shown a significant increase in
rainfall while one station, in the lower part of the
catchment (EE01), has shown a significant decrease.
The area under forest cover decreased betwe
en 1970’s and 1986 by 6.4% in the northwest and south of
the catchment. But between the 1980’s and the 2000’s there
was an increase in
area under forest cover
by 41.3%. Agricultural land use showed
an increase in areal coverage between 1970’s and 1986 by
6.7%, but in the year 2000’s the agricultural activities declined by
4.6%. The area under
bushland/shrubland/riverine agriculture increased between the 1970’s, 1986 and the 2000’s by about
123.4% and 11.10% respectively. This could be as a
result of an expansion in riverine agriculture | en_US |