| dc.description.abstract | The purpose of the study was to investigate the
hydrological characteristics of the arid and semi-arid
lands (ASAL) of Kenya with a view of establishing the water
resources potential of ephemeral streams in providing
supplementary water needs. Laikipia district which has
undergone remarkable land use changes resulting in water
use stress of perennial river abstractions and ground
water exploitation in the recent years has been used as a
model of the Kenyan ASAL. Long term availability of
hydrological records was limited due to poor spatial
distribution of the hydrological network over the Ewaso
Ngiro basin. Further, the river flow data was only
available on the perennial streams of the basin. Five
sub-catchments areas in the Upper Ewaso Ngiro basin were
chosen for experimental design. The period of experimental
study extended from January 1989 to December 1991.
The available hydrological records and the data obtained
from the experimental study were subjected to spatial and
temporal analyses. Spatial analyses included correlation
and principal component analysis (peA) while temporal
analysis involved the determination of diurnal, seasonal,
cyclical and inter-annual characteristics including
spectral and cross spectral techniques .
simple transfer functions and complex Transfer Function
Noise, TFN (r,b,s) models were used to quantify the
rainfall-runoff and runoff-sediment relationships for the
individual experimental sub-catchments. The univariate
ARIMA p,d ,q) models were also used to quantify the
annual long term hydrological records. The quality of all
the records was first investigated before carrying out the
above analyses.
The study delineated the ASAL area into five
hydrologically homogenous zones based on results from
peA. The physical significance of the delineated zones
were confirmed through inter-correlation and vector plot
analyses together with the geophysical characteristics of
the area. In each of these zones, a representative
network was established, for which additional hydrological
data is now available for-ASAL water resources planning in
the light of increased settlements and population growth.
Runoff episodes for individual sub-catchments indicated
remarkable variations and contrasting responses among the
sub-catchments. A 160 mmjh storm intensity caused a runoff
peak rate of 1.456 m3jsjkm2 in Sirima while only 57.2 mmjh
peak storm intensity caused a peak runoff rate of 6.052
m3jsjkm2 in Mukogodo. A minimum of 17 runoff events were
observed in Matanya while a maximum of 67 events were
obtained in Ngenia and the ephemeral channels were
continuously dry for maximum periods ranging from 118 to
338 days during the experimental period. Although the
ephemeral catchments have a surface water supply potential
of at least 8,000 m3 per km2 per year, major threats were
identified as sediment transport, high evaporation rates
and intensified land use changes.
The linear rainfall-runoff models for events showed
initiation of runoff at threshold rainfall of between 8 mm
and 18 mm . which characterised sub-catchment storage.
Whereas the annual perennial streamflows were fitted by
ARIMA (1,0,1) model, the annual rainfall indicated ARIMA
(3,0,1) model to be appropriate due to the inherent
complex rainfall-runoff relationships. The TFN (r,b,s)
models based on 15 minutes rainfall-runoff event time
series identified TFN (1,1,1) as the optimum model while
the TFN (2,0,1) model fitted the corresponding runoffsediment
series. Whereas runoff lagged the input rainfall
by one time unit, the runoff and sediment occurrences were
instantaneous.
Ephemeral streams therefore have a high potential of water
harvesting which could serve as supplementary water
sources in the ASAL parts of Kenya. Its utilization
however requires proper conservation measures which may
enhance replenishment of the ground water systems. | en |
