| dc.description.abstract | For engineering design and planning purposes,
forecasts and synthesis of hydrological information is
often very necessary. These are usually obtained through
the use of hydrological models.
This study aims at developing and evaluating the
skill of a rainfall-runoff model over the upper Athi
river catchment. The model is based on catchment subareas
centred about lines of constant time of travel for water
to the outlet (isochrones).
The study was subdivided into several parts. The
first part of the study concentrated on the estimation of
missing rainfall and discharge records using arithmetic
mean method and the isopleth method. This was followed
by testing the quality of both the estimated and observed
records using mass curve analysis. Only single straight
lines could be fitted to all mass curves at all
locations. The homogeneous records were subjected to
Principal Component Analysis (PCA) inorder to determine
spatial similarities in the characteristics of rainfall
within the catchment. The Principal Component Analysis
(PCA) results, based on Kaiser's criterion (Kaiser,
1959), indicated that only one Principal Component was
dominant over the catchment. The magnitude of the
loading of this component was greater than 0.85 at all
locations. The point rainfall records were then areally
averaged using arithmetic mean method.
The second part of the study investigated the
statistical characteristics in the discharge and rainfall
records through time series analysis. The statistical
parameters which were computed included the
autocorrelation, auto-spectra, cross-correlation, cospectra,
quadrature spectra, coherence and phase. The
spectral analysis results indicated the dominance of
quasi-periodic fluctuations during the major rainfall
seasons and dry months. These were centred around 2-2.5
and 2.8-3.5 days. The 5-6 days cycle was also observed
during the dry seasons. Cross-spectral analysis
indicated that rainfall was leading discharge. The lead
time ranges from 1 to 7 days. The quasi-periodic
fluctuation were still evident from cross-spectral
analysis.
Finally the model was developed through plotting of
the isochrones using the cross-correlation and coherence
functions. The Kernel functions for the model were then
computed through matrix solution using coincident
rainfall and discharge records. The longest rainfalldischarge
response was also centred within 1 to 7 days.
Peak response values were however centred within 1 to 5
days. Tests of skill based on residual error test, using
the data which were not used during the model
development, indicated that the model gave reliable
forecasts of discharge during most of the cases a part
from few extreme cases. | en |
