| dc.description.abstract | The Green-Ampt model was used to evaluate the infiltration
parameters by testing it on Sambret and Lagan catchments of Kericho
district, Kenya. The comparison of observed and predicted surface
runoff formed the basis for evaluating the parameters in the
Green-Ampt infiltration model. The model parameters were obtained
from texture based nomographs emanating from United States
Department of Agriculture-Soil Conservation Service (USDA-SCS).
Values of antecedent soil moisture were obtained from available
records. The nomographs have not been previously examined in Kenya.
The storm hyetographs and hydrographs provided the necessary
hydrologic information in the analysis.
For the selected events In all catchments, surface runoff was
over estimated by the model both in its original form, for ponded
conditions and the modified form, for predicting rainfall
infiltration. This was regardless of the influence of interception
which was estimated as a percentage of storm rainfall for each
event considered. Poor prediction by the original Green-Ampt model
was attributed to its poor representation of infiltration during
rai~fall events. It assumes immediate ponding at the onset of rain
so chat; surface runoff starts at the moment rainfall comnences. The
result is an ove~rediction of surface runoff. Overorediction of
sur:::acer..moffby the modified Gree!1-Fmptmodel was at~ributed to
the low values of saturated hydrauiic conductivity (Ko) and
x
effective porosity (85) predicted from the nomographs. A low value
of ~ implies that less water is allowed to infiltrate resulting in
high surface runoff volume. A low effective porosity (85) means a
reduced moisture deficit due to the small difference between the
porosity and antecedent soil moisture.
Attempts were made to establish a correct value of ~ for each
catchment through an optimization and validation exercise. K, lS
the most variable and unpredictable parameter. 85 is a function of
soil texture and varies only slightly with land use changes. The
measured value of 85 (85=0.7crn3/crn3)obtained from records was used
In the procedure of optimization and validation. Saturated
hydraulic conductivity (~) for Sambret, Sambret sub-catchrnent , and
Lagan catchments were found to be 0.03, 0.04, and 0.05crn/h
respectively. Since these optimized values were those obtained with
interception taken into consideration, the results therefore show
that interception has a significant effect on the rainfallinfiltration-
runoff process in all the catchments studied. These
results also show that the nomographs require modifications before
they can be used on Kenyan catcr.ments.
The infiltration rate curve derived from the original Green-~npt
model was compared to the curve obtained from double ring
infiltrometer measurements. The results indicaced that the
concencric cylinder measurements yield highly variaele and
UDxeliable results which must be used cautiously. | en |
