dc.description.abstract | In this study, a regionalized distributed parameter model, Agricultural Non-Point Source
Pollution (AGNPS) and lumped parameter models, Soil Conservation Services - Curve Number
(SCS-CN) and Rational Formula Methods, were utilized in predicting surface runoff (volume and
peak flow rates) of specific rainstorm events in a semi-arid Watershed, Iiuni, Machakos, Kenya.
The models were applied using input parameters derived from watershed characteristics
(conditioning factors such as soil type, vegetative cover, slope, land use and causative factors like
rainfall amount, distribution and duration). The watershed characteristics were obtained from a
resource survey that was conducted and subsequent interpretations of topographic map, aerial
photographs and Spot Imagery.
Results of runoff volume prediction showed that SCS-CN and AGNPS models gave
satisfactory results under the wet antecedent moisture condition (AMC III). The regression lines for
the models were close to the 1:1 line with acceptable coefficients of determination (r). Likewise all
statistics of comparison (goodness of fit criteria, R2;percentage error, %E; and t statistic) that were
computed were acceptable with R2 being above 70% indicating good model predictions. When
predicting peak runoff discharges, the AGNPS and Rational Formula models performed poorly and
gave low values of R? ofless than 30%.
This study provided some insight into semi arid watershed runoff dynamics through the
determination of the distribution of runoff (e.g. direct runoff and baseflow) within the watershed.
Analysis of watershed runoff data showed that a small fraction 10% (mean) of rainfall constituted
direct runoff (rainfall excess) and direct runoff was a large fraction 66% (mean) of total runoff
Consequently, most of the rainfall is retained within the watershed as expected due to the soil
conservation measures in place. Therefore, it was possible to determine the proportion of storm
runoff outflow (of large rainstorms) that could be conserved for dryland crop production within the
watershed. | en |