https://repository.seku.ac.ke/handle/123456789/83 2026-04-04T15:08:54Z 2026-04-04T15:08:54Z Wambua, Raphael M. https://repository.seku.ac.ke/handle/123456789/7197 2023-11-30T09:58:52Z 2019-05-17T00:00:00Z

Title: Spatio-temporal drought characterization and forecasting using indices and artificial neural networks. A case of the Upper Tana River basin, Kenya Authors: Wambua, Raphael M. Abstract: Drought is a critical stochastic natural disaster that adversely affects water resources, ecosystems and people. Drought is a condition characterized by scarcity of precipitation and/or water quantity that negatively affects the global, regional and local land-scales. At both global and regional scales, drought frequency and severity have been increasing leading to direct and indirect decline in water resources. Increase in drought severity and frequency in the upper Tana River basin, Kenya, water resources systems have been adversely affected. Timely detection and forecasting of drought is crucial in planning and management of water resources. The main objective of this research was to formulate the most appropriate models for assessment and forecasting of drought using Indices and Artificial Neural Networks (ANNs) for the basin. Hydro-meteorlogical data for the period 1970-2010 at sixteen hydrometric stations was used to test the performance of the indices in forecasting of the future drought at 1, 3, 6, 9, 12, 18 and 24-months lead times, by constructing ANN models with different time delays. Drought conditions at monthly temporal resolution were evaluated using selected drought indices. The occurrence of drought was investigated using non-parametric Man-kendall trend test. Spatial distribution of drought severity was determined using Kriging interpolation techinique. In addition, a standard Nonlinear-Integrated Drought Index (NDI), for drought forecasting in the basin was developed using hydro-meteoroogical data for the river basin. The results of spaial drought show that the south-eastern parts of the basin are more prone to drought risks than the north-western areas.

2019-05-17T00:00:00Z Wambua, Raphael M. https://repository.seku.ac.ke/handle/123456789/6891 2023-11-30T09:58:01Z 2022-10-24T00:00:00Z

Title: Drought assessment and forecasting using indices and artificial neural networks for the upper Tana River basin, Kenya Authors: Wambua, Raphael M. Abstract: Drought is acritical stochastic natural disaster that adversely affects water resources, ecosystems and people. Drought is a condition characterized by scarcity of precipitation and/or water quantity that negatively affects the global, regional and local land-scales. At both global and regional scales, drought frequency and severity have been increasing leading to direct and indirect decline in water resources. For instance, increase in drought severity and frequency in the upper Tana River basin, Kenya, water resources systems quantity and quality have been adversely affected. Timely detection and forecasting of drought is crucial in planning and management of water resources. The main objective of this research was to formulate the most appropriate models for assessment and forecasting of drought using Indices and Artificial Neural Networks (ANNs) for the basin. Hydro-meteorlogical data for the period 1970-2010 at sixteen hydrometric stations was used to test the performance of the indices in forecasting of the future drought at 1, 3, 6, 9, 12, 18 and 24-months lead times, by constructing ANN models with different time delays. Drought conditions at monthly temporal resolution were evaluated using selected drought indices. The occurrence of drought was investigated using non-parametric Man-kendall trend test. Spatial distribution of drought severity was determined using Kriging interpolation techinique. In addition, a standard Nonlinear-Integrated Drought Index (NDI), for drought forecasting in the basin was developed using hydro-meteoroogical data for the river basin. The performance of the drought forecasting models at the selected lead times were assessed using Mean Absolute Error (MAE), correlation coefficient (R), Nash-Sutcliffe Efficiency (NSE), Ratio of mean square error (RSR) and modified index of agreement (d1). The results of spaial drought show that the south-eastern parts of the basin are more prone to drought risks than the northwestern areas. The Mann-Kendall trend test indicates an increasing drought trend in the south-eastern and no trend in north-western areas of the basin at both 90 and 95% significant levels. Another output of this research was the development of Surface Water Supply Index (SWSI) function, NDI and characteristic curves defining the return period and the probability of different drought magnitudes based on Drought Indices (DIs). In addition, drought Severity-Duration-Frequency (SDF) curves were developed. The formulated NDI tool can be adopted for a synchronized assessment and forecasting of all the three operational drought types in the basin. The results can be used in assisting water resources managers for timely detection and forecasting of drought conditions in prioritized planning of drought preparedness and early warning systems.

2022-10-24T00:00:00Z Nassar, Shamim O. https://repository.seku.ac.ke/handle/123456789/6801 2023-11-30T09:57:33Z 2022-06-02T00:00:00Z

Title: Miniaturised multilayer RF and microwave circuits Authors: Nassar, Shamim O. Abstract: Ceramic and laminate multilayered technologies are explored in the design of novel circuit topologies and in novel implementations of classical circuit topologies. A cross-slot coupled filter topology implemented in folded substrate integrated waveguide (FSIW) is proposed and is shown to exhibit properties that make it favourable for diplexer design. A C-Band diplexer design is presented. The diplexer is fabricated in both Liquid crystalline Polymer (LCP) and Printed Circuit Board (PCB) multilayed technology. The viability of both processes for this type of circuit is analysed and performance is verified by simulation and measurement. A ’ridge-like’ folded substrate integrated waveguide resonator is proposed. A comparative analysis of this resonator and a traditional ridge waveguide resonator structure in substrate integrated technology is presented. For rectangular waveguide resonators with identical outer dimensions, the former is shown to achieve lower operational frequencies relative to the latter. Two X-band filters are designed using the ’ridge-like’ FSIW resonator. Both filters are fabricated in PCB multilayered technology and performance is verified by both simulation and measurement. The measurement results of the first, a second order filter, show a maximum insertion loss of 2.23 dB for the primary band and a wide frequency range of 7.5 GHz between the first passband and the second. The second filter is a fourth order filter which achieves a maximum measured insertion loss of 4.7 dB for the primary passband with the second passband occuring over 8.5 GHz away. A classical sequence asymmetric RC polyphase filter (PPF) is implemented in low temperature co-fired ceramics (LTCC) technology. The novel implementation realises a miniaturised structure comprising embedded components interconnected using planar transmission lines. Verification of the structure’s performance is by simulation of a three segment PPF. The LTCC PPF is shown to achieve an image suppression of 35 dB over a frequency range of 100 MHz to 300 MHz and a gain error of 0.14dB between its quadrature outputs. The final design is a novel implementation of a miniaturised two-stage PIN Diode limiter-switch in LTCC. The structure comprises both embedded components and surface mount components interconnected using planar transmission lines and vias. Circuit viability is assessed through simulation of a multilayered L-Band limiter-switch design. An insertion loss of 0.25 dB and a return loss of 25.34 dB at a center frequency of 1.3 GHz are obtained when the switch is in its off-state. When a large 1 ms input pulse signal of 45 dBm is applied at the input of the limiter-switch, the resulting output pulse has a flat leakage of approximately 16.4 dBm. Description: Doctor of Philosophy in Engineering, 2016

2022-06-02T00:00:00Z Kituu, Gareth M. https://repository.seku.ac.ke/handle/123456789/872 2023-11-30T10:07:16Z 2015-02-10T00:00:00Z

Title: Application of Genetic Algorithms in the Optimization of a Solar Tunnel Fish Dryer Design and Performance Authors: Kituu, Gareth M. Abstract: The fish industry in Kenya, with a production of over 350,000 metric tonnes earns USD 105 million which accounts for about 5% of the national Gross Domestic Product, provides 3% of skilled and unskilled employment. However, this industry is threatened as the fish is harvested at high moisture content of about 5kg/kg, dry basis, and at this moisture content, if not preserved, fish undergoes spontaneous spoilage in 24 hours. At the artisanal fishermen level, the most viable preservation option is solar drying, in which fish is enclosed in a solar dryer, shielding it from contamination, and destruction. Depending on the conditions in the dryer, fish can either be over-dried or under-dried, resulting in heavy losses at household and national level, and therefore, a conducive environment must be provided within the dryer to avoid destruction of fish during drying. Based on the above observations, studies were conducted with the objective of optimising the design parameters and performance of a solar tunnel dryer, using genetic algorithms. This involved, initially, developing computer simulation models for prediction of global solar radiation incident on the dryer, the amount of solar energy harnessed and the drying of fish. The models were then validated, based on actual data, and thereafter were used in the optimisation process. The original (non–optimised) solar dryer was then modified based on the obtained optimised design parameters. The optimised solar tunnel dryer was then tested to evaluate its performance in the harnessing of solar energy and the drying of tilapia fish.The results of a two–way Student’s t–test at 5% level of significance, show that there were no significant differences between simulated and actual data for global solar radiation (tstat = 0.17,3tcrit = 1.65), plenum chamber temperature (tstat = 0.55, tcrit = 1.72) and for moisture ratio of the drying fish (tstat = 0.96, tcrit = 2.06). The subsequent performances of the models in the prediction of the above parameters were 78.4, 83.3 and 81%, respectively, at 10% absolute residual error interval. This implies that the developed models can be used to predict the global solar radiation, the harnessed energy and the drying of fish in a solar tunnel dryer. The optimization process resulted in the heating chamber dimensions of 2.44m long, 1.22m wide and 0.11m high as compared with the non–optimised of 2.44m long, 1.22m wide and 0.54m high. Higher temperatures (14.2 to 57.6oC) in the plenum chamber were obtained for the optimised solar tunnel dryer (OSTD) as compared with those (12.1 to 42.5oC) for the non-optimised solar tunnel dryer (NOSTD). This indicates that the OSTD harnessed more energy than the NOSTD. The results further show that the OSTD took 15 hours as compared to 28 hours for the NOSTD to dry fish to equilibrium moisture. A two–factor Analysis of Variance at 5% level of significance confirmed the existence of significant difference in plenum temperatures developed by the two dyers (F=36.83, Fcrit,α=0.95 = 3.26).The mean values of protein, fat, carbohydrates and ash content of fish dried under NOSTD and OSTD were 69.60%, 8.00%, 1.01μg/g and 8.41% (for OSTD only), respectively, 69.70%, 5.92%, 1.00 μg/g and 17.6%, (for NOSTD only), respectively, and 71.10%, 7.3%, 0.73 μg/g and 18.11% (for open sun drying, Osd), respectively. This indicates that the drying process had no significant influence on the nutritive value of fish dried in both the OSTD and NOSTD solar dryers. In addition, based on TBARS analysis, the quality of fish dried in the OSTD was found to be acceptable at 2.3μg(MA)/kg, while that for NOSTD (5.3μg(MA)/kg) was close to the4unacceptable level of 6 μg(MA)/kg, though within the acceptable range. Finally, the TVB–N results show “very good” putrefaction values (11.14–12.74mg/100g) and these were not significantly different for the two treatments. Based on these results, it is recommended that appropriate designs and optimisation principles and models for solar dryers should always be developed and adopted as has been established in this study. This would result in effective and efficient energy harnessing and quality enhancement of solar dried food material, with the possibility of reducing food losses, improving food security and raising the level of income at farm level. Description: Doctor of Philosophy in Bioprocessing Engineering, 2011

2015-02-10T00:00:00Z