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University of Nairobi (2018)

Effects of urbanization on rainfall and temperature over the city of Nairobi, Kenya

Ndolo, Isaac J

Titre : Effects of urbanization on rainfall and temperature over the city of Nairobi, Kenya

Auteur : Ndolo, Isaac J

Université de soutenance : University of Nairobi

Grade : Doctor of Philosophy in Climatology 2018

Résumé
Urban areas depending on their size : Surface areas, population, industrial activities, building density, alter the microclimate and impede greening of any city, Nairobi included. The problems associated with urbanization in the city of Nairobi are poorly documented. This formed the basis of the current study. The main objective of this study was to determine the influence of the urbanization on the urban canopy layer and microclimate of Nairobi City. Monthly rainfall and temperature data for the period 1961 to 2008 were obtained from four Kenya Meteorological department’s ground stations located within Nairobi City namely ; Dagoretti, Wilson Airport, Jomo Kenyatta International Airport (JKIA) and Moi Airbase. Data on population census conducted at stipulated intervals within the study period were obtained from Kenya National Bureau of Statistics. Satellite data on Normalised Difference Vegetation Index (NDVI) was also obtained from NASA satellite. Spatial analysis of temperature across the city pointed out that the most representative station in studying the urban canopy characteristics of the city of Nairobi is Moi Airbase. The minimum temperature across the city showed a significant positive trend. This may be attributed to urbanization effect leading Urban Heat Island (UHI) which is manifested at night by elevated temperature characteristics. Of the four stations, only Dagoretti exhibited a significant positive trend of maximum temperature. The microclimate of the city of Nairobi exhibited spatial variability, with the wettest part being to the west where Dagoretti Meteorological station is situated and the driest being to the south east where JKIA Meteorological station is located. Time series analysis on rainfall data revealed that the most frequent monthly rainfall distribution over Nairobi was between 49.0 mm and 99.0 mm with rainfall being experienced in all the months of the year. The short rains season is ideally received in November and December (ND), contrary to previous studies which have reported the season as October, November and December (OND). The correlation between rainfall anomalies with NDVI anomalies at time lag of one month was significant, r= 0.75 with 56.3% of the variation in NDVI anomaly being explained by rainfall anomaly. The greenness of the city, therefore, was deemed to be driven by rainfall. Further, analysis of NDVI data revealed that irrigated greening measures were not statistically significant. The study therefore recommended the utilization of prevalent storm water for greening the city of Nairobi. Comparative analyses of temperature and rainfall across the city of Nairobi depicted heterogeneity among the four weather stations, with Moi Airbase being the hottest. Dagoretti, being located at a higher altitude and in a peri-urban setting of the city was the coldest. On the other hand, Dagoretti received the highest annual rainfall amount with JKIA receiving the lowest of the same. A similar rainfall pattern exists during the MAM rainfall season. Therefore the microclimate of Nairobi is not homogeneous. Minimum temperature in all stations is significantly and positively correlated with population. This is attributed to the Urban Heat Island (UHI) phenomenon. Urbanization is impacting on minimum temperature over the city of Nairobi through heat generation by the urban population. The results suggest that the UHI may cover a region beyond the analysed stations. Further investigations on the influence of urbanization upon the microclimate of the city of Nairobi in future are recommended once more weather stations in and around the city of Nairobi are established.

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