Diagnosis of Urbanization and seasonal Rainfall over the City of Nairobi

Isaac J. Ndolo*, Nzioka John Muthama**, Christopher Oludhe***, John Kinyuru Nganga***, Richard S. Odingo*

*Department of Geography and Environmental studies, University of Nairobi

**Wangari Maathai Institute for Peace and Environmental studies, University of Nairobi

***Department of Meteorology, University of Nairobi 

Corresponding Author

Isaac Ndolo

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(Received 2 June 2017, received in revised form 3 July 2017, Accepted 4 August 2017)


Urbanization modifies corresponding local climate through alteration of vegetative cover, high population density, anthropogenic emissions and other processes such as infrastructural development. This paper seeks to investigate the influence of the urbanization on the rainfall patterns over the city of Nairobi. Rainfall data from four Kenya Meteorological Department’s ground stations: Dagoretti Corner, Wilson Airport, Jomo Kenyatta International Airport (JKIA) and Moi Airbase, for the period January 1961 to December 2008 were obtained.  Nairobi population census data were obtained from Kenya National Bureau of Statistics. The data were subjected to trend and correlation analyses to determine temporal patterns and possible causal relationships. Rainfall spatial variability was apparent, with the driest part being to the south east, where JKIA station is located and the wettest to the west where Dagoretti Station is located. Time series analysis on rainfall data revealed that the most frequent monthly rainfall distribution over Nairobi is between 49 mm and 99 mm. In addition, rainfall is experienced every month of the year. The short rains season is ideally received in November and December (ND)   in contrast to previous studies which have reported the season as October, November and December (OND), suggesting the length of the short rains season is shortening. The study also found out that rainfall occurrence in recent years has been above normal than previous years as evidenced by the latter years having more positive anomalies than the former years. The existence of extreme rainfall occurrences across the four stations after every 4 to5 years is notable It is especially beneficial for urban planning?  Finally, a shift in the inter-annual rainfall pattern over Moi Airbase is observed, suggesting an increasing influence   of the changing density of built- up area around the station, therefore changing atmospheric composition, which leads to alteration of the rainfall pattern.

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