Spatial – Temporal Characteristics of Rainfall Events in Kenya

Abstract

Rainfall is very important for all life on earth because it is the main source of fresh water which is essential for survival of mankind, plants and animal. However as human populations continue to grow, the search of better livelihoods has seen an increase in the number of people living in urban centres. This has left communities vulnerable to extreme rainfall events. During the rainy season’s road, bridges and other infrastructure are damaged by rainfall events, costing the government a lot of money, all of which could be avoided by research for development in this study. Many countries around the world have done a lot of research on the spatial and temporal characteristics of rainfall in their regions. Countries such as Nepal (Devkota 2018), Denmark (Madisen et al 2000), and Italy (Languise & Venaziano 2000) among others have published studies detailing the spatial temporal characteristics of rainfall in their regions. In Africa, Rwanda (Wangesho & Claire 2016), Nigeria (Antiga & Ogarekpe, 2013), Benin (Agbazo et al., 2016), Egypt (Awadalla et al., 2017) have all published studies of the rainfall characteristics for their region. This study seeks to understand the spatial and temporal characteristics of rainfall events using hourly data from the Trans-African Hydro Meteorological Observatory, TAHMO automatic weather AWS stations all around the country. This study sought to understand how long rainfall events last (duration), the intensity (mm/hr) of rainfall received, and the variation rainfall events in Kenya (thresholds) and to establish intensity duration frequency curves of rainfall events in Kenya. Percentage distribution was used to categorise the duration from one to eight hour and determine the most prevalent durations. The Average intensities of the rainfall events were also analysed and categorised into drizzles (>0.9mm/hr), light rains (1-4.9mm/hr), moderate rains (5-9.9 mm/hr), heavy rains (10-19.9 mm/hr) and very heavy rains (<20mm/hr) and percentage distribution was used to show the variation of these categories. Curve analysis was used to establish the peak durations and the average intensities at the peak durations. These were then plotted and sufer graphical analysis was used to show their variations all around the country. Gumbels’ Extreme Value analysis was used to calculate the intensities of rainfall for durations of 1 to 8 hours at return periods of 10, 25, 50, 100, 500, 1000 and 10,000 years. These intensities for every station were then plotted together for each station to show the IDF curves for that given region. This study has shown that most of the rainfall events recorded in Kenya last one to two hours. Light to moderate rainfall events are the most common ones experienced (<10mm/hr). However significant number of extreme rainfall events are experienced (>20mm/hr). The rainfall thresholds v (peak duration and average intensity at peak duration) vary greatly all across the country. The peak duration varies from 0.7 hours to 1.5 hours with the average intensities varying greatly all across the country. This is because the topographical variation of the country’s terrain. The duration as to when the peak rainfall intensity is achieved varies all around the country. With regions such as north eastern Kenya, achieving the peak duration in the shortest periods while the highlands in central Kenya take the longest duration to achieve their peak durations. Maximum rainfall intensities were analysed for durations of one to eight hour for the return periods of 10, 25, 50, 100, 500, 1000 and 10000 years. These were then plotted together to show the IDF curves for different regions of our country. Each region showed its unique characteristics