International Journal of Research and Innovation in Applied Science (IJRIAS) | Volume IV, Issue VIII, August 2019|ISSN 2454-6194 www.rsisinternational.org Page 65 Use of Mann Kendall Method to Study Annual Rainfall Trends in Narok and Kisii Counties Derick L. Kesuna 1 , Moses M. Manene 2 and Muriungi Robert 1 1 Meru University of Science and Technology, P.O. Box 972-60200, Meru, Kenya 2 University of Nairobi, P.O. Box 30197-00100, Nairobi, Kenya Abstract:-Rainfall is the main source of water which is an important basic requirement for a living. It is therefore important to have proper planning and water resources management in any county. In this study, the purpose was to study annual trends of rainfall in Narok and Kisii counties. Specifically the study sought to detect trends in the rainfall time series data, to determine the significance and magnitude of the trends and to compare the trends behavior for Narok and Kisii counties. The data used for this study was obtained from the Kenya Metrological Department for the period 1963 to 2015 inclusive for Narok and Kisii counties. Studying trends will help the residents of the two counties to cope with the climate change in the area. Mann Kendall and Sen’s Slope were used to analyse rainfall data. The results showed that Kisii County had a significant increasing trend while Narok County had a significant decreasing trend. Key words: Rainfall trends; Mann Kendall; Yearly I. INTRODUCTION ainfall is the natural source of water for domestic activities, agricultural practices, construction and many other areas. Deforestation, global warming and some other factors have led to reduced rainfall. This has become a serious issue in the world today. Prolonged drought has led to loss of life and destruction of property (Nyakundi, 2017). Majority of Kenyans makes their living on land as small scale holders who depend on rain fed agriculture. Rainfall trend analysis has been of great concern for the past century because of the attention that is given to global climate change from the scientific community. (IPCC, 1996), indicated that a large area of the world is characterised by negative trends and small global positive trends. (Kansiime,2013), studied perceived and actual rainfall trends and variability in Eastern Uganda. They found out that there is a significant increasing trend in seasonal and annual rainfall in the highlands but in the low lying areas, there is a decreasing trend. In Kenya, there is generally a decrease in rainfall trend which is accompanied by significant increases in average air temperatures, (USAID,2010). Narok county practices crop farming for commercial and subsistence and livestock keeping as the main economic activities. Kisii county also practices agriculture as the main economic activity (Tea, Bananas, Maize, Coffee and dairy farming). In both regions, the activities are highly dependent on rainfall. Therefore, there is a need to detect rainfall trend and determine significance and magnitude of the trends. This will guide in advising farmers on how to prepare before and after rainfall and how to adjust with the change in amount of rainfall. In this research rainfall trend was studied. Mann Kendall test and Sen’s slope were used to detect trends of rainfall in Narok and Kisii regions. The magnitude of the trend line explains the behaviour of rainfall. The study has used rainfall data for Narok and Kisii counties. Narok County is situated in Kenya along the Great Rift Valley. It is named after, Enkare Narok, the river flowing through Narok town. It covers an area of 17,944 square kilometers and has a population of 850,920. The temperature range is 12 0 C to 28 0 C and the average rainfall range from 500 to 1,800 mm per annum. The Maasai Mara National Park, an important tourist destination, is located in Narok County. It is home to the Great Wildebeest Migration which is one of the “Seven New Wonders of the World”. It constitutes 6 sub- counties namely: Kilgoris, Narok North, Narok South, Narok East, Narok West and Emurua Dikirr. Narok town is the capital Head Quarters of Narok County and stands as the major center of commerce in the county. It is inhabited mostly by the Maasai community. Narok County is livestock farming, practiced in both local and adaptive exotic breeds. Livestock farming in Narok County contribute around 10 % of the county's Gross Domestic Product. (www.narok.go.ke/about- narok) Kisii County is in the former Nyanza Province in southwestern Kenya. Its largest town is Kisii. The region is inhabited mostly by the Gusii Community. The total area is 1,317.9 km square while the total population is 1,152,282.The maximum temperatures in the region range between 21°C and 30°C while the minimum temperatures range between 15°C and 20°C. Kisii County is characterized by a hilly topography with several ridges and valleys. It can be divided into three main topographical zones. The first zone cover areas lying below 1,500m above sea level located on the western boundary and include parts of Suneka, Marani and Nyamarambe. The second zone covers areas lying between 1500-1800m above sea level located in the Western parts of Keumbu and Sameta divisions, Eastern Marani and Gucha River basin. R