Ministry of Agriculture- National Agricultural Research Laboratories-Kenya Soil Survey No m14 1986

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Ministry of Agriculture
Kenya has a wide range of annual rainfall averages, from 180 mm in parts of Turkana to 2400 mm or more on the Nyambeni Hills and the slopes of Mount Kenya. For the identification of the seasonal patterns of rainfall distribution, the large variation in amounts of rainfall is often a hindrance. By using the rainfall per month as a percentage of the annual total instead of monthly amounts of rainfall, this problem can be overcome. Moreover it was found during the preparation of a chapter on climate for the Kindaruma report (Braun, 1975), that for thirteen stations between Meru and Kibwezi, the percentage of the average October-December rainfall which falls in October, is well correlated with the latitude expressed in degrees and minutes south of the North Pole (linear correlation coefficient r = -0.92). Similarly, for the same stations a good correlation was found between the percentage of the average March-May rainfall which falls in March and the station's latitude (r = +0.84). The negative sign of the correlation coefficient for October is in agreement with the southward movement of the Inter-Tropical Convergence Zone (ITCZ) and of the accompanying rain belt in October, while the positive sign of the coefficient for March agrees with the northward movement of ITCZ and rain belt in March. The coefficients for the correlation between the AMOUNT of rainfall in October or March and latitude are -0.92 and -0.41 respectively. The coefficient for March has a low value and it has the wrong sign, i.e. the coefficient would indicate a movement of the rain belt in southern direction. These examples indicate that for some purposes it can be an advantage to express the monthly rainfall as a percentage of the annual or seasonal total. In this report some comparisons based on rainfall percentages will be made for various areas in Kenya (and Tanzania). The use of rainfall percentages is complementary to, and not instead of, the normally used average amounts of rainfall.