A Simple Method of Calculating The Reliability of Rainfall

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Robinson P.
Glover J.
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The accumulation of records by the E,A. Meteorological Department now makes it possible to investigate rainfall variation from year to year. The farmer in East Africa, who suffers more from too little rather than too much rain, is much more concerned with its reliability than any other aspect. While investigating the variation we therefore concentrated on the problem of measuring how often the farmer can expect a certain minimum total of rain in a season. As everyone knows, averages of rainfall are very misleading and they are certainly of very limited value in comparing one place with another. For example, two farms may each have an average of 20 inches of rain in the same "rainy" season, yet on one farm the records show the extremes of the seasonal rains during, say, the last 20 years to be from 10 to 30 inches while on the other, over the same period, the range may be only 15 to 25 inches. Of the two, the second farm has undoubtedly the more satisfactory rainfall, yet it is not shown in a statement of averages. Consider the same two farms and suppose both grow a crop which requires a minimum of about 15 inches of rain in the season to give an adequate yield. The second farm is likely to get this frequently, for as the figures show it has not failed in the 20 years the records have been kept. The first farm, however, must sometimes fail to receive it, for in one year at least it received only ten inches. It is obvious, therefore, that the owner of this farm is, or should be, concerned with the question of how frequently the farm will receive less than 15 inches of rain in the season for it may mean the difference between profit and loss.
East African Agricultural And Forestry Journal, 19, p. 11-13