Analyses of Annual Droughts in Kenya Using an Objective Annual Rainfall Drought Index
Journal of Kenya Meteorological Society
(JKMS)
Volume 4:2
Analyses of Annual Droughts in Kenya Using an Objective Annual Rainfall Drought Index
Francis M. Mutua*, Abdel A.F. Zaki**
*University of Nairobi, **UNESCO-Cairo Office, Cairo, Egypt
CORRESPONDING AUTHOR
Prof. Francis M. Mutua,
Department of Meteorology, University of Nairobi
P. O. Box 30197 00100 NAIROBI KENYA
Email : This email address is being protected from spambots. You need JavaScript enabled to view it.
(Manuscript received 27 July 2010, in final form 01 November 2010)
Abstract
Droughts are naturally imbedded in the climate system and therefore inevitable and can be essential regulators of the climate-driven environments. However, droughts in Africa are some of the most common and often misinterpreted natural hazards. When left improperly managed for an extended period of time, they cause unparalled suffering to the poor communities, mostly by catalyzing other community and nation-level vulnerabilities. However, droughts do not have to be disastrous; they can even have potential benefits when properly managed. As a hazard, inadequate rainfall is usually the primary driver in the formation and development of droughts, although temperature, wind and soil moisture can also be significant causative factors. Ironically, there is no one single and universally accepted definition of drought, an attribute which often leads to the frequent and unfortunately incorrect diagnosis of drought, particularly within the developing countries, leading to ineffective management of droughts in these countries. Most commonly, droughts are studied through the use of drought indices which are often used to assess the degree of drought severity. There are a few drought related indices which exist in the literature today. However, most of them have pertinent weaknesses which are either related to their structure or to their limited application in the data scarce countries such as is the case in most of the developing countries. In this study, an annual rainfall drought index is developed using an objective antecedent precipitation concept, based on monthly rainfall data. The study utilizes monthly rainfall data from 26 meteorological stations in Kenya. The temporal as well as the spatial distribution of the index is also analyzed. Further, the study compares the adequacy of the two commonly used 3-parameter extreme-analysis distributions, namely, the General extreme Value (GEV) and the normal probability distributions to investigate the temporal as well as the spatial characteristics of the index over Kenya. The Akaike Information Criteria and the L-moment ratio tests are used to assess the goodness-of-fit of these these distributions. Both tests show that the GEV DECEMBER 2010 J.Meteorol. Rel. Sci., 4 21–33 (2010) 19 distribution fits the logarithmically transformed annual drought indices better that the normal distribution. The patterns for the distribution of the drought estimates for different return periods show that drought intensities are strongest in the wet regions of the country while the weakest intensities occur in the dry and semi-arid regions of the country. The index performs well in objectively identifying the drought severity and distribution in Kenya. The index can also be applied in other regions or countries over which snowmelt and/or ice-melt do not significantly contribute to the water balance.
Keywords Aridity, Drought definitions, drought types, drought severity, drought indices, Akaike Information Criterion, L-Moments
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