Horn of Africa: annual probability of drought

This map illustrates the likelihood that any cell in the grid will sustain a drought in any given year.

Eastern and western Ethiopia and western Kenya are the regions of the HOA most frequently affected by severe drought (Figure 1). Based on historical rainfall records, there is greater than 40% likelihood that these regions will experience severe drought within the rainy seasons of any given year. The regions of Eritrea, Djibouti, and Somalia that lie along the Gulf of Aden and the Red Sea are the least prone to severe drought, with a less than 20% annual probability.

Moderate to severe drought is defined, in this study, to have occurred in a given month when the average rainfall over a three-month period centered on that month is at least 1.5 standard deviations below the long-term monthly average rainfall. This approach compares the rainfall in a given month to the rainfall in that same month over the past 21 years. The calculation is performed independently for each grid cell, so the analysis should not be biased spatially or temporally. For example, 10 cm of rain in a typically wet area could compute as a drought, where the same rainfall in an arid region would not. Similarly, in the same area, 10 cm of rainfall could compute to being a drought if it occurs during a rainy season, but not if it occurs during a dry season.