Trends and Spatial Patterns of Drought Incidence in the Omo-Ghibe River Basin, Ethiopia

The objective of this paper is to evaluate trends and spatial patterns of drought incidence across the Omo-Ghibe River Basin using monthly rainfall data from eight stations for the period 1972-2007. It also aims to estimate the probability of drought episodes for a 100-year period. Drought indices were generated using the Standard Precipitation Index (SPI) computed at 3-, 6-, 12- and 24-month time-steps for three intensity classes: moderate, severe and extreme drought events. The Mann-Kendall's trend test and Sen's slope estimator were employed to detect temporal changes. The results show complex spatial patterns on the frequency and magnitude of drought events across the study area for all timescales and intensity classes. However, the total number of drought events for the three intensity classes for all timescales were larger in the southern lowlands, where there exists a serious water scarcity for the rain-fed pastoral system, than in the northeastern part (around Wolaita Sodo area). In contrast to this, the longest and most extreme (SPI<-4.0) drought events for all timescales were observed at Wolaita Sodo station. In a 100-year period one could expect 57-69 drought events with 3 months' duration, 19-34 events with 6 months' duration, 9-16 events with 12 months' duration and 5-9 events with 24 months' duration. The SPI values show negative rainfall anomalies in the 1980s while positive anomalies have occurred in the 1990s and 2000s, which implies tendency towards decreasing drought events. The Mann-Kendall's trend test for the 12- and 24-month timescales and for seasonal events also confirms this general trend.