By Chris Funk, Diego Pedreros, and Diriba Korecha
An analysis of the best-available satellite and rainfall station data for Ethiopia confirms continued drying during the February-June Belg regions. Ninety percent of the area examined exhibited declining rainfall trends. The average decline for these areas appears to be about 50mm, or 20%. This 20% decline is enough to greatly increase the frequency of poor rainy seasons, which appear to be two-to-three times more frequent now than in the 1980s and early 1990s. An analysis of growing season lengths indicates more localized reductions, with the north-eastern Belg growing areas experiencing large 20-to-40 day declines in growing season length, and up to 60% reductions in seasonal rainfall totals. These substantial rainfall reductions, as well as the as the shrinking length of the growing season, hinder natural phenological vegetation growth over agropastoral and pastoral regions of northeastern, eastern, southern, and southeastern Ethiopia.
This brief analysis summarizes an analysis of trends in the Ethiopian Belg rains. The Belg season in Ethiopia extends from February to mid-June, and contributes 50-65% of the annual rainfall totals for the southern and southeastern parts of the country, and up to 40% for the northeastern and eastern sectors of the country. This study examines trends in Belg rainfall over the past 39 years (1981-2019), documenting changes in rainfall characteristics, including start, end, and duration of the season, spatial and temporal distribution, and rainfall totals.
The rainfall analyzed combines the Climate Hazard Center InfraRed Precipitation with Stations (CHIRPS) product with a dense set of additional gauge observations provided by the Ethiopian National Meteorological Agency. Our analysis was restricted to pastoral and agro-pastoral regions that historically receive Belg season rainfall (Figure 1). Time series of dekadal (10-day) rainfall were extracted for each administrative zone. For agropastoral areas, the start of the Belg season was defined as when a dekad with 25 mm rainfall totals was followed by two consecutive dekads, with cumulative rainfall totals of 20 mm. For pastoral regions, the start of the Belg season was defined as when a dekad with 10 mm rainfall totals was followed by two consecutive dekads with cumulative rainfall totals of 5 mm. The end of the Belg season was identified as the last dekad before the second dekad of June, with rainfall totals of at least 10 mm for agro-pastoral, and 5 mm for pastoral regions. Length of the Belg rainfall season was calculated from these dates, and seasonal rainfall totals were calculated for the period from the first dekad of February to the second dekad of June.
Figure 1. Agropastoral (blue) and pastoral (magenta) regions examined in this study.
Results – Changes in Belg rainfall totals
Changes in regional rainfall, expressed as a percent of the long-term median, are presented in Figure 2. Ninety percent of the area examined exhibited declining rainfall. Of the 47 zones examined, 79% (37 zones) exhibited declining rainfall trends. In south-central Ethiopia Guji, Liben, Afder, East Shewa, and Arsi all exhibit declines exceeding 20%. The central-eastern highlands (West and East Haraghe) exhibit large declines of 25 to 29%. Most zones in the eastern pastoral Somali region exhibit declines of 13 to 24%. The northern Belg regions tend to exhibit the largest percent declines, but the Belg rainfall totals in this region are generally lower on average, as well.
Figure 2. Trends in zonal Belg February-June rainfall, expressed as a percent of the long-term median rainfall.
We next present a time series of rainfall, averaged over the 37 zones with declining rainfall trends (Figure 3). Also shown on this plot are averages for 1981-1996 and 1997-2019. This region has experienced a large (50 mm or 20%) decline in rainfall. Dry seasons, with average rains equal to or less than ~227 mm have become the new normal. The FEWS NET/NMA data shown here indicate 17 such events since 1997 and 6 events since 2011, frequencies of 74 and 67%. In the 16 years between 1981 and 1996, there were only 4 such events. These data suggest that the frequency of dry Belg seasons has doubled or tripled in frequency. This has likely eroded farm and livestock productivity and contributed to increased food insecurity. These repetitive shocks may be eroding resilience.
Figure 3. Time series of average Belg rainfall in eastern Ethiopia.
We can place the current dry conditions in deeper context by combining 1900-1980 values from the FEWS NET Centennial Trends data set with the 1981-2019 Belg totals presented above. Time series of Belg-region averages of the station-only Centennial Trends and the blended station CHIRPS data set are highly correlated (R=0.92), which allows us to combine 1900-1980 Centennial Trends data with the 1981-2019 presented in Figure 3. Figure 4 presents these combined data, smoothed with 20-year running averages. While there are some decadal variations, the recent decline appears unprecedented in the historical record. Our research (a, b) has suggested that this decline is associated with anthropogenic warming in the western Pacific.
Figure 4. A long-time series of Belg season rains, smoothed with 20-year running averages.
Results – changes in the length of the Belg season
We next present changes in the length of the Belg seasons (Figure 5). The number of zones with downward trends in the growing season is smaller (34), and the overall area exhibits declines in the length of the Belg season is smaller, ~65% of the area examined. These regions tend to appear in the northern parts of the Belg area. Changes in some of these northern areas appear to be very large: 20, 30, or even 40 days in some cases. For farmers, these decreases in the length of the rainy season may make it difficult to achieve substantial crop yields, especially in cooler areas with slow maturation rates. These decreases in season length may also impact important and productive “long cycle” crops, which are planted during the Belg, but harvested after the Kiremt rains come in the summer. For pastoralists, these much shorter Belg rainy seasons likely indicated longer lean times and reduced fodder and water availability.
Figure 5. Changes in the length of the Belg season (in days).
A time series of Belg season growing season length, averaged over the 34 zones with declining growing season lengths, is shown in Figure 6. The difference in the length of the season from year to year is notable. Good seasons average about 90 days. Poor seasons can have less than 70 days of rainfall. Overall, there appears to be a decline of about 10 days, on average. As noted above, however, this decline appears to preferentially be impacting areas in the northern Belg regions.
Figure 6. Average length of the Belg season in zones with negative length of season trends