Analysis of Rainfall Trends for the Belg Season in Ethiopia

The Belg/Gu/Ganna season in Ethiopia stretches from February to June and contributes 50-65% of the annual rainfall for the south-southeastern and up to 40% to the northeastern-eastern parts of the country. Our analysis finds decreases in seasonal rainfall totals in many parts of the Belg regions, based on extensive station data blended with the Climate Hazards Group Infrared Precipitation with Stations (CHIRPS) data. This study examines trends in Belg rainfall over the past 37 years, documenting changes in rainfall characteristics including the start and end of the season, the duration, spatial and temporal distribution, and rainfall totals. We used a combination of CHIRPS improved rainfall data using the GeoCLIM tool, with stations provided by National Meteorological Agency (NMA). Dekadal (ten-day totals) rainfall time series (1981-2017) were extracted for administrative zones. Annual start and end of season dekad for pastoral and agro-pastoral regions were then calculated. Criteria used for start of season for agro-pastoral (pastoral) areas was a dekad with 25 mm (10 mm) rainfall followed by two consecutive dekads with cumulative rainfall of 20 mm (5 mm). The end of season was identified as the last dekad with 10 mm or more before the 2^nd dekad of June. Annual length of the rainfall season was calculated from these dates. Also, seasonal totals were calculated for the period 1^st dekad of February to the 2^nd dekad of June. The results show that there are areas, both pastoral and agro-pastoral, in the north-east region of Ethiopia that have lost up to 5 dekads from the total length of rainy season and up to 60% of the median rainfall. These substantial rainfall declines highlight that other changes to growing season climate, e.g. increasing evaporative demand, might have also occurred and additionally harmed vegetation growth. We investigate this using a reference evapotranspiration dataset based on MERRA-2 reanalysis.