CLIMATIC CONTROLS OVER VARIATION OF STABLE ISOTOPES OF PRECIPITATION IN ETHIOPIA

Understanding the variation of stable isotopes of precipitation (δ¹⁸O and δD) in Ethiopia is critical for their applications in water resource management and interpretation of paleoclimate records in this region. For this purpose, precipitation samples were collected from 2012/08-2013/11 on daily, weekly and monthly basis at four sites in Ethiopia to cover different climate zones, and subsequently analyzed for their isotopic compositions. We examine the inter- and intra-seasonal variations of the precipitation isotopes, and how they are affected by a variety of local and regional climate factors, such as temperature, precipitation amount, relative humidity, convective intensity (estimated by the Outgoing Longwave Radiation (OLR)), and fraction of connective precipitation, sources of moisture, regional wind circulation patterns, and moisture flux and transport. Our results show that Ethiopian precipitation has distinct seasonal isotope patterns, with depleted δ¹⁸O in wet season precipitation and enriched δ¹⁸O for dry season precipitation. The wet and dry season isotope values also have different d-excess and slope values for their respective local meteoric water lines. In terms of climatic controls, a weak amount effect is observed in all stations on the western highland, but absent in the southeast highland. Temperature effect is not observed. The precipitation isotope values are significantly correlated (at 0.05 level) with convective intensity (OLR) in all stations, and with the fraction of convective precipitation at some stations, suggesting that weather types (e.g. convective vs. stratiform precipitation) also have an impact on the isotopic composition. Finally, different moisture sources also affect the isotopic composition of precipitation. Our results show that even within the same wet season (June-September), moisture comes from different source regions (northeast for June, south for July and August, and north for September), causing differences in isotope values because of the distance of transport and convective intensity of source regions. The high-resolution Ethiopian precipitation isotope data provide new insights into the climatic controls on seasonal to intra seasonal isotope variability and help to link weather patterns to water supply in Ethiopia.