Spatial and temporal variation in the isotopic composition of Ethiopian precipitation

Abstract

Stable isotopes of oxygen (δ18O) and deuterium (δ2H) in precipitation are used as tracers of the hydrologic cycle in studies of both past and present climate. In Ethiopia, a drought sensitive region where direct observations of climate are limited, understanding the stable isotopic composition of precipitation can provide an integrated view of the hydroclimate today and help interpret records of climate variability in the past. To date, the isotopic composition of precipitation in Ethiopia is known primarily from monthly precipitation collections from one long-term monitoring station in Addis Ababa. Here we report δ18O and δ2H values of precipitation from a 16-month long sampling campaign at four stations in Ethiopia that represent different climate regimes. Precipitation samples collected at daily, weekly and monthly intervals between August 2012 and November 2013 at these stations exhibit a large range of δ18O (−12.6 to +10.3‰), δ2H (−91.8‰ to +80.8‰), and d-excess values (−9.9‰ to +29.9‰). The four stations have similar amount-weighted mean δ18O values, but exhibit different seasonal patterns in δ18O values as clearly shown in daily and weekly data. The most striking feature of this dataset is the synchronous decrease in δ18O values in daily and weekly precipitation samples from the three stations in the northwestern highlands (Gondar, Debre Markos, Jimma) during the first three weeks of August 2013. This decrease in δ18O values does not correlate strongly to precipitation amount but it is likely due to a combination of increased storm intensity and deep convection at the time of lower δ18O values of precipitation. Our high-resolution dataset provides new insights into the controls on the isotopic composition of rainfall in Ethiopia and how we might use it to understand hydroclimate variability today and in the past.