Spatial and temporal variation in the isotopic composition of Ethiopian precipitation

Abstract

Stable isotopes of oxygen (delta O-18) and deuterium (delta H-2) 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 delta O-18 and delta H-2 values of precipitation from a 16month 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 delta O-18 (- 12.6 to +10.3 parts per thousand), delta H-2 (- 91.8 parts per thousand to +80.8 parts per thousand), and d-excess values (- 9.9 parts per thousand to + 29.9 parts per thousand). The four stations have similar amount-weighted mean delta O-18 values, but exhibit different seasonal patterns in delta O-18 values as clearly shown in daily and weekly data. The most striking feature of this dataset is the synchronous decrease in delta O-18 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 delta O-18 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 delta O-18 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.