The δ18O and δ2H enrichment of Ethiopian lakes

SummaryUp-to-date overview of 18O and deuterium isotope composition of Ethiopian (low latitude) lakes is presented and the suitability of the isotope balance method in water balance estimations of these lakes is assessed. To determine the water budgets of the lakes and to investigate the factors that influence isotopic enrichment in these lakes we used a method whereby the calculated (theoretical) local evaporation line (CLEL) is compared with the evaporation line derived from fitting of the measured isotopic compositions of the lakes in the region. The CLEL is specific to a given region and is determined from representative relative humidity, the isotopic composition of the ambient moisture and the isotopic composition of inflow waters. The comparison between CLEL and the measured isotopic composition of the lakes, and the departures of the isotopic compositions of these lakes from the calculated values, were interpreted in terms of the lakes water budget and their catchment characteristics. This comparison revealed how far the existing empirical parameters in the isotope balance method predict evaporative enrichment in lakes under low latitude setting. Significant discrepancies were noted when CLEL was compared with the measured isotope enrichments of lakes. In this regard the low latitude lakes show similar enrichment behavior to that of other lakes in high latitude settings. Satisfactory agreement between the calculated and the measured local evaporation lines was obtained through adjusting the theta value which accounts for humidity build-up over the evaporating lake. All the investigated lakes are evaporation-dominated systems, with evaporative water losses exceeding 50% of total water loss.