Comprehensive hydrological and hydrogeological study of topographically closed lakes in highland Ethiopia: The case of Hayq and Ardibo

SummaryA comprehensive conventional hydrological and hydrogeological study was conducted in two topographically closed crater lakes in the northeastern highlands of Ethiopia. Particular emphasis is given to the estimation of the water balance and assessment of groundwater and surface water interactions. The detailed bathymetric map of the lakes has also been established for the first time. The result revealed the importance of both groundwater and surface water components in the water balance of the lakes. Field hydrogeological observations and the conventional water balance indicate outflow and inflow of groundwater from both lakes. The lakes leak groundwater through large faults, which also control the occurrence and distribution of springs. The leakage of the lakes through faults is independently validated by hydrochemical analysis; indicating lower ionic concentration and freshness of both lakes in contrast with closed terminal lakes, which are commonly characterized by higher salinity. Limited environmental isotopes together with major ion analysis revealed that shallow fresh groundwater recharged from highland rainfall is one of the main driving forces of the hydrology of the lakes and their catchments. There is no indication of deep groundwater with high ionic concentration and residence time that feeds the lakes. From erratic lake level record and bathymetric survey, it was found that lake Hayq has declined in level over the last 60 years. But, recently due to human influences (return flow from irrigation), the level has shown slight rise at the expense of the leaking lake Ardibo located at relatively higher topographic position. These recent anthropogenic influences demand careful assessment of the complex nature of the hydrology and the water balance. In this regard, this study is believed to have laid the foundation for future detailed work that may help for sustainable water management practise.