THE ECOHYDROLOGICAL CHANGE OF LAKE BESEKA AND ITS CONSEQUENCE

The Main Ethiopian Rift is endowed by number of fresh and mineralized lakes. One of the peculiar phenomenons of these lakes is their shrinking and expanding characteristics whereby some of them are continuously diminishing while others are growing. Lake Beseka is one among the growing lakes swelled from 2.6 km² and expanded to 67 km² over forty years with volume increased nearly 22 fold while its salinity decreased from 75,000 μScm^-1 to 5,500 μScm^-1 over the same period.

Previous studies give various explanations for the causes of the lake level rise including climate change, anthropogenic and tectonics factors. The increase in volume of the lake has impacted the area in various forms: a) submerging agricultural land, b) inundating urban settlement and utilities, c) engulfing Ethio-Djibouti rail road and highway. In addition, the alarming concern is its threat to impair Awash River if the lake overflows its natural levee and spills to Awash River on its own path-–thereby leading to damages on the downstream large scale irrigation projects and urban water supplies which are entirely contingent to this river.

Geochemical and isotope hydrology have been used to investigate cause for the lake level rise and propose mitigation measures. Furthermore, geophysical 2D tomography has been conducted around the lake to understand the extent of lake intrusion and locate flow zones. Result from isotope hydrology shows that shallow ground waters around the lake and Awash River water have strong isotope compositional similarity suggesting that the lake is receiving water mainly from excess irrigation water whereas 2D tomography reveals that the lake invades vast portions of the area east and southeast of the lake. Hydrochemical monogram for the lake and river waters mix ratio has been established which is used as tools for optimum release of the lake to Awash River.