Groundwater recharge of Robit - Bata Experimental Watershed, Lake Tana Basin, Ethiopia [Abstract only]

Irrigation development is a policy priority for the Laka Tana basin. However available baseflow can irrigate less than 3% of area. The irrigated area can be increased from water stored in constructed reservoirs, pumping water from Lake Tana and by using groundwater. While groundwater has the greatest potential for increasing irrigation in the near future, the main drawback is a lack of information on sustainable groundwater use and specifically the ground water recharge. Therefore the objective of this research is to calculate the annual recharge to the ground water. The study is conducted in Robit-Bata, an experimental watershed of 911 ha, located at the south-eastern edge of Lake Tana. Farmers have excavated more than 300 hand dug wells for irrigation and we used 50 shallow groundwater wells and observed water table fluctuation and precipitation for the past one year. The annual recharge was estimated using water – level fluctuation method. Specific yield was taken to be the difference of porosity and field capacity of the subsurface formation. The annual average areal groundwater recharge of the watershed was 200 mm/year, which is 12% of the rainfall and ranged from 80mm to 500mm for the various locations in the watershed. The greatest recharge amounts were found at the foot of the hillside in river course plane areas consisting mostly weathered basalt rock. Here the groundwater was up to 15 m deep and rose steadily during the rainy monsoon phase. Smaller amount of recharge occurred both near the top of the hills with tough rock formation and in the, flat areas near to stream sandy and clay deposits and groundwater at, shallow well depth. Our study indicates that the current use of the groundwater is sustainable, but will become unsustainable if many more wells are constructed. Sufficiency and sustainable use of the shallow groundwater needs further research.