Impacts of human intervention through irrigation on hydrological responses of Lake Tana sub-basin, upper Blue Nile, Ethiopia.

Lake water balance studies in Ethiopia are commonly conducted for natural conditions without considering the impact of water abstraction. However, hydrological changes are a function of changes caused by human interventions as much as through natural processes. The Lake Tana sub-basin is one of the important basins in Ethiopia as the source of the Blue Nile river and supporter of various livelihoods including considerable number of small-scale farmers. With Ethiopia’s ambitious plan to expand irrigation at different scales to feed the growing population, the Lake Tana sub-basin is one of the selected areas by the federal government for medium scale irrigation. Moreover, the basin is experiencing booming small-scale irrigation users due to initiations by local governments, NGOs and individual farmers. Small-scale community managed irrigation schemes and farmer-led irrigations schemes are common in the basin. While communities are expected to benefit from the expanding irrigation use, there is limited information on how this irrigation expansion impacts the water balance of the sub-basin.

In this study the area under small-scale irrigation is estimated through inventory survey of woredas (districts) that are within the Lake Tana sub-basin and cover the four main tributaries (Gilgel Abay, Ribb, Gumara, and Megech).  The inventory from 16 woredas showed more than 30,000 ha is under small-scale irrigation in the Lake Tana sub-basin in 2020. The dominant water source for irrigation is found to be surface water through diversion from rivers and streams. This accounts for 80% of irrigation water source. The other 20% comes from shallow groundwater sources, which are mainly located in the eastern part of the sub-basin in the floodplain in the Fogera area.

This study investigates the current situation of irrigation water abstraction through water abstraction surveys in selected locations in the dry-season and evaluates the impact of water abstraction on the long-term water balance of the sub-basin. Previous studies indicate that Gilgel Abay sub-basin, for instance, shows a decreasing dry season flow in the past decades, which might be associated with dry season water abstraction. The current survey provides the gross diversion of water from rivers to canals in the selected schemes. Preliminary results show large amount of water is being diverted by medium irrigation schemes but also collectively by small-scale irrigation schemes. Continuation of such water abstraction levels will be a concern to the water balance of the sub-basin as it increases losses without productive use and calls for better water management practices.