Flow regime change in an endorheic basin in southern Ethiopia
- 1UNESCO-IHE Institute for Water Education, P.O. Box 3015, 2601 DA Delft, the Netherlands
- 2Deltares, P.O. Box 177, 2600 MH Delft, the Netherlands
- 3University of Leeds, School of Civil Engineering, Leeds, UK
- 4Arba Minch University, Institute of Technology, P.O. Box 21, Arba Minch, Ethiopia
- 5Department of Water Resources, Delft University of Technology, P.O. Box 5048, 2600 GA Delft, the Netherlands
- 6Ethiopian Institute of Water Resources, Addis Ababa University, P.O. Box 150461, Addis Ababa, Ethiopia
Abstract. Endorheic basins, often found in semi-arid and arid climates, are particularly sensitive to variation in fluxes such as precipitation, evaporation and runoff, resulting in variability of river flows as well as of water levels in end-point lakes that are often present. In this paper we apply the indicators of hydrological alteration (IHA) to characterise change to the natural flow regime of the Omo–Ghibe Basin in southern Ethiopia. Little water resource infrastructure has been developed in the basin to date, and it is considered pristine. The basin is endorheic and is the main source of flow to Lake Turkana in the East African Rift Valley. The water level in Lake Turkana shows significant fluctuation, but increase of its level can be observed over the past 20 years. The reasons are currently not well understood.
Of the five groups of hydrological characteristics in the IHA (magnitude, timing, duration, frequency and variability), only those related to magnitude were found to show significant trends, with the main trend being the increase of flow during the dry season. This trend was not reflected in climatological drivers such as rainfall, evaporation and temperature (which shows a positive trend), but rather is attributed to the substantial changes in land use and land cover in the basin. The change in the basin hydrology is apparent mainly in the more humid part of the basin. The significant shift from forest and woodland to grassland and cropland results in a decrease of actual evaporation and subsequent increase in (dry season) runoff. The long-term trend of the increasing levels in Lake Turkana are related to these trends in dry season flows, while shorter-term fluctuations of the lake levels are attributed primarily to anomalies in consecutive wet and dry season rainfall.