Analyzing runoff processes through conceptual hydrological modeling in the Upper Blue Nile Basin, Ethiopia

Understanding runoff processes in a basin is of paramount importance for the effective planning and management of water resources, in particular in data-scarce regions such as the Upper Blue Nile. Hydrological models representing the underlying hydrological processes can predict river discharges from ungauged catchments and allow for an understanding of the rainfall-runoff processes in those catchments. In this paper, such a conceptual process-based hydrological model is developed and applied to the upper Gumara and Gilgel Abay catchments (both located within the Upper Blue Nile Basin, the Lake Tana sub-basin) to study the runoff mechanisms and rainfall-runoff processes in the basin. Topography is considered as a proxy for the variability of most of the catchment characteristics. We divided the catchments into different runoff production areas using topographic criteria. Impermeable surfaces (rock outcrops and hard soil pans, common in the Upper Blue Nile Basin) were considered separately in the conceptual model. Based on model results, it can be inferred that about 65% of the runoff appears in the form of interflow in the Gumara study catchment, and baseflow constitutes the larger proportion of runoff (44-48%) in the Gilgel Abay catchment. Direct runoff represents a smaller fraction of the runoff in both catchments (18-19% for the Gumara, and 20% for the Gilgel Abay) and most of this direct runoff is generated through infiltration excess runoff mechanism from the impermeable rocks or hard soil pans. The study reveals that the hillslopes are recharge areas (sources of interflow and deep percolation) and direct runoff as saturated excess flow prevails from the flat slope areas. Overall, the model study suggests that identifying the catchments into different runoff production areas based on topography and including the impermeable rocky areas separately in the modeling process mimics the rainfall-runoff process in the Upper Blue Nile Basin well and yields a useful result for operational management of water resources in this data-scarce region

Geomorphology of the Lake Tana basin, Ethiopia

The geomorphological map of the Lake Tana basin (15,077 km2, Nile basin, Ethiopia) presented in this paper was prepared from fieldwork data, maps and satellite data that were processed with a geographic information system (GIS). It contains four major components: (i) hydrography, (ii) morphology and morphometry, (iii) materials and (iv) processes at a scale of 1:500,000. The geomorphological setting of the basin consists of lavas that erupted from fissures or (shield) volcanoes during the Tertiary and Quaternary eras, were uplifted and eroded primarily by water. Lake Tana emerged through a combination of a lava barrier blocking the Blue Nile to the south and by epirogenetic subsidence. When the lake reached its maximum extent, extensive lacustrine plain (e.g. Fogera and Dembia plains) were created, river valleys and basins were filled with sediment and higher lying topography was eroded. Today, the lake plays a lesser role in landscape formation because of a decreased extent (3041 km2) compared to the ancient maximum (6514 km2). Dominant processes today are fluvial and subaerial. Recent (1886–2010) changes in the lake coastline are small with the exception of the delta formed by Gilgel Abay, which has increased disproportionally over the last 15 years. This indicates a large input of sediment which is mainly due to rivers flowing through Quaternary lavas. The recent sediment input increase is most probably related to human induced land-use changes