Abstract
Abstract. 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.
Highlights
The Upper Blue Nile Basin, the largest tributary of the Nile River, covers a drainage area of 176 000 km2 and contributes more than 50 % of the long-term river flow of the Main Nile (Conway, 2000)
In Soil and Water Assessment Tool (SWAT), a watershed is divided into homogenous hydrologic response units (HRUs) based on elevation, soil, management and land use, whereby a distributed parameter such as hydraulic conductivity is potentially defined for each HRU
A simple conceptual semi-distributed hydrological model was developed and applied to the Gumara and Gilgel Abay catchments in the Upper Blue Nile Basin, Lake Tana sub-basin, to study the runoff processes in the basin
Summary
The Upper Blue Nile Basin, the largest tributary of the Nile River, covers a drainage area of 176 000 km and contributes more than 50 % of the long-term river flow of the Main Nile (Conway, 2000). Mishra et al (2004) and Conway (1997) developed grid-based water balance models for the Blue Nile Basin, using a monthly time step, to study the spatial variability of flow parameters and the sensitivity of runoff to climate changes In both models, the role of topography was not incorporated, and in the model of Conway (1997), soil characteristics are assumed spatially invariant. This paper further investigates the contribution of such landscapes in the rainfall–runoff process by including a class for these impermeable rock and hard soil surfaces in the conceptual hydrological model This approach has not yet been tested in the Upper Blue Nile Basin. These outcomes positively add to the existing knowledge and contribute to the development of water resources plans and decision making in the basin
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
