A multi basin SWAT model analysis of runoff and sedimentation in the Blue Nile, Ethiopia

Abstract

Abstract.A multi basin analysis of runoff and erosion in the Blue Nile Basin, Ethiopia was conducted to elucidate sources of runoff and sediment. Erosion is arguably the most critical problem in the Blue Nile Basin, as it limits agricultural productivity in Ethiopia, degrades benthos in the Nile, and results in sedimentation of dams in downstream countries. A modified version of the Soil and Water Assessment Tool (SWAT) model was developed to predict runoff and sediment losses from the Ethiopian Blue Nile Basin. The model simulates saturation excess runoff from the landscape using a simple daily water balance coupled to a topographic wetness index in ways that are consistent with observed runoff processes in the basin. The spatial distribution of landscape erosion is thus simulated more correctly. The model was parameterized in a nested design for flow at eight and sediment at three locations in the basin. Subbasins ranged in size from 1.3 to 174 000 km 2 , and interestingly, the partitioning of runoff and infiltrating flow could be predicted by topographic information. Model predictions showed reasonable accuracy (Nash Sutcliffe Efficiencies ranged from 0.53-0.92) with measured data across all sites except Kessie, where the water budget could not be closed; however, the timing of flow was well captured. Runoff losses increased with rainfall during the monsoonal season and were greatest from areas with shallow soils and large contributing areas. Analysis of model results indicate that upland landscape erosion dominated sediment delivery to the main stem of the Blue Nile in the early part of the growing season when tillage occurs and before the soil was wetted up and plant cover was established. Once plant cover was established in mid August landscape erosion Correspondence to: Z. M. Easton (zme2@cornell.edu) was negligible and sediment export was dominated by channel processes and re-suspension of landscape sediment deposited early in the growing season. These results imply that targeting small areas of the landscape where runoff is produced can be the most effective at controlling erosion and protecting water resources. However, it is not clear what can be done to manage channel erosion, particularly in first order streams in the basin.

Critical review of SWAT applications in the upper Nile basin countries

“…The original model gives a base flow factor between 0.5-0.65 (Setegn et al, 2009a), while the modified SWAT-WB computes a base flow factor between 0.9 and 0.95 (Easton et al, 2010). The base flow factors that are obtained after using base flow filter programs on observed flow time series range between 0.49 and 0.6 and are more in line with the results of the original SWAT model (see Fig.…”

“…The results showed a very high spatial variability for the obtained annual sediment yields, ranging from 0 to more than 150 t ha −1 . Easton et al (2010) simulated the hydrologic balance and sediment loss for the Blue Nile watershed that lies mainly in Ethiopia using SWAT-WB, a modified SWAT model that captures variable source area hydrologic phenomena. Predicted runoff losses (averaged across the entire subbasin) varied from as low as 13 mm yr −1 for the entire Blue Nile basin to 44 mm yr −1 in Anjeni.…”

“…One of these developments that have been applied within the Nile basin is the SWAT-WB model that represents a hydrology that is driven by saturation excess processes as an alternative to the curve number of the SWAT model that represents infiltration-excess processes (Easton et al, 2010).…”

“…Two different SWAT-based modelling concepts are used in the modelling of the Blue Nile basin: the original SWAT model that uses the curve number (SWAT-CN) and a modified version that contains a newly developed water balance concept (SWAT-WB) that uses the topographic index to define the generation processes of the surface runoff (Easton et al, 2010;White et al, 2011).…”

Assessing Climate Change Impact on Gilgel Abbay and Gumara Watershed Hydrology, the Upper Blue Nile Basin, Ethiopia

“…At the national level, it has great potential for irrigation, hydroelectric power generation, crop production, livestock production, and ecotourism. At the regional level, the lake is the head of the Blue Nile River, which contributes 80 -85% of the inflow to the Nile River (Easton et al 2010). The lake is the water tower of the Ethiopia's Grand Renaissance Dam on the Blue Nile.…”