HYDROLOGICAL PROCESSES Hydrol. Process. 23, 3728–3737 (2009) Published online 16 November 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/hyp.7513 Predicting discharge and sediment for the Abay (Blue Nile) with a simple model Tammo S. Steenhuis, 1,2 * Amy S. Collick, 1,2 Zachary M. Easton, 1 Elias S. Leggesse, 2,3 Haimonote K. Bayabil, 3 Eric D. White, 1 Seleshi B. Awulachew, 4 Enyew Adgo 5† and Abdassalam Abdalla Ahmed 6 1 Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY, USA 2 Department of Water Resources Engineering, Bahir Dar University, Bahir Dar, Ethiopia 3 Integrated Watershed Management and Hydrology Program, Cornell University, Bahir Dar, Ethiopia 4 International Water Management Institute Sub Regional Office for Nile Basin and Eastern Africa, Addis Ababa, Ethiopia 5 Department of Natural Resources, ARARI, Bahir Dar, Ethiopia 6 UNESCO Chair in Water Resources (UNESCO-CWR), Khartoum, Sudan Abstract: Models accurately representing the underlying hydrological processes and sediment dynamics in the Nile Basin are necessary for optimum use of water resources. Previous research in the Abay (Blue Nile) has indicated that direct runoff is generated either from saturated areas at the lower portions of the hillslopes or from areas of exposed bedrock. Thus, models that are based on infiltration excess processes are not appropriate. Furthermore, many of these same models are developed for temperate climates and might not be suitable for monsoonal climates with distinct dry periods in the Nile Basin. The objective of this study is to develop simple hydrology and erosion models using saturation excess runoff principles and interflow processes appropriate for a monsoonal climate and a mountainous landscape. We developed a hydrology model using a water balance approach by dividing the landscape into variable saturated areas, exposed rock and hillslopes. Water balance models have been shown to simulate river flows well at intervals of 5 days or longer when the main runoff mechanism is saturation excess. The hydrology model was developed and coupled with an erosion model using available precipitation and potential evaporation data and a minimum of calibration parameters. This model was applied to the Blue Nile. The model predicts direct runoff from saturated areas and impermeable areas (such as bedrock outcrops) and subsurface flow from the remainder of the hillslopes. The ratio of direct runoff to total flow is used to predict the sediment concentration by assuming that only the direct runoff is responsible for the sediment load in the stream. There is reasonable agreement between the model predictions and the 10-day observed discharge and sediment concentration at the gauging station on Blue Nile upstream of Rosaries Dam at the Ethiopia–Sudan border. Copyright  2009 John Wiley & Sons, Ltd. KEY WORDS model; erosion; sedimentation; rainfall-runoff; monsoonal climate Received 14 October 2008; Accepted 22 September 2009 INTRODUCTION The Abay (Blue Nile) River in Ethiopia contributes sig- nificant flow and sediment to the Nile River. Thus, a better understanding of the hydrological processes, ero- sive losses and sedimentation mechanisms in the vari- ous watersheds in the headwaters of the Nile River is of considerable importance. There is a need to improve and augment current resource management and develop- ment activities in areas with heavy degradation and low productivity, particularly in Ethiopia, where it is gener- ally believed that only 5% of surface water is utilized (Weiß and Schaldach, 2008). There is a particular need to develop further existing hydropower and irrigation poten- tial of the Abay (Blue Nile) for socio-economic develop- ment in Ethiopia, while maintaining sustainable operation * Correspondence to: Tammo S. Steenhuis, Department of Biological and Environmental Engineering, Riley Robb Hall, Cornell University, Ithaca, NY 14853, USA. E-mail: tss1@cornell.edu † Present address: Department of Natural Resources, Bahir Dar University, Bahir Dar, Ethiopia. of water infrastructure systems downstream in Sudan and Egypt. Sustainable operation is dependent in large part on preventing silting up of reservoirs. This paper focusses on characterizing the rainfall-runoff–sediment relationships for the Ethiopian portion of the Blue Nile River. The majority of the sedimentation of rivers in the basin occurs during the early period of the rainy season and peaks of sediment are consistently measured before peaks of dis- charge for a given rainy season. Typical erosion models based on stream power would predict the greatest con- centration to occur when the velocity and discharges are at their maximum (e.g. soil and water assessment tool (SWAT), annualized agricultural non-point source pollu- tion (AnnAGNPS), general watershed loading function (GWLF)). Thus, innovative models are called for to pre- dict erosion and sedimentation that are consistent with the hydrology of the region. Once developed, these models can be used for managing and or mitigating the sedimen- tation of newly constructed reservoirs. A review by Awulachew et al. (2009) shows that the number of models simulating the discharge from Copyright  2009 John Wiley & Sons, Ltd.