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

RESEARCH IWMI R E P O R T I n t e r n a t i o n a l Water Management I n s t i t u t e Research ReportsThe publications in this series cover a wide range of subjects-from computer modeling to experience with water user associations-and vary in content from directly applicable research to more basic studies, on which applied work ultimately depends. Some research reports are narrowly focused, analytical and detailed empirical studies; others are wide-ranging and synthetic overviews of generic problems.Although most of the reports are published by IWMI staff and their collaborators, we welcome contributions from others. Each report is reviewed internally by IWMI's own staff and Fellows, and by external reviewers. The reports are published and distributed both in hard copy and electronically (www.iwmi.org) and where possible all data and analyses will be available as separate downloadable files. Reports may be copied freely and cited with due acknowledgment. About IWMIIWMI's mission is to improve the management of land and water resources for food, livelihoods and the environment. In serving this mission, IWMI concentrates on the integration of policies, technologies and management systems to achieve workable solutions to real problems-practical, relevant results in the field of irrigation and water and land resources.

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 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 two subbasin locations in the basin. Subbasins ranged in size from 4.8 to 174 000 km², 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 shallow soils. 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 before the soil was wetted up and plant cover was established. Once plant cover was established in mid August landscape erosion 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

The Soil and Water Assessment Tool (SWAT) uses the popular Curve Number (CN) method to determine the respective amounts of infiltration and surface runoff. While appropriate for engineering design in temperate climates, the CN is less than ideal in monsoonal climates and areas dominated by variable source area hydrology. The CN methodology is based on the assumption that there is a unique relationship between the average moisture content and the CN for all hydrologic response units, a questionable

SUMMARYEthiopia has an irrigation potential of 5.3 million ha (Mha) of which 3.7 Mha can be developed using surface water sources, and 1.6 Mha using groundwater and rainwater management. Irrigation contributes to rapid transformation of agriculture as present-day agriculture is dominated by rainfed single crops. The current irrigation development in Ethiopia is about 0.7 Mha, and the performance of the existing schemes is not well understood. As the country is planning to expand irrigated agriculture in the next five years, it will be useful to review existing performance and to identify areas for interventions that help revitalize underperforming systems. In this paper, we have investigated the performances of irrigation at three levels: (a) national level for broad performance, (b) regional level for small-scale irrigation and (c) scheme level for large-scale irrigation. National level indicators measure (i) the relative proportion of operating schemes, (ii) ratio of actually cultivated area to planned command area and (iii) relative number of benefited to targeted number of households. The result shows that 86.5% of schemes are operating, 74.1% of command area is under cultivation and only 46.8% of the planned beneficiaries have benefited from implemented irrigation. For regional level irrigation performances, the regions of Southern Nations and Nationalities and Oromia Regions were investigated. We used technological (structural) and management factors as measures of performances. Sixty-four underperforming schemes were sampled from the two regions to analyse the causes. About 30 parameters were identified as causes of underperformance. Watershed degradation related problems that are causing erosion and sedimentation of water control and conveyance structures are found to be the major cause for structural failures, while lack of sustainable funding, extension of agronomic practice, and post harvest technologies are identified as the top management-related problems. For evaluating performances of large-scale schemes, we used irrigation water delivery performance and output performance indicators applied to six large-scale schemes. Scheme level performance indicators results showed that all of the schemes considered have supplied adequate to excess amounts of water during the period. The Wonji scheme that uses pump diversion showed higher water use efficiency than other schemes that are using simple gravity diversion types. In this case it might be the running costs of pumps that have encouraged efficient management of water. In terms of output performance, sugarcane based irrigation schemes are superior and up to ten times that of banana, cotton, and maize or tobacco production. The results of these assessments are useful for decision-makers and disclosed the low performance of the existing irrigation schemes. They also indicated the need to revitalize existing schemes to improve performance in parallel to the implementation of new projects. The paper also provided new indicators of evaluation of performance with respect to national level, structural and management related performance.