Tewodros proposal
- 1. ASSESSMENT OF SURFACE WATER POTENTIAL IN THE OMO-GIBE RIVER BASIN, ETHIOPIA Tewodros Assefa Nigussie 1. INTRODUCTION 1.1 Background and justification With an estimated total annual surface flow of 123 billion cubic meters per year, Ethiopia is said to have huge water resources. Despite this, however, very little of it has been developed for various purposes such as agriculture, industry and hydropower. According to the Ministry of Water Resources (2001), a very small portion of the potential irrigable land in the country is (3.7 million ha) has been developed so far. Much of the water resources in Ethiopia are not utilized to meet the food demand. It is a dilemma why Ethiopia is starving while it has huge amount of surface water and perennial rivers. This is mainly because of lack of well-organized research on water resource management and finance (Tadesse, 2006). As a result, the country could still face million tons of cereal deficits by in the future (UK Trade and Investment 2004). This is in addition to the problems related to domestic and industrial water supply. The Omo-Gibe River Basin is one of the most important river basins of Ethiopia as it is key for development of new hydropower and irrigation projects. Like many of the other basins, this basin is facing water related challenges, including water scarcity (both economic and physical) and competition for water between different sectors and states. The solution to these problems is proper water management, which should be based on for water resource assessment. This is because water resources assessment at a basin scale is one of the key activities to provide insight into water available for various purposes (Abdulla et al., 2002; Al-Adamat et al., 2010). In addition, the information generated via the assessment can be used to support current and potential future investments in managing water resources in a basin (US Army Corps of Engineers, 1998). A comprehensive assessment of the water resources available in a region or a river basin is essential for finding sustainable solutions for water-related problems concerning both the quantity and quality of the water resources. It is a pre-requisite to undertake an analysis of the stress on the water resources and to subsequently adopt appropriate management strategies to avoid adverse environmental effects and reconcile conflicts between users (Xu and Singh, 2004). Accurate assessment of Surface water potential and demands are critical to solve water related problems in basins (Dereje Atnafu, 2015). Knowing the potential and availability of surface water in a basin is crucial for wise use of the resource, designing economical and suitable hydraulic structure for water supply, hydropower, irrigation and other purposes (Asmerom, 2015). Such a study is, thus, a necessity to deal with water resource management problems in basins like Omo-Gibe River Basin. In this regard, the Omo-Gibe River Basin Master Plan was developed in 1992. The studies in the master plan were, however, carried out based only on observed flow
- 2. data at terminal sites for various purposes. In addition, lumped approach was used in estimating water availability at terminal sites of the basin, and there were no hydrological models used for assessing water resources availability. The WATBAL model was used by Bogale (2011) to simulate stream flow at an ungauged watershed using regionalization technique in the Omo-Gibe River Basin. The study was carried out just in one watershed and, thus, did not assess the water resource at basin scale. Moreover, the study indicated some limitation of the WATBAL model and recommended the use of more powerful models such as the Soil Water Assessment Tool (SWAT) for similar studies. Antafu (2015) carried out a study to quantify basin scale water wealth of the Basin using the SWAT model. However, the author divided the basin into only 13 sub-basins and calibrated the model using data obtained from one gauged station. This study, thus, cannot be taken as reliable and as detailed as the required level. It can be seen from the above reviewed papers that there has not been a study that assessed the water resource of the Omo-Gibe River Basin to the required detail and accuracy. On top of these, the irrigation potential of the basin differs between studies: FAO (2016) estimated 383,000 ha and the Omo-Gibe Master Plan (2001) estimated 348,000 ha. Such an ambiguity needs to be cleared and assessing the surface water potential is vital for that. This study is, therefore, initiated to assess the surface water potential of the Omo-Gibe River Basin by dividing the Basin into a number of watersheds so that the assessment is carried out the required depth using the SWAT model. 1.2 Objectives of the study 1.2.1 General objective The general objective of this study is to assess and map the surface water potential of the Omo- Gibe River Basin. 1.2.2 Specific objectives The specific objectives of these study are to: delineate all rivers and derive basin characteristics of gauged and ungauged rivers in the basin screen and map stream flow data for gauged stations in the basin, and compute minimum and maximum stream flow for gauged and ungauged stations in the basin
- 3. 2. MATERIAL AND METHODS 2.1 Description of the study area This study will be conducted in the Omo-Gibe River Basin. The Basin is located in the southwestern part of Ethiopia extending between 4o00’ North and 9o22’ North, and 34o44’ East and 38o24’ East geographical coordinates (Figure 1). The basin covers an area of 79,000 km2 with a length of 550 km and a mean width of 140 km. These sizes make it the second largest basin in Ethiopia. The river flows from north to south and drains into Lake Turkana and its two major tributaries are the Gibe and Gojeb Rivers. Figure 1. The Omo-Gibe River Basin with its tributaries and locations of hydropower projects (source: Boulos, 2017).
- 4. The Omo-Gibe River Basin, abbreviated as OGRB from now on, is bounded by the Baro-Akobo River Basin in the west, the Blue-Nile River Basin in the north and northwest, and the Rift Valley Lakes Basin in the east. A small area of the OGRB is bounded by the Awash River Basin in the northeast (Woodroofe and Associates, 1996). About 25% of the area of the basin falls within the State of Oromiya and the remaining 75% falls within the Southern Nations, Nationalities and People’s Regional State (SNNPR) (ITAB-CONSULT PLC, 2001). Topography According to ITAB-CONSULT PLC (2001), the highest point in the Basin has an elevation of 4,200 meters above sea level (m a.s.l.) and lowest point has an elevation of 368 m a.s.l. In general, the elevation in the basin decreases from East to West and from North to South. The basin can generally be divided into highlands and lowlands. According to Mays and Yung (1992), about 38% of the lowlands have slopes of less than 2% while a further 20% of the area falls between 2% and 10% slope. Rainfall and Temperature The rainfall amount in the basin is very diverse; annual rainfall amount ranges from 400 mm in the lowlands located in the southern part of the basin to 1900 mm in the highlands that dominate the northern part of the basin. This shows that, in general, the rainfall amount in the basin increases with an increase in elevation. Based on monthly rainfall distribution regimes, OGRB is classified into three distinct rainfall regimes: a bimodal rainfall distribution pattern, which is found in the central eastern part of the Basin, an asymmetric bimodal rainfall distribution pattern found in the north-eastern part of the Basin, and a unimodal rainfall distribution pattern that dominates the western mountain slopes of the basin. Similar to the rainfall in the Omo-Gibe River Basin, the mean annual temperature in the basin is very diverse, ranging from 16oC in the highlands located in the of the northern part of the basin to more than 30oC in areas of the lowlands dominating the southern part of the basin. The basin, according to the traditional classification system, can be classified into three agro-ecology zones based on the temperature of the basin together with the altitude: Dega (cool zone), Weyna-Dega (temperate zone) and Kolla (hot zone). Soils The dominant soils in the basin are well drained red and reddish brown clay looms over clays that have deep to very deep depth. The soils are, however, wide spread over the whole of the northern part of the Basin. On the highlands of the basin (northern boundary of the basin), well drained soils that developed from volcanic parent materials, often with pumice layer, and having depth ranging from moderately deep to deep, dark brown to dark reddish brown colored, and having textures of sandy clay loams to clays tend to be dominant. In the southern part of the Basin, which is characterized by lower rainfall, the soils developed are characterized by the
- 5. presence of a course sand fraction. These soils are moderately deep to very deep, well drained yellowish brown course grained sandy loams to sandy clays. Landuse and vegetation The northern part of the basin is characterized by extensive cultivation with increased land pressure. According to Atnafu (2015), forest areas are now confined to areas too steep and inaccessible to farm. 2.2 Data collection and analyses To be copied from the other proposal