Developing a Hydrological-GIS Data Base System in the Blue Nile Basin: A Support for the Irrigated Agriculture

Transcription

1 R ( ) Proceedings of the International Workshop on Participatory Management of Irrigation Systems, Water Utilization Techniques & Hydrology A Session of the 3 rd World Water Forum, Theme: Agriculture, Food & Water, March 2003 Developing a Hydrological-GIS Data Base System in the Blue Nile Basin: A Support for the Irrigated Agriculture Anil Mishra 1*, Takeshi Hata 1, and A.W. Abdelhadi 2 Abstract Though the river Nile in the North-Eastern Africa is one of the most studied river in the world, flow gauge dating as far back as 620 AD have been collected from the river, very little information about the Blue Nile catchment and its hydrology are available, especially in its upper basin within Ethiopia. The Blue Nile originates in the highlands of Ethiopia, as do other two major tributaries of the Nile, the Atbara and the Sobat. About 80% of the Nile's water originates in Ethiopia and mainly drain towards downstream through Blue Nile. The down stream states of Egypt and Sudan are heavily dependent on irrigated agriculture for food/cotton production, and use 94 % of the available Nile water. Pressure on Nile resources is likely to increase dramatically in the coming years as a result of high population growth rates in all riparian states, and increasing development-related water needs in Ethiopia. This part of the study focuses on the Blue Nile river basin flowing through Ethiopia and Sudan. Sharing of information, improvement on monitoring, research and mainly establishment of hydrological system database are essential steps in order to improve knowledge on Blue Nile water and its resources. The size and complexity of the Blue Nile, together with the lack of hydrological data, is therefore a severe constraint to the application of sophisticated hydrological models. The establishment of global databases of geographic, climatic and other physical parameter is one of the exciting, new 1 Division of Regional Environment, Department of Global Development Science, Kobe University, Japan. mishra@ans.kobe-u.ac.jp 2 Agricultural Research Corporation, Agricultural Engineering Research Proram, Wad Medani, Sudan 86

2 developments in Geographic Information Systems. Public domain datasets have become increasingly available on the Internet. These data are free, easy to obtain, and often more up-to-date than those from local sources. These data sets can be used as inputs to establish hydrological database for such basins. In this study a GIS- hydrological data base for the Blue Nile catchment is developed and it is hoped that the data base system might help in managing Blue Nile water to fulfill irrigation and hydropower needs. Introduction The Nile basin, 3 million km 2, covers about 10% of the area of Africa, and 2.3% of the world s land surface area. Its tributaries are united in the Sudan and flow through a virtually rain free desert into the Mediterranean Sea. The Blue Nile is by far the largest tributary in terms of water flow. Its share of the water that reaches Aswan in Egypt is 55%. The other tributary, which also originate from Ethiopian Highlands, Atbara brings 12% of the water, whereas the contribution of the longest arm, the White Nile, which also includes contribution from Sobat (Ethiopia) is around 33%. Table 1. The contribution of the main Nile tributaries Source Average annual flow (Km 3 ) Contribution (%) Blue Nile (Ethiopia) Atbara(Ethiopia,) White Nile including Sobat (Equatorial lakes region 7- countries, + Ethiopia) Sources: Shahin 1985, Klot 1994, Sutcliffe and Parks 1999 The Blue Nile River originates at Lake Tana, in the Ethiopian Highlands. The highland plateau has been deeply incised by the Blue Nile tributaries. Much of the plateau is above 2600 m, with several peaks up to 4000 m or more. Lake Tana is a natural lake with a surface area of about 3150 km 2 and is fed by a number of streams. From Lake Tana the Blue Nile flows in a southwestern direction for 35 kilometers until the Tissiasat Falls where it drops 50 meters. Below the falls the river enters a canyon, which deepens progressively downstream, in places the Blue Nile flows 1300 m below the level of country on the either side. At the foot of the plateau the plain slopes gradually westwards down into Sudan from a height of about 700m. The Blue Nile provides a vital source of freshwater to the downstream riparian users, Sudan and 87

3 Egypt. However, only a few studies on its upper basin within Ethiopia have been conducted and the information available on the climatic, hydrologic, topologic, and hydraulic characteristics of the river and the basin is incomplete (Gamachu, 1977; Conway 1997). Thus the hydrologic assessment of the upper Blue Nile is dependent on downstream measurement. The Blue Nile is characterized by severe seasonality and about 70% of the runoff occurs between July and September. Peaking in August, after which the Blue Nile start to fall as rain water supply to the river begins to decline (Mishra et al., 2003a). Flow in the river rises again in March-May because of equatorial spring rains on the southern tributaries. Khartoum Sudan N Sennar Ethiopia Lake Tana El Deim 0km 300km Fig. 1. The Blue Nile basin and its tributaries Lake Tana outflow is only 7.7 % of the flow at Roseires dam in Sudan and the difference between the peak month in August at Roseires/El Deim and in September at the outfall of Lake Tana shows the relatively slight effect of lake storage (Sutcliffe and Parks, 1999). The average annual rainfall over the Blue Nile basin is about 1600 mm, which increases from about 1000 mm near Sudan border to about mm over the parts of the upper basin near lake Tana (Sutcliffe and Parks, 1999). The seasonal distribution of runoff varies considerably owing to differences in the seasonality of rainfall and catchment physiography. These runoff patterns reflect the variation in 88

4 rainfall distribution in the basin (Conway, 2000). The topography of the catchment is derived from the US Geological Survey s 30 arc second resolution HYDRO1K (GTOPO30) Digital Elevation Model (DEM). The delineated upper Blue Nile catchment is shown in Figure 1. The catchment area is about 175, 000 km 2. Water resources management in the Blue Nile Basin The hydrological data required for water resources planning depend on the type of development. The development of water resources within the Blue Nile Basin is to a large extent dominated by irrigation and hydro-electric power; while water demand for domestic and industrial purposes is by comparison smaller. The Blue Nile course in Ethiopia is mostly in an inaccessible and poorly mapped area. Irrigation dominates the consumptive use of the Blue Nile waters in Sudan. About two thirds of the irrigation requirements in Sudan are satisfied from the Blue Nile. The aim of the Sudanese authorities is to maximize irrigation, hydropower and water supply while minimizing sedimentation within the framework of 1959 River Nile water agreement between Egypt and Sudan. Several attempts have been made to forecast the Blue Nile low flow for this purpose (Abdelhadi et al. 2000, Mishra et al., 2003a, 2003b). There are two reservoirs in the Sudanese territory along the Blue Nile, Roseires and Sennar, with a combined storage capacity of 3 km 3. The reservoirs are operated in series. Operating rules have been developed to minimize sedimentation and to optimize hydropower production in terms of water availability and irrigation demands (Hamad, 1993). The problem is complex because energy demand increases during the recession time and doesn t coincide with irrigation releases (Sutcliffe and Parks, 1999). The water stored from the previous season in the two reservoirs is essential for irrigation and for hydropower generation. Since the combined capacity of the two reservoirs falls short of the total requirements the gap has to be supplemented from the natural river flow during the recession time. Preceding flood information holds indispensable information about the foregoing hydrological processes (Mishra et al., 2003a). The length of flow volumes and low flows are both important for water resources assessment. The need for flood estimates for design and operational forecasting purposes makes high flow measurement necessary (Sutcliffe and Lazenby, 1996). As available information on the rainfall and water balance within Ethiopia is limited, establishment of hydrological system database, 89

5 research, monitoring and sharing of information are essential steps in order to improve knowledge and management of the Blue Nile water. Hydrological- GIS database for the Blue Nile Basin Computer simulation models have been used extensively for analyzing river systems. However, many of these models are limited in application to the particular river basins and require the collection of large amounts of data. The size and complexity of the Nile, together with the lack of hydrological data, is therefore a severe constraint to the application of sophisticated hydrological models. Despite the numerous data problem, Nile countries have the greatest need for models that enable them to better manage their common natural resources. The establishment of global databases of geographic, climatic and other physical parameters are one of the exciting, new developments in Geographic Information Systems. Based on Hydrological-GIS (Geographical Information System) database, modeling framework for basin boundaries can be implemented in a grid-based model. Grid-based hydrological models may be integrated with Global Climate Model (GCM). GIS have recently been used in deriving raster and vector representation of watersheds and rivers for hydrologic modeling. GIS based approaches have also been used for deriving hydrologic parameters from grided representation of the earth. GIS thus provides an ideal environment for developing and parameterizing large scale routing. From the use of DEM (digital elevation model), hydrologic features of the terrain (i.e. flow direction, flow length, stream network and drainage areas) can be determined using any GIS software. This study aims to contribute to this goal by establishing a Hydrological-GIS data base system for the upper Blue Nile Basin. Data sets from public domain Data sets from public domain are free, easy to obtain, and often more up-todate than those from local sources (Lacroix et al., 2000). These data can be used as inputs in to hydrological models. - USGS HYDRO1K DEM data HYDRO1k is a geographic database developed to provide comprehensive and consistent global coverage of topographically derived data sets, including streams, drainage basins and ancillary layers derived from the USGS' 30 arc-second digital 90

6 elevation model of the world (GTOPO30). The HYDRO1K is a hydrologically corrected version of GTOPO30 DEM, which is available for downloading from the USGS website: - Global climate data Global data sets of mean monthly temperature and precipitation interpolated to a 0.5 by 0.5 grid are available from University of Delaware. These data are from the "Global Air Temperature and Precipitation Data Archive" compiled by D. Legates and C. Willmott. These monthly precipitation estimates were previously corrected for gage bias. Data from 24,635 terrestrial stations and 2,223 oceanic grid points were used to estimate the precipitation field. The data can be downloaded from; - Water holding capacity data Global estimates of plant-extractable water capacity have recently become available on a 0.5 grid (Dunne and Willmott, 1996). As used in this study, the term plant-extractable water capacity is equivalent to available water-holding capacity. Grid water-holding capacity estimates can also be obtained from FAO. Values in the grid are compiled in the CD-ROM Digitized Soil Map of the World (FAO). The GIS based water balance and water transport models A macro- scale grid based model has been developed and organized as a set of interacting components of water balance and water transport sub models together with the Digital Elevation Models (DEM). The catchment of the upper Blue Nile Basin is represented by a digital elevation model HYDRO1K (GTOPO30) from which the stream network is derived. Using climatic and topographic data from the public domain a simple water balance model was used to generate monthly surplus for each 0.5 o x 0.5 o grid. A fraction of precipitation is extracted and declared runoff, before remaining precipitation is passed to the soil. The runoff for each grid is then routed directly from its source of generation to the outlet, the approach is also known as source to sink (Olivera et al., 2000). Thus for the upper Blue Nile 58 sources and one sink at Roseires El Deim are identified. The model was verified by comparing simulated flows with the recorded monthly flows at the outlet of four sub-catchments and the total simulated flow with the recorded monthly flow measured at the outlet of the catchment at El/Deim station located near Sudan Ethiopia border. Thus for the upper Blue Nile catchment a 91

7 GIS- hydrological data base and a grid based water balance and water transport model was developed using data sets from the public domain. Further extension of the study into the whole Blue Nile and Nile basin remains a scope for future research. El Deim station Conclusion Fig. 2. Upper Blue Nile watershed with 58-source and sink at El Deim Increased competition for water will be amongst the most important issues of the next few decades. Similarly, water scarcity for agriculture and the resulting problem of food security must be addressed. Establishment of the proper set of Hydrological-GIS (Geographic Information System) database, which can query both the hydrological and geographic information of the Nile Basin, will be a milestone in developing long-term cooperation and better integration of common issues: to achieve sustainable socioeconomic development through the equitable utilization of, and benefit from, the common Nile Basin water resources, which is also a motto of Nile Basin Initiatives. The study is an attempt to model the upper Blue Nile basin using data set only from the Internet. The GIS based Hydrological data set and the model will be a useful decision support system for water resources planners, managers, stake holders and policy makers which will be eventually helpful for better management of scarce water resources. 92

8 References Abdelhadi A. W., O. E. Hamad and Takeshi Hata A Recession forecast model for the Blue Nile River. Nordic Hydrology, 31 (1). Pp Conway D A water balance model of the Upper Blue Nile in Ethiopia. Hydrological Sciences Journal 42: Conway D The Climate and Hydrology of the Upper Blue Nile River. The Geographical Journal 166: Dunne KA. Willmott CJ Global Distribution of Plant-extractable Water Capacity of Soil, International Journal of Climatology, 16, Gamachu D Aspects of climate and water budget in Ethiopia. Addis Ababa: Addis Ababa University Press Hamad OE Optimal operation of a reservoir system during a dry season. Ph.D. Thesis. University of Newcastle upon Tyne, Department of Civil Engineering Lacroix M. KiteG. And Droogers P Using Datasets from the Internet for Hydrological Modeling: An Example from the Menderes Basin, Turkey, IWMI Research Report No. 40. Olivera F. Famiglietti J. and Asante K Global-Scale flow routing using a source - to- sink algorithm, Water Resour.Res. No. 36 (8) Klot N The geopolytics of the monopolized division of the Nile Waters. Water Resources and Conflict in the Midle East. Routledge London. Mishra A, Hata T, Abdelhadi AW, Tada A, and Tanakamaru H,. 2003a. Recession flow analysis of the Blue Nile River. Hydrological Processes, Special issue: Japan Society of Hydrology and Water Resources, Vol. 17 Mishra A, Hata T, Abdelhadi AW. 2003b. Models for recession flows in the upper Blue Nile River. Hydrological Processes, (Accepted). Shahin MMA Hydrology of the Nile Basin. Developments in water Sceince, 21, Elsevier, Amsterdam. Sutcliffe JV. Parks YP The Hydrology of the Nile. IAHS Special publication No:5. Sutcliffe JV. Lazenby, JBC Hydrological data requirement for planning Nile management. In: The Nile: Sharing a scarce Resource (ed. By PP Howell & JA Allan), Cambridge University Press, UK 93