I4. Irrigation Area Suitability Mapping by Multi-criteria Evaluation Technique for the Case of Lake Tana Basin, Upper Blue Nile, Ethiopia

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The study was carried out in the Lake Tana Basin, the upper portion of the Blue Nile Basin in Ethiopia. It has a total catchment area of around 15,000 km2, of which the lake covers approximately 3,060 km2 at an average altitude of 1786 m amsl. Although the lake has developmental potential, until recently, there has been only one water resource development situated at the mouth of the lake to control the outflow for harnessing hydroelectricity down stream on the Blue Nile River. Besides hydroelectric power, expansion of surface irrigation is of great interest to the basin farmers whose livelihoods are heavily dependent on cereal and other rainfed crop production. Therefore, in this study, the suitability of surface irrigation within the lake basin was evaluated by employing a GIS-based Multi Criteria Evaluation (MCE) analysis of available spatial data. The main objective was to identify suitable medium scale (between 200 and 3000ha) and large scale (greater than 3000ha) irrigation areas in the basin by considering factors, such as meteorological information (temperature, humidity, rainfall etc), river proximity, soil type, land cover, topography/slope and market outlets. Using the daily metrological data from 1992 to 2006, the long-term average rainfall and potential evapotranspiration raster map was computed through interpolation based on Thiessen polygons. The monthly rainfall deficit map (rainfall-evaporation) was aggregated to the annual rainfall deficit map. The major perennial river network segment map and slope raster map was derived from an SRTM DEM of the basin, and then the drainage network map was interpolated using the Euclidian distance tool in ArcGIS. Major potential marketing towns and the main paved road were digitized manually from Google Earth and interpolated. The interpolated maps were reclassified into four groups of suitability by an equal interval ranging technique. The soil and land use map collected from the Ethiopian Ministry of Water and Energy (EMWE) was reclassified to four major classes of FAO land suitability. Weighting of the decision factors was accomplished by comparing three approaches: ranking technique, pairwise comparison and equal weighting. Approximately 10%, 7% and 5% of the basin was suitable for surface irrigation according to the pairwise, equal weighting and ranking technique, respectively.

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