Browsing by Author "Alemu, Mersha"

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    Comparative Analysis of the Effects of Land Use/Land Cover Changes on Hydrology and Sedimentation: the Case of Wabeand Meki Catchments, Central Ethiopia
    (Addis Ababa University, 2017-08-03) Alemu, Mersha; Ayenew, Tenalem (Professor)
    Wabe and Meki, located in southwestern and central part of Ethiopia, are adjacent catchments and flowing to Omo-Gibe and Rift Valley Lakes basins respectively.The total area coverage for Wabe and Meki catchments are 1763km2 and 2168km2 respectively.The land use/land cover changes observed in the catchments over the past 25 yearshave significantly impacted the hydrologic and sedimentation conditions of the catchments. In order to assess these impacts, land use/land cover maps of three-time series data were generated from landsat images of 1986, 1999, and 2011, using remote sensing and GIS tools. Based on the three land use/land cover maps and hydrogeological inputs, SWAT model was appliedto analyze the effects of these changes on hydrolologic components and sediment yields of the catchments. Hydrogeological characterization of the catchments was carried out using conventional hydrogeological, techniques. The results of change detection showed that,cultivation is expanding mainly at the expense of forested land and enchroachments into range lands for the periods of 1986-1999 and 1999-2011.The mean annual surface runoff increased from 164mm to 188mm and from 256mm to 259mm for Meki and Wabe catchments respectively. However, the baseflow decreased from 216mm to 145mm and from 370mm to 352mm for Meki and Wabe catchmentsrespectively. Similarly, the average sediment yield generated for Meki catchment decreased from 51t/ha to 31t/ha in the first period (1986-1999), but increased to 47t/ha in the second period (1999-2011). Whereas, it decreased from 37t/ha to 31t/ha in the first period, but increased to 49t/ha in the second period in the case of Wabe catchment.The general increase in surface runoff and sediment yields and decrease in baseflow from 1986 to 2011 is largely due to the expansion of agricultural lands by 10% and 8% as well as deforestation by 15% and 11%for Wabe and Meki catchments respectively. For prediction years of 2012 to 2026, agricultural land expanded by 17% and 11%, but deforestation is increased by 30% and 5% in Wabe and Meki catchments respectively. Conversely, surface runoff shows a decline in both catchments whereas the predicted baseflow indicates an increasing trend in Meki, but a decreasing in Wabe catchmenet due to large variation in the rate of deforestation between the catchments. However, the effects on both surface runoff and baseflow can be attributed to changes inbothland use/land cover and climatic factors.
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    Flood Hazard, Vulnerability and Risk Assessments:Itang Area of Gambella
    (2007-06) Alemu, Mersha; Ahmad, Syed (PhD)
    The town of Itang is located in the lower part of the Baro-Akobo basin. The geographical position and the topographical composition have made the area vulnerable to flooding. The heavy rainfall of the area and the high discharges of Baro River, with the source from high lands, are determining factors for the occurrences of flooding in the area. The uncontrolled populating to flood plain area and the river channel characteristics also contribute for the severity of the flooding. The geomorphologic units, land use/ land cover, the digital elevation models (DEM) and the frequency analysis of hydrologic data for the study area are utilized as inputs for this study in order to deal with the nature and characteristics of the Baro river flooding. The floodwater depths and the flooding extents of different return periods on the basis of hydrologic data and actual flood event of 1988 have been analyzed. GIS provides the broad rang of tools for determining areas affected by floods and for predicting areas that are likely to be flooded due to high water level. This method also helps to assess vulnerability of different elements in addition to flood hazard zonation maps. This involves a combination of flood scenarios of different return periods, elements at risk and DEM of the area through the application of vulnerability functions. The function relates the floodwater depth and the degree of loss or damage to each elements or group of elements on the basis of flooding extents. Based on vulnerabilities, probabilities and degree of damage, methods for risk assessments for the three return periods have been developed. And finally, recommendations were forwarded in order to reduce the damages and losses that would arise from flooding