In this study Hydrologic Engineering Center’s Geospatial Hydrologic Modeling System together with Arc-Hydro tools of ArcGIS are applied for catchment delineation, determination of watershed characteristics and developing methods to model a Curve Number values for Watersheds. This study is aimed at to model and map the Curve Number of Meki watershed considering Enset land use system for Hydrologic Engineering Center’s Hydrologic Modeling System based Hydrological simulations, Rift valley, Ethiopia. In this study, Earth Resources Data Acquisition System (ERDAS Imagine) 2014 used for land use data preparation and soil data is acquired from Ministry of Water, Irrigation and Electricity and compared with Harmonized World Soil Data base (HWSD) and verified on the field using its textural class using field finger analysis technique and used it as an input for Curve Number generation in ArcGIS 10.1 and Soil Conservation Curve Number algorithm is used to achieve estimation of the direct runoff. Meki watershed covers 2325.3 km2 which is dominantly covered by Agricultural land use system (41.5%) followed by Bush and Chat land use system (25.6%). Enset land use system comprises of 10.65% of the watershed while Forest and plantations together cover (14.14%) while urban and built up land use system covers 7.4% with minimal portion is covered by water bodies including wetland (0.75%). The watershed has an average curve number of 78.21 considering Enset land use has its own CN matrix while a curve number of 81.72 by considering Enset land use as an agricultural land. Hence a difference of runoff occurs due to consideration of Enset land use system by 3.59mm which can cumulate to a volume of runoff from the whole 2325.3km2 Meki watersheds is given as 8340916m3. Finally, there is an improvement of average CN from 81.72 to 78.21 for the whole Meki watershed and the volume of water infiltrated in the fields due to the consideration of Enset land use system is 8340916m3. A significant volume of water is infiltrated due to the presence of Enset land use system in the watershed which can indicate the improvement of the hydrological characteristics of the watershed by increasing the infiltration capacity of the soil and also by increasing the canopy cover of the area which reduces the rain drop impact not to cause erosion problem which can also contribute to protect the lake against sedimentation. Therefore, increasing the coverage of Enset land use system can contribute to reduce the runoff volume which could be the cause of flooding in different parts of the watershed and also protect the lake against sedimentation
Keywords: Curve number (CN), Catchment delineation, ERDAS, HEC-GEOHMS, HEC-HMS, model, Soil Conservation Curve Number (SCS-CN) & sedimentation.