Assessment of rainfall-runoff process using Arc-GIS and HEC-HMS in Gelana Watershed, Rift Valley basin Ethiopia.

Abstract Estimating runoff from rainfall is necessary to know the amount of water available in a river basin and to better understand the occurrence of flooding. In different basins, hydrological models have been used to determine hydrological processes and available water resources. This study investigates the use of distributed hydrological models to estimate rainfall-runoff processes in the Gelana River basin in West Guji Region,Ethiopia. The hydrological model HEC-HMS (Hydrologic Engineering Center, HydrologicalModeling System) is used in conjunction with the geospatial hydrological model extension, HEC-GeoHMS in the ArcGIS environment. DEMs (Digital Elevation Models)of the study areas were processed using HEC-GeoHMS, an ArcView GIS extension for watershed delineation, terrain preprocessing, and watershed processing was used to estimate the HEC-HMS continuous rainfall runoff simulation. The meteorological models were developed within the HEC-HMS from the recorded daily rainfall data for the hydrological years 2000 to 2010. In HEC-HMS, the basin model includesthree important processes; loss, conversion and basic flow. The control specifications have been defined for the specified time period and time step within a day. Soil conservation service curvenumber (SCS-CN), SCS unit hydrology method, and Muskingum methodwere used to calculate losses, conversion,and routing, respectively. For model calibration the coefficient of determination, Nash-Sutcliffe coefficient, and PBIAS resulted in 0.75, 0.72 and 3.4 respectively for calibration and 0.79, 0.77 and -7 respectively for validation. The results indicate that HEC-HMS can be used for the hydrological modeling of the watershed which will be necessary for soil and water conservation of the area.