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Arc Hydro is a geospatial and temporal data model for water resources that operate within ArcGIS (Maidment, ESRI, 2002). Arc Hydro is not a simulation model in itself. It helps to structure data to support the hydrologic simulation model. HRUs can be delineated automatically using the digital elevation model (DEM) or triangular irregular network (TIN) together with ArcHydro. The drainage path is defined based on the eight-direction pour point model, hence the creation of a flow direction grid. This model is discussed in detail (Arc Hydro, Maidment, 2002). Theflow accumulation grid is developed from the flow direction grid. By defining the threshold drainage area (flow accumulation value), the stream links are defined. The catchment grid is developed by combining the flow direction grid and stream links. These raster grids - stream links, outlet cells and catchment grid were vectorized to drainage lines, drainage points and catchment polygons respectively and stored in Arc Hydro. Watershed and subwatersheds were delineated by selecting outlet points of interest.