Before carrying out terrain pre-processing, the input terrain data DEM was distinguished using DEM reconditioning. After this process, the DEM was pre-processed to originate sub-basins and drainage network of the catchment. The steps comprised were fill sinks, flow direction, flow accumulation, stream definition, stream segmentation, catchment grid delineation, catchment polygon processing, and adjoin catchment processing. After terrain pre-processing, HEC-HMS project was created. At first, a project point was defined at the downstream end of the watershed based on which the software delineated the project area. The resulting project area for Giriulla Badalgama catchment was 64.8 square kilometers.
The delineated sub-basins and rivers were merged based on river junctions. For each of the sub-basins and river, physical physiognomies were computed based on the
refined DEM. The computed characteristics for river included river length and river slope and
for basin included basin slope, longest flow path to the basin, basin centroid, centroid
elevation and centroid longest flowpath. To calculate basin slope, watershed slope was
required which was calculated using Arc Hydro tool.