Raster and Vector Datasets

28 28 3.4.2 Model Parameter and Input 3.4.2.1 Schematic Watershed Model – Watershed Delineation First step in hydrological modeling is delineating watershed boundaries and discretize to hydrology response unit. The aim of the watershed delineation process is to determine the boundary of the watershed and also to break it into smaller management unit sub-basin if necessary. The watershed boundary was derived from Suttle Radar Terrain Mission SRTM data with 90 by 90 meters of spatial resolution. It divided into seven sub-basins as shows in Figure 3.12. In additional to determining the catchment boundary and its sub-basins area, the watershed delineation process is also determined the stream network and its related parameters such as basin slope, river slope, basin distance, river length, etc. The watershed delineation process was done by using HEC-GeoHMS tool that is can be integrated as plug-in on ArcMap. Connector Reach Junction OutletSink Sub-Basin Figure 3.12 Schematic network element of HEC-HMS model in Palu catchment 29 29

3.4.2.2 Sub-Basin Lag Time and Peaking Coefficient

This research was use the Snyder transform method to transform the rainfall into unit hydrograph. The Snyder transform method requires the lag time and peaking coefficient information to determine the peak time. Lag time is defined as the time difference between the peak of the rain event and the peak discharge. Based on the Snyder theory, the rainfall transformations into unit hydrograph are determined by some basin parameter, in which those parameters can be measured based on the physical form of the watershed Hartanto, N. 2009. Figure 3.13 Hydrograph element; lag time illustration. Basin lag time can be obtained by using some lag time equation. This study is using Tulsa rural lag time equation, in which the equation was developed by US Army Corps of Engineers for Tusla district. = … … … … … … … … … … … … … … … … … … … … … … . 6 Where Ct is a coefficient for natural watershed with value 1.420, Tp is lag time, L is sub-basin length, Lca is length to sub-basin centroid, S is maximum river slope, and m is a power coefficient with default value of 0.33. The Snyder peaking coefficient can be calculated by using following relationship for the peak flow rate that can be used solve for Snyder’s peaking coefficient. = 640 … … … … … … … … … … … … … … … … … … … … … … … … … … . 7