Abstract
The limitation of hydrograph data availability becomes an obstacle for the design of water resources. The obstacle makes the Synthetic Unit Hydrograph give the big enough benefit for the design of water resources. Ideally, every watershed has its unit hydrograph. The Nakayasu Synthetic Unit Hydrograph is one of the synthetic unit hydrographs frequently used in the analysis of design flood discharge in Indonesia. However, the Nakayasu Synthetic Unit Hydrograph concept has been widely used and evaluated in some areas in Indonesia, and there is bias in determining the watershed characteristic coefficient. This research intends to investigate the characteristic of the observed hydrograph in every watershed and the whole watershed in the South Sulawesi Province-Indonesia. The research aims to build the alpha formula using alpha modeling with the affected parameters of watershed area, river length, river bed slope, shape factor, and the time concentration. Based on the ArcGIS version 10.7, most of the watershed’s shapes are elongated. The methodology consists of analyzing observed unit hydrograph by using the Collins method and Nakayasu Synthetic Unit Hydrograph. Then the model is built by using the statistical analysis of SPSS version 23 software for obtaining the affected level of each parameter that is determined. However, the modeling analysis uses the non-linear regression after there is carried out the descriptive test. The alpha formula for the elongate watershed in the Jeneberang River is proposed.