359 Journal of the Civil Engineering Forum, September 2021, 7(3): 359-368 DOI 10.22146/jcef.64782 Available Online at http: https://jurnal.ugm.ac.id/jcef/issue/archive Rainfall-Runoff Simulation Using HEC-HMS Model in the Benanain Watershed, Timor Island Wilhelmus Bunganaen 1 , John H. Frans 1 , Yustinus A. Seran 1 , Djoko Legono 2 , Denik Sri Krisnayanti 1* 1 Department of Civil Engineering, Nusa Cendana Universitas, Kupang, INDONESIA Jalan Adi Sucipto Penfui Kupang 2 Department of Civil and Environmental Engineering, Universitas Gadjah Mada, Yogyakarta, INDONESIA Jalan Grafika No 2 Yogyakarta * Corresponding authors: denik.krisnayanti@staf.undana.ac.id SUBMITTED 18 March 2021 REVISED 11 July 2021 ACCEPTED 02 August 2021 ABSTRACT Floods in a watershed area are caused by reduced water recharge due to changes in land use, increasing their discharge volume. Benanain watershed is an extensive area with many tributaries. Watershed morphometrics provides initial information about the hydrological behavior and the hydrograph shape of flooding in these areas. Furthermore, rainfall-runoff modeling uses as a unit to approach the hydrological values of the flooding process. This study determines the physical characteristics of the Benanain watershed based on curve number () values, land cover, peak discharge, and peak time. It was conducted on the Benanain watershed with 29 sub- watersheds covering 3,181.521 km 2 . Data were collected on the rainfall experienced for 13 years from 1996 to 2008 and analyzed using the Log Pearson Type III method, while the HEC HMS model was used for flood discharge analysis. HEC-HMS model must calibrate by adjusting the model parameter values until the model results match historical data such as initial abstraction, lag time, recession, baseflow values, and curve number. The results show that the curve number values range from 56.55 - 73.90, comprising secondary dryland forest and shrubs. Moreover, the rock lithology in the Benanain watershed is dominated by scaly clay and other rock blocks. This means the area has low to very low permeability, which affects the volume of runoff. The return period of a 1000-year flood discharge obtained a peak of 5,794.50 m 3 /s, with a peak time of ± 14 hours. Morphometry of the Temef watershed with large catchment, radial shape pattern, an average of steep slope river, and meandering affects the peak of flood discharge hydrograph and the peak time of the flood. KEYWORDS Run off; Curve Number; Radial; Permeability; Rock Lithology. © The Author(s) 2021. This article is distributed under a Creative Commons Attribution-ShareAlike 4.0 International license. 1 INTRODUCTION A watershed is a catchment area with a unit river and tributaries accommodating, storing, and flowing water from rainfall to lakes or the sea. The land boundary is known as topography, while the sea boundary consists of the water areas affected by land activities (UU No. 7, 2004). Additionally, a watershed could be considered a hydrological unit because it converts rainfall (input) into runoff (output), sediment, and nutrients. The rainfall- runoff is a very complex scientific process influenced by several factors. Chow et al. (1998) explained that climatic and physiographic factors have the most significant influence on rainfall- runoff. Climatic factor includes the relation between rainfall and evapotranspiration, while physiographic factor influences comprise river and watershed characteristics. Rainfall is an important meteorological parameter to determine river flow discharge. Furthermore, the runoff is influenced by the watershed's physical characteristics, including land use and soil. The watershed physical parameter could be expressed as an index of a curve number (). According to McCuen (1998), the  value shows the effect of hydrology on soil, land use, and soil moisture. Surface runoff is needed to determine the potential of water resources in the watershed. Tivianton (2008) stated that river flow discharge is the total surface runoff, rainwater directly falling on the river body, intermediate flow, and baseflow, measured at the river outlet point. The