RAINFALL-RUNOFF ANALYSIS USING HEC-HMS 4.11 PROGRAM IN THE TUKAD PETANU WATERSHED AREA, GIANYAR REGENCY, BALI

Abstract

The Tukad Petanu Watershed is one of the main rivers in Gianyar Regency, Bali, playing a critical role in irrigating agricultural areas and supporting regional water needs. A significant challenge facing this watershed is a water deficit of −0.65 m³/s, driven by increasing water demand and growing conflicts over water allocation. To address this, rainfall–runoff transformation modelling was applied to better represent hydrological field conditions and support water resource planning. This study aims to determine the flow discharge characteristics and compare the differences between HEC-HMS simulation results and observational data. The rainfall–runoff simulation used observed discharge, rainfall, land-use maps, soil type maps, and Digital Elevation Model (DEM) data as model inputs, covering the 2022 analysis period. A composite Curve Number (CN) value of 81.31 was derived from land-use and soil type analysis. The modelling results produced a Root Mean Square Error (RMSE) value of 3.7, indicating a reasonable level of simulation accuracy. The peak simulated discharge occurred during the maximum rainfall event on October 17, 2022, with 87 mm of rainfall and a simulated peak discharge of 59.4 m³/s. In contrast, the observed peak discharge occurred on November 8, 2022, with a value of 11 m³/s. The discrepancy between simulated and observed peak discharges reflects the influence of weirs and water abstraction structures within the watershed that were not fully captured by the model. These findings indicate that HEC-HMS can represent the general runoff pattern of the Tukad Petanu Watershed, providing insights to enhance hydrological modelling accuracy for tropical watersheds and support sustainable water management strategies in Bali.