Model calibration for predicting runoff coefficients in a sub-basin

Rational method and HEC-HMS (Hydrological Engineering Center-Hydrologic Modeling System) rainfall-runoff modelling are required to be calibrated, verified, and validated to determine peak flow. Therefore, this study aims to find C value using rational method. During the analysis, runoff coefficient was calculated based on rainfall data, land coverage, and water level data from an automatic water level recorder, which was used to evaluate the results in calibration and validation models using HEC-HMS. Model simulations produced runoff coefficients of 0.6, 0.33, and 0.4 for Lanang Watershed, Pandantoyo, as well as Ngadirejo sub-basin, respectively. A validation of Q rational was performed during the analysis using this model. The K1 constant rate was defined by dividing the flow calculated by rational method and the flow from HEC-HMS model. Additionally, primary data that was the measurement of the water depth using a logger/AWLR was assumed to be more accurate than data simulation. The validation process improved the advanced HEC-HMS parameters, and the sensitivity analysis identified the most accurate values. The performance of HEC-HMS was assessed using RMSE (Root Mean Square Error) and Nash-Sutcliffe Efficiency, and calibration also assessed the accuracy of the modeling/simulation. The flow calculated using rational method and HEC-HMS should be calibrated, verified, and validated with observed flow data before determining the K value. In conclusion, values obtained from rational formulas provided a more accurate coefficient C based on empirical data.