Wastewater discharge inversion under data scarcity using a coupled HEC-RAS–DREAM framework: a synthetic case study

Estimation of wastewater discharge in developing urban areas is often limited by insufficient monitoring data, leading to substantial uncertainty in conventional calibration approaches. This study evaluates a coupled HEC-RAS–DREAM inverse modeling framework through a synthetic case study in the Cibarani Channel, Bandung, Indonesia. Due to the absence of continuous downstream water stage observations, discharge estimates from a previous study were used in HEC-RAS unsteady-flow simulations to generate synthetic water surface elevation (WSE) data. These data served as calibration targets for DREAM-based inverse modeling to assess whether alternative discharge configurations could reproduce similar hydraulic responses. The results show substantial differences between DREAM-derived and conventional trial-and-error discharge estimates, with discrepancies reaching fivefold at downstream locations. Nevertheless, both configurations produced nearly identical WSE profiles, demonstrating strong equifinality under limited data conditions. These findings highlight how deterministic calibration may yield satisfactory model fits while concealing substantial parameter uncertainty. The near-perfect performance indicators reflect the synthetic nature of the calibration targets rather than predictive validation. Therefore, this study demonstrates the applicability of inverse modeling for identifying uncertainty and equifinality in data-scarce urban drainage systems, emphasizing the need for uncertainty-aware approaches to support urban water quality and pollution management.