Proposing an alternative approach based on channel network topology to determine Instantaneous unit hydrographs for ungauged Indian catchments

Estimation of flood response in ungauged catchments remains a critical challenge in hydrology, particularly in regions with heterogeneous physiography and limited observational data. In India, the Central Water Commission (CWC) provides regional empirical equations for deriving unit hydrographs within predefined, contiguous hydrological zones. Although widely applied, this zonal framework does not explicitly account for variations in internal catchment structure and drainage network organization. The present study proposes an alternative approach for flood response estimation based on catchment topological characteristics, with application to South Indian catchments located within CWC zones 3d, 3e, 3f, 3g, 3h, and 3i. Initially, unit hydrographs were computed using the CWC regional relationships and subsequently converted into instantaneous unit hydrographs (IUHs). Given the contiguous nature of the selected CWC zones, a topology-based classification of catchments was then introduced to better represent hydrological response mechanisms. Catchments are grouped according to their drainage network configuration, and empirical width functions were derived for each group. Since the width function describes the spatial distribution of contributing areas with respect to flow travel distance, it provides a physically meaningful representation of the instantaneous unit hydrograph of a catchment. A comparative analysis was conducted between IUHs derived from CWC-based unit hydrographs and those obtained directly from width functions. The results show good agreement between the two approaches in terms of hydrograph shape, peak timing, and overall response dynamics, indicating that catchment topology exerts a dominant control on flood response. Based on these findings, new regional relationships were developed using topological classification rather than contiguous geographic zoning. The proposed framework offers a physically based and alternative approach to existing CWC methodologies for estimating ungauged instantaneous unit hydrograph for Indian catchments. By emphasizing drainage network structure over zonal continuity, the approach enhances applicability across catchments with similar topological characteristics and provides a robust tool for regional flood estimation and hydrological modeling in data-scarce regions.

Keywords: Instantaneous Unit Hydrograph, Catchment Topology, Width Function, Regionalization, Ungauged Catchments.