Abstract:
The absence of a nationally integrated watershed classification system has led to inconsistencies
in hydrological studies, fragmented datasets, and inefficiencies in water resource management
in Sri Lanka. It makes it extremely difficult to locate, reference, and access information
about particular watersheds. Addressing this significant gap, this study presents a conceptual
model for a regionally compatible, hierarchical watershed numbering system developed for Sri
Lanka’s hydrological and topographical characteristics. The study integrates geospatial datasets
and remote sensing technologies, including Shuttle Radar Topography Mission (SRTM) Digital
Elevation Models (30 m resolution), HydroBASINS data, and Sentinel-2 imagery. Boundary
delineation for streams and watersheds was carried out using QGIS SAGA Nextgen hydrological
tools, which enabled hydrological conditioning of DEM, flow accumulation, and stream
extraction to generate accurate watershed boundaries. HydroBASINS-based boundaries were
also analysed for validation and comparison. The results confirmed that DEM-derived delineation
offers better accuracy in reproducing real hydrological behaviour and more precise subwatershed
structures compared to global datasets, which do not catch the variability at the local
scale. The developed numbering system symbolises multiple parameters into a multi-digit code,
integrating the country identifier, main river basin (based on Arumugam’s 1969 classification of
103 basins), HydroBASINS level, sub-watershed number following Pfafstetter principles, elevation
range, slope classes, and dominant land cover derived from the Vegetation - Impervious
- Soil (VIS) model. This combination allows for a more reliable classification that reflects both
natural processes and human activities of land cover change. Because of its hydrological significance,
ecological diversity, and steep terrain, Belihul Oya, a tributary of the Walawe River,
was chosen as the pilot watershed. The study demonstrated that the proposed numbering system
enhances hydrological modelling, disaster risk assessment, and long-term water resource
planning by improving the clarity, scalability, and usability of watershed identification. It also
highlights the value of incorporating higher resolution data sources, such as LiDAR and UAVbased
terrain models to further increase accuracy in upcoming applications. The study provides
the foundation for Sri Lanka’s national watershed numbering system. In addition to its technical
contributions, the model offers useful information for database development, policymaking,
and institutional collaboration, and database construction that facilitates sustainable watershed
management and informed national and regional decision-making.
