subcatchments

Subcatchments are the smaller drainage areas that collectively compose a larger catchment. Each subcatchment contributes surface runoff, groundwater discharge, and sediment to a defined outlet in the drainage network. They are delineated by topographic boundaries such as ridges and divides, and their boundaries are often determined using digital elevation models and hydrological analysis. Subcatchments may be nested within larger catchments, enabling hierarchical analysis and modeling of spatial variability in rainfall-runoff responses.

Delimitation relies on terrain data, stream networks, and sometimes land use and soils to reflect differing hydrologic responses. A drainage network is mapped, an outlet for the subcatchment is defined, and attributes such as area, slope, soil type, land use, and rainfall-runoff parameters are assigned.

In hydrological models, subcatchments serve as the basic computational units. They aggregate rainfall, infiltration, evapotranspiration, and other processes to produce runoff and pollutant loads that are routed through the drainage system to the outlet. Examples of modeling frameworks that use subcatchments include SWAT, HEC-HMS, and SWMM. Subcatchments are used for flood forecasting, water resource planning, sediment and nutrient transport, and ecological assessments.

Limitations include resolution of input data, misalignment with real drainage boundaries, and temporal changes due to land use and urban development. Accurate delineation requires consistent data, calibration, and validation with observed flows and water quality data.

See also: catchment, drainage basin, watershed management, digital elevation model, hydrological modeling.