Identifying optimal subwatershed delineation thresholds using variance-based segmentation in the Central Himalaya

There is no consensus on the appropriate number of subwatersheds to delineate in a basin, nor a widely accepted methodology for identifying optimal delineation thresholds. Past studies indicated that neither very fine subwatershed discretization nor overly simplified representations consistently perform better in hydrological and nutrient modeling. Consequently, the development of an efficient and objective methodology to reduce the labor-intensive trial-and-error process of determining optimal subwatershed thresholds remains a challenge. This study applies a variance-based segmentation technique to the sequences of subwatersheds delineated using variable drainage area thresholds. The method is validated using hydrological simulation in the Soil and Water Assessment Tool (SWAT). The approach is evaluated across 18 mountainous sub-basins on the southern slope of the central Himalaya to identify statistically optimal upstream contributing area (UCA) thresholds. At the regional scale, an optimal UCA threshold of 45 km² was identified, while sub-basin-specific thresholds ranged from 39 km² to 51 km². SWAT simulations at the case study basin support the statistically identified threshold, yielding Nash-Sutcliffe Efficiency (NSE) of 0.72 to 0.81, coefficient of determination (R²) of 0.74 to 0.82, and satisfactory error metrics. The identified thresholds exhibit a power-law relationship with basin size when normalized by basin area, indicating that drainage area thresholds are scale-dependent rather than fixed values. Larger basins were associated with smaller threshold-to-basin-area fractions, whereas smaller basins exhibited higher fractions. Across the study region, the optimal threshold consistently corresponded to less than 5% of the total basin area. The proposed approach provides a statistically robust and computationally efficient alternative to the conventional method of identifying the optimal number of subwatershed delineations. This framework offers objective guidance for basin subdivision in hydrological modeling and watershed management and is adaptable to regions with differing geomorphic and hydrological characteristics, particularly in mountainous environments.