Surface runoff modeling at the event scale, incorporating sensitivity and uncertainty analysis within a foothill watershed

BACKGROUND AND OBJECTIVES: Foothill regions along the Himalayan belt are hydrologically complex due to rugged terrain, highly variable rainfall, and rapidly changing land cover, making them particularly prone to flash flooding. Accurate runoff estimation in such ungauged basins is therefore essential for effective watershed management and flood mitigation. The study objectives were to develop a spatially distributed, uncertainty-aware framework for runoff evaluation in the ungauged Pagladiya River basin in the Himalayan foothills of Baksa District, Assam, India using the geographic information system -based soil conservation service curve number method.METHODS: The basin was subdivided into five physiographically distinct sub-watersheds to capture spatial heterogeneity. Geospatial datasets, including land use/land cover, hydrological soil groups, slope, and rainfall records (۲۰۰۸–۲۰۲۳), were integrated within a geographic information system environment to derive distributed curve number ranging from approximately ۶۰ to ۹۲. Event-based runoff depths were estimated using the Soil conservation service curve number model under varying antecedent moisture conditions. Sensitivity analysis was performed to identify dominant controlling factors, and the predictive uncertainty associated with curve number variability was evaluated using a Monte Carlo simulation with ±۵ percent perturbations to curve number values.FINDINGS: Peak runoff depths ranged from about ۴۰ to ۱۶۰ millimeters, indicating strong event-scale variability across the basin. Runoff generation was found to be predominantly rainfall-driven, with Thiessen-weighted areal rainfall explaining over ۹۹.۸۸ percent of the variability in runoff (R² = ۰.۹۹۹۷). Land-soil interactions also significantly influenced hydrological response, with land use/ land cover, hydrologic soil groups, and antecedent moisture condition acting as secondary controls; antecedent moisture condition-III produced the highest discharge under saturated conditions. Runoff coefficients ranged from ۵۹.۶۹ percent to ۷۹.۵۷ percent, indicating high runoff efficiency during intense storms. Sensitivity testing confirmed rainfall, curve number, and antecedent moisture condition as primary governing parameters, while morphometric characteristics had a moderate influence, mainly on flow concentration.CONCLUSION: The integrated geographic information system-based Soil conservation service curve number framework provides a decision-support tool for runoff estimation and flood risk assessment in ungauged Himalayan foothill basins. The approach supports watershed management and is compliant with other data-scarce mountainous regions facing similar hydrological challenges.

کلیدواژه ها:

Baksa District ، Foothill Region ، Pagladiya River ، Runoff Estimation ، Soil conservation service curve number (SCS-CN) ، Ungauged watershed.