Urbanization-Induced Shifts in Hydrological Response of a Metropolitan Watershed

Rapid urban expansion in semi‑arid metropolitan areas alters hydrological response, yet the shift in the minimum rainfall depth required to initiate surface runoff—the runoff initiation threshold—has received limited quantitative attention. This study assesses this temporal shift in a part of Tehran metropolitan using Landsat‑based land‑use mapping, Random Forest classification in Google Earth Engine, and the SCS‑CN hydrological model. The novelty lies in three aspects: explicitly targeting the runoff initiation threshold as a primary response variable, isolating land‑cover change effects from interannual precipitation variability, and providing a replicable framework for data‑scarce semi‑arid catchments. Impervious surfaces expanded from 38.2% to 64.7% of the study area, while pervious cover declined from 56.8% to 31.2%. Consequently, the runoff initiation threshold decreased from approximately 10 mm to 8 mm, meaning that smaller and more frequent rainfall events now generate surface runoff. Direct runoff depth increased from 282 mm to 314 mm, although annual precipitation varied between 227 mm and 289 mm without a consistent upward trend. The Mann–Kendall test confirmed the statistical significance of these changes at the 95% confidence level. These findings demonstrate that urban expansion has measurably lowered the rainfall threshold for runoff generation, increasing flash flood risk under previously innocuous events. For District 22, low‑impact development interventions—permeable pavements, rain gardens, and infiltration trenches—are recommended, particularly in the intensively developed western and central sectors. The proposed framework can support evidence‑based stormwater management in rapidly growing cities with limited hydrological data.