Modeling Infiltration Behavior to Assess the Reliability of Surface Flow Generation in Forested and Deforested Watersheds

Understanding differences in infiltration traits between forested and deforested watersheds is vital for evaluating the ecological and hydrological impacts of deforestation. This study addresses a notable research gap by investigating runoff generation in paired watersheds in Calicut, India, one forested and the other deforested, using a physical infiltration model integrated with a probabilistic framework. Infiltration dynamics are simulated with four classical models: Horton, Philip, Kostiakov, and Green–Ampt. The Green–Ampt model performs best, achieving the lowest root mean square error and percent bias and the highest Nash–Sutcliffe efficiency across sites. Runoff probabilities are assessed using the mean-value first-order second-moment method under five rainfall intensities (SR1–SR5, from light to extreme) based on Indian Meteorological Department classifications. Under low to moderate rainfall (SR1∶ 5 mm/h; SR2∶ 21.55 mm/h), the forested watershed shows high reliability (β=16.36, 12.86) and zero runoff probability. The deforested watershed also shows zero runoff probability but lower β values (8.67, 8.17), reflecting reduced resilience. Under higher intensities (SR3∶ 50 mm/h, SR4∶ 90 mm/h, SR5∶115.6 mm/h), the runoff probability rises sharply, reaching 84%–100% in forested and 79.5%–100% in deforested watersheds, despite antecedent rainfall being far greater in forested sites (119.5 versus 1.75 mm). At SR3–SR5, both watersheds are highly vulnerable to infiltration-excess runoff, but the deforested watershed responds more rapidly and severely, with more negative reliability indices (−0.82 to −38.5) compared to the forested watershed (−1.02 to −29.62). Soil organic matter is a principal factor, ranging from 2.1% to 2.3% in forested versus 0.9% to 1.8% in deforested sites. Higher organic matter enhances the soil structure, aggregation, and moisture retention, delaying runoff initiation. These findings highlight the critical role of forest vegetation in regulating runoff and emphasize forest conservation and restoration as strategies to mitigate flood risks and strengthen watershed resilience under climatic variability.