Event-Based Hydrological Modeling Using HEC-HMS with a Focus on the Infiltration Mechanism: A Case Study in Iran

This study examines the influence of flood event types on the accuracy of unit hydrograph estimation. The research utilizes the soil moisture ratio (θ) and soil storage capacity (S) to assess the reliability of a linear performance function. This function offers a probable range for the maximum flood occurrence in watersheds. A probabilistic approach addresses the uncertainty of estimating the C _p parameter in the Snyder Unit Hydrograph Artificial method. Importance sampling is used, considering the probability distribution equations of SCS-CN (Soil Conservation Service-Curve Number) runoff losses. The C _p parameter is refined by determining the threshold for variations in the potential maximum flow rates within the watershed. The foundation of this method lies in identifying the sensitivity of the shape parameter (a) in the SCS-CN runoff infiltration rate equations. A new cumulative probability distribution of soil moisture capacity specific to the watershed is introduced. The results obtained for the Shadegan watershed in southwestern Iran indicate a 21% variation in the probability of failure within different probability classes of the infiltration distribution relationship for the shape parameters (a). These threshold variations result in an estimated flow of 14.93 mm in the Shadegan watershed, representing a difference of 12.41 mm under minimum conditions and 27.52 mm under maximum conditions compared to the initial estimate using the SCS-CN base method.