This study investigates the effectiveness of combining the Snyder Synthetic Unit Hydrograph (SUH) with three flood hydrograph models—Alexeyev, Gama I, and Nakayasu—in estimating High Direct Runoff (HDRO) across six sub-watersheds located in the southern region of the Siak River Basin, Pekanbaru City, Indonesia. The methodology integrates watershed characterization using Geographic Info…