Accurate discharge prediction is the backbone of flood mitigation, water resources planning, and watershed management — yet spatial rainfall variability and limited gauge data remain major challenges. This research applies HEC-HMS for rainfall-runoff modeling in the Mayang Watershed, Jember Regency (2012–2021), comparing three areal rainfall methods (Arithmetic, Thiessen Polygon, and Isohyet) alongside satellite data from GPM-IMERG and TRMM, validated using NSE, RMSE, and PBIAS. Key finding: the Isohyet method delivers the best spatial rainfall representation, and satellite data significantly improves discharge prediction — making this model highly applicable for small and ungauged watersheds.