HEC-HMS

HEC-HMS (Hydrologic Modeling System) from the US Army Corps of Engineers' Hydrologic Engineering Center is the default rainfall-runoff model for design hydrology in much of the world. It covers event and continuous simulation with a deep menu of methods: SCS Curve Number, Green-Ampt and Deficit-Constant losses; Clark, Snyder and SCS unit hydrographs; Muskingum and Muskingum-Cunge routing; frequency and hypothetical design storms; snowmelt; and erosion/sediment options.

Recent versions added GIS tooling that used to require HEC-GeoHMS — terrain preprocessing, subbasin delineation and grid-based modelling now live inside the program. Because it is free, documented to an exceptional standard, and accepted by virtually every reviewing agency, HEC-HMS is usually the safest choice for a defensible design hydrograph.