HEC-HMS

HEC-HMS (Hydrologic Modeling System) is rainfall-runoff software from the US Army Corps of Engineers Hydrologic Engineering Center (HEC). It is free and widely used to estimate flood flows, design storms and the effect of land use change on runoff. Where HEC-RAS tells you how high water will rise in a river, HEC-HMS tells you how much water will arrive and when. A project has four main parts. The basin model is a network of subbasins, reaches, junctions and reservoirs. The meteorologic model supplies rainfall, snowmelt and evapotranspiration. Control specifications set the start, end and time step. Time series data hold gauge records. For each subbasin the user picks methods for four steps: loss (how much rain soaks in, for example SCS curve number, Green and Ampt, or initial and constant loss), transform (how excess rain becomes direct runoff, for example a Clark, Snyder or SCS unit hydrograph), baseflow, and channel routing (for example Muskingum, Muskingum-Cunge, lag or modified Puls). Inputs are subbasin areas, loss and transform parameters, rainfall by gauge or grid, and reach and reservoir data. Outputs are hydrographs at every element, peak flows, runoff volumes and summary tables. The software also has optimisation tools to fit parameters to observed flows. For example, to size a culvert, an engineer can build a basin model for the catchment above the road, apply a design storm for the chosen return period, and read the peak discharge at the culvert. The hydrograph can then go into HEC-RAS. HEC-HMS is flexible, free and accepted for many design studies. Its limits: most methods are lumped by subbasin, so detail inside a subbasin is lost. Parameter choices strongly affect results, and continuous simulation needs more data and care than single-event runs.