The computation engine draws on over 30 years experience with hydrologic simulation software. Many algorithms from HEC-1 (HEC, 1998), HEC-1F (HEC, 1989), PRECIP (HEC, 1989), and HEC-IFH (HEC, 1992) have been modernized and combined with new algorithms to form a comprehensive library of simulation routines. Future versions of the program will continue to modernize desirable algorithms from legacy software. The current research program is designed to produce new algorithms and analysis techniques for addressing emerging problems.

The initial program release was called Version 1.0 and included most of the event-simulation capabilities of the HEC-1 program. It did introduce several notable improvements over the legacy software including an unlimited number of hydrograph ordinates and gridded runoff representation. The tools for parameter estimation with optimization were much more flexible than in previous programs. The maiden release also included a number of "firsts" for HEC including object-oriented development in the C++ language and multiplatform support in a program with a graphical user interface.

The second major release was called Version 2.0 and focused on continuous simulation. The addition of the soil moisture accounting method extended the program from an event-simulation package to one that could work equally well with event or continuous simulation applications. The reservoir element was also expanded to include physical descriptions for an outlet, spillway, and overflow. An overtopping dam failure option was also added.

The third major release was called Version 3.0 and introduced new computation features and a brand new graphical user interface. The meteorologic model was enhanced with new methods for snowmelt and potential evapo-transpiration simulation. The basin model was enhanced with additional methods for representing infiltration in the subbasin element, and additional computational options in the diversion and reservoir elements. The new graphical user interface was designed to simplify creating and managing the many types of data needed for hydrologic simulation, and to increase user efficiency with a better-integrated work environment.

The fourth major release was called version 4.0 and focused primarily on new computation features and integrating a GIS framework. A broad range of surface erosion and sediment transport features were added to the subbasin, reach, reservoir, and other elements. A preliminary capability was also added for nutrient water quality simulation. New simulation component was added to to facilitate real-time forecasting operations. Finally, an integrated GIS framework was added to the program that allows modelers to delineate subbasins and reaches, discretize subbasin elements for precipitation and process based computations, estimate model parameters and visualize results. 

Enhancement of the program is ongoing. HEC has a strong commitment to continued research in emerging needs for hydrologic simulation, both in terms of simulation techniques and representation of physical processes. Future needs are identified by conducting our own application projects, speaking with program users, and monitoring academic journals. HEC also has a strong commitment to continued development of the program interface. Plans are already underway to add new features in a future version that will make the program easier to use by providing more flexible ways to accomplish work. New visualization concepts are also being developed. Look for future versions to continue the tradition.