HEC-1 is software that was developed by the US Army Corps of Engineers to estimate river flows as a result of rainfall. It was written in the FORTRAN language and until 1984 could only be run on a mainframe computer. When desktop computers became popular the program was ported to the PC. Engineers and scientists still use it today because of its ability to model a wide range of natural hydrologic systems.
References
HEC-1 Wikipedia(Text) CC BY-SA