The Hydraulics file is an unformatted binary file used to store the results of a hydraulic analysis. Results for all time periods are stored, including those at intermediate times when special hydraulic events occur (e.g., pumps and tanks opening or closing because control conditions have been satisfied).

Normally it is a temporary file that is deleted after the EN_deleteproject function is called. However, it will be saved if the EN_savehydfile function is called before that.

Likewise, a previously saved Hydraulics file can be used if the command HYDRAULICS USE filename appears in the [OPTIONS] section of the input file, or if the EN_usehydfile function is called.

When the Toolkit function EN_solveH is used to make a hydraulic analysis, results are automatically saved to the Hydraulics file. When the EN_initH - EN_runH - EN_nextH set of functions is used, the initFlag argument to EN_initH determines whether results are saved or not. The need to save hydraulic results is application-dependent. They must always be saved to the Hydraulics file if a water quality analysis will follow.