While a reach element conceptually represents a segment of stream or river, optional modeling of interactions with the subsurface is performed by a Loss/Gain Method contained within the reach. A Loss/Gain Method represents losses from the channel, additions to the channel from groundwater, or bi-directional water movements depending on the specific implementation of a method. A total of three different Loss/Gain Methods are provided. Each method included in the program provides a different level of detail and not all methods are equally adept at representing a particular stream. Further, because of differing data requirements, some Loss/Gain Methods are only compatible with certain Routing Methods.

Four different channel loss/gain methods are available for use within HEC-HMS.  These methods include:

Constant Loss/Gain

Deficit Constant Loss/Gain

Green Ampt Loss/Gain

Percolation Loss/Gain