Available Loss Methods

The following loss methods are included within HEC-HMS:

• Initial and Constant Loss Model
• Deficit and Constant Loss Model
• Green and Ampt Loss Model
• Layered Green and Ampt Model
• Linear Deficit and Constant Model
• SCS Curve Number Loss Model
• Exponential Loss Model
• Smith Parlange Model
• Soil Moisture Accounting Loss Model

With each method, precipitation loss is found for each computation time interval and is then subtracted from the precipitation depth for that interval. The remaining depth is referred to as excess precipitation. This depth is considered uniformly distributed over a subbasin or grid cell, depending upon the chosen method, so it represents a volume of runoff.

Some of the loss methods included in the program are gridded. These methods presume a subbasin is composed of regularly spaced cells with uniform length and width. These methods permit the user to specify initial conditions and parameters for each grid cell separate from the neighboring cells. All other loss methods simulate the entire subbasin with one set of initial conditions and parameters.

When using a unit hydrograph transform method, the excess on pervious portions of the watershed is added to the precipitation on directly-connected impervious area, and the sum is used in runoff computations. With the ModClark method, the excess from the pervious and impervious portion of each cell is combined and routed to the outlet. With the kinematic wave transform method, directly connected impervious areas may be modeled separately from pervious areas if two overland flow planes are defined.