# WMS:HEC-HMS Loss Methods

The Compute HMS Loss Method Attributes dialog

The loss rate method defines the equations used in the HMS simulation to separate precipitation volumes from runoff excess. Each of the following methods require one or more input parameters to be defined.

## Green & Ampt

The Green and Ampt method requires the following variables to be defined:

• Initial loss (abstraction) in inches (mm).
• Volumetric moisture deficit. If this value is 0, then the method reduces to the initial loss equal to IA and a constant loss equal to XKSAT.
• Wetting front suction in inches (mm). If this value is 0, then the method reduces to the initial loss and a constant loss equal to the saturated hydraulic conductivity.
• Hydraulic conductivity at natural saturation in inches/hour (mm/hour).
• Percentage of drainage basin that is impervious.

Using methods defined by the Maricopa County Flood Control District, Green-Ampt parameters can be determined from GIS data layers automatically in WMS.

## Initial/Constant

This loss method uses an initial value and a uniform value to define infiltration losses. Input parameters are as follows:

• Initial rainfall/snow melt loss in inches (mm) for snow free ground.
• Uniform rainfall/loss (in inches/hour or mm/hour) which is used after the starting loss has been satisfied.
• Percentage of drainage basin that is impervious.

## SCS Curve Number

The SCS curve number method uses the following parameters:

• Initial rainfall abstraction in inches (mm) for snowfree ground. If value is 0, then initial abstraction will be computed as 0.2*(1000/CRVNBR-10).
• SCS curve number for rainfall/ losses on snowfree ground.
NOTE: Composite Curve Numbers can be computed automatically when this method for computing losses is chosen and a terrain model is present.
• Percentage of drainage basin that is impervious.
• Composite curve numbers can be computed from digital land use and soils files with an accompanying CN value mapping table.

## Gridded SCS Curve Number

With the gridded SCS curve number method a grid defining the CN value must be defined. An initial abstraction ratio must also be defined as well as the potential retention scale factor. The default initial abstraction ratio as originally suggested by the SCS is 0.2, but later research has shown that for many watersheds this value could be as small as 0.05. This method should only be used with the ModClark unit hydrograph transform method.

## Deficit/Constant

The Deficit/Constant method is much like the Initial/Constant uniform where a total losses volume and an initial losses volume are used to specify an "initial" value. This method uses the following parameters:

• Initial deficit in inches (mm).
• Constant loss rate in in/hr (mm/hr).
• Maximum deficit in inches (mm).
• Percentage of drainage basin that is impervious.

## SMA

Soil moisture accounting uses a five layer model that includes evapotranspiration calculations. The five layers include:

• Canopy storage layer
• Surface depression storage layer
• Soil profile layer
• Groundwater 1 layer (shallow groundwater)
• Groundwater 2 layer (deep percolation groundwater)

To define the SMA method, first define the capacity of each layer as well as the initial storage as a percent of that capacity. In addition, infiltration rates for the soil and groundwater layers have to be defined (based on soil types) and the tension zone capacity for the soil profile layer and storage coefficients for the groundwater layer must also be defined. Composite area-weighted parameters can be computed for each drainage basin by overlaying soil and land use coverages with the drainage basins using the GIS calculator.

## Gridded SMA

The gridded SMA method uses the same parameters as the SMA, except that they are defined on a gridded basis rather than by sub-basin. This method should only be used with the ModClark unit hydrograph transform method. Use the Compute HMS Grid Parameters... menu command to compute the parameters required for each grid cell.