# WMS:HEC-HMS Transform

Transform properties are defined in the *HMS Properties* dialog when the "Sub-basins" type is active.

## Clark

The parameters for the Clark method are as follows:

- Time of concentration in hours for the unit hydrograph. Several different equations exist for determining the time of concentration. The list of basin geometric attributes computed automatically when basins have been delineated from a terrain model can be useful in many of these equations. These attributes can be viewed from within the
*Unit Hydrograph Method*dialog by choosing the**View Basin Geometrical Attributes**button. Time of concentration can be computed from one of several equations using these attributes, or by using a time computation coverage. These options are accessed from the**Compute Parameters – Basin Data**and**Compute Parameters – Map Data**buttons respectively.

- Clark storage coefficient in hours.

A synthetic time area curve is used in the HMS Clark method, but it is possible to use the HEC-1 model if wanting to derive a time area curve that is more specific to the sub-basin.

## Kinematic Wave

Distributed outflow from a basin may be obtained by utilizing combinations of three conceptual elements: overland flow planes, collector channels, and a main channel. These elements can be defined if the kinematic wave option is specified.

The first and second kinematic wave records can be used to distinguish between different properties such as pervious/impervious (grass/pavement). For each record, the following parameters can be supplied.

- L – Overland flow length.

- S – Representative slope.

- N – Manning's roughness coefficient.

- A – Percentage of sub basins area that this record represents (The total of the two records must sum to 100).

In addition to the kinematic wave records, collector channels and a main channel must be defined. The main channel must be defined, whereas the two collector channels are optional. The following parameters are used for each channel:

- L – Channel length.

- S – Channel slope.

- N – Manning's roughness coefficient for the channel.

- CA – Contributing area to the channel.

- SHAPE – The characteristic shape of the channel.

- WD – Channel bottom width or diameter.

- Z – Side slopes if the channel type requires it.

## ModClark

The modified Clark method, or ModClark, is a linear quasi-distributed unit hydrograph method which can be used with gridded meteorologic data.

## Snyder

Parameters for the Snyder unit hydrograph are as follows:

- Lag time in hours – Several different equations have been published to determine the lag time of a basin. Many of them use some of the geometric attributes computed automatically when a terrain model is present. These attributes can be viewed by choosing the
**View Basin Geometrical Attributes**button. Lag time can be computed from one of several equations using these attributes, or by using a time computation coverage. These options are accessed from the**Compute Parameters – Basin Data**and**Compute Parameters – Map Data**buttons respectively (see Computing Travel Times).

- Peaking coefficient.

A synthetic time area curve is used in the HMS Snyder method, but it is possible to use the HEC-1 model if wanting to derive a time area curve that is more specific to the sub-basin.

## SCS

Parameters for generating a unit hydrograph using the SCS dimensionless method include:

- SCS lag time in hours – Several different equations have been published to determine the lag time of a basin. Many of these use some of the geometric attributes computed automatically when a TIN is present. Lag time can be computed from one of several equations using these attributes, or by using a time computation coverage. These options are accessed from the
**Compute Parameters – Basin Data**and**Compute Parameters – Map Data**buttons respectively.

## User Specified S-Graph

The user specified S-Graph method allows entering the exact specification of the empirical relationship between on unit of excess rainfall and the resulting direct runoff. The relationship is defined using a dimensionless s-graph in the time series editor.

## User Specified Unit Hydrograph

The user specified unit hydrograph option allows defining the exact unit hydrograph for the basin in the time series editor.

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