WMS:Lag Time and Time of Concentration

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Lag time and time of concentration are variables often used when computing surface runoff using unit hydrograph methods available in the hydrologic models supported in WMS. These variables indicate the response time at the outlet of a watershed for a rainfall event, and are primarily a function of the geometry of the watershed. WMS provides two powerful methods of computing travel times for lag time and time of concentration from the geometric data being used for basin delineation and parameter estimation.

The first method is to use one of several empirical equations (or user defined) based primarily on the basin data computed by WMS when using a DEM or TIN for basin delineation. Many different equations have been developed for different watersheds, and most of these equations are a function of the geometric parameters computed from digital terrain models.

The second method allows creating a time computation coverage in the map module and then define the "representative" flow paths within each basin using arcs that are used to determine lag or time of concentration. A travel time equation can then be assigned to each arc (length and slope are automatically determined from the arc when a DEM or TIN is present) and the sum of the arc travel times within a basin used for time of concentration or lag time. Pre-defined equations such as are used by the FHWA or in TR-55 can be selected or user defined equations developed.

There is no distinct advantage of one method over the other. Each allows a certain amount of customization and the ability to generate a summary report in a text file or by copying to the clipboard so that these critical input data can be well documented. In general, if time of travel can be determined from a single empirical equation then computing using the basin data will be more convenient whereas if the time of concentration or lag time is determined by combining the time of travel across one or more flow path segments, (overland flow, shallow concentrated flow, channel flow, etc.) then the map data method will likely work best.

Each of the hydrologic models supported by WMS that require a lag time, time of concentration, or channel travel time allow selecting either of the two methods. Buttons adjacent to the input fields allow access to the different methods and the computed result is used to define the input value for the model being working on (i.e. TR-55 time of concentration, TR-20 lag time, etc.). It is also possible to compute travel times for selected basins or outlets using the Compute Travel Time command from the Calculators menu. This dialog will allow choosing between the two methods for a selected basin (only the Map module method is available for a selected outlet). The computed travel time can then be assigned to the relevant input parameter for the selected hydrologic model (the hydrologic model corresponds to the current default model and can be changed using the drop-down combo box).

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