All of the available Reach Routing Methods include an initial condition at the start of the simulation. The initial condition may be used to set discharge equal to outflow or specified as an absolute flowrate (m3/s or ft3/s). The flow determined from the initial condition typically recedes until runoff from upstream elements begins generating inflow to the reach.

Observed flow provides one good source for calculating the initial outflow condition at the start of the simulation. In some cases observed flow may be available at the upstream end of a reach and can be used directly as the outflow initial condition. It is more common that there will be multiple elements between observed flow locations but this does not negate the use of observed flow as a means to estimate the outflow initial condition. When there are multiple elements between observed flow locations, a ratio can be applied to the observed flow to calculate the outflow initial condition for each routing reach. One approach to determining the ratio is to consider the upstream contributing drainage area for each reach within a zone and compare against the total contributing drainage area at the observed flow locations. The reach outflow initialization using the Reaches editor can be accessed on the Compute menu by selecting the Forecast Initial Reach Outflow menu option. An example of the Reaches editor is shown in the following figure.

This Reach Outflow Initialization Feature is optional. When reach outflow initialization is unused, reach outflow is calculated purely from the basin model parameters, zone adjustments, and element overrides. The initialization feature may be applied at no reaches, only one reach, several reaches, or all reaches. Using the initialization feature supersedes the basin model parameters, zone adjustments, and element overrides.

Using observed flow to automatically calculate initial reach outflow

The Reaches editor uses the Reach Zone Configuration to assist in organizing data entry. The current zone is selected in the upper left corner of the editor. Each reach in the zone is shown on a separate row in the editor.

A Discharge Gage may be selected for each reach. Discharge gages must be created in the Time-Series Gage Manager before they can be selected for a reach. Usually the selected gage will be at the upstream or downstream end of the reach or at some nearby location. The selected gage should be representative of initial flow conditions for the reach. The observed flow rate at the gage will be shown for the start time of the forecast alternative.

A Minimum and Maximum Flow Rate may be entered to qualify the use of the observed flow gage for calculating the initial reach outflow. The minimum and maximum values are used in conjunction with the Default Flow Value. If the observed flow is less than the minimum, then the default value will be used instead. The default will also be used if the observed flow is greater than the maximum. The minimum or maximum can be left blank if the observed flow should be used without a lower or upper limit. Finally, the default value will be used if the observed flow is missing at the start time of the forecast alternative.

The Observed Flow from the selected gage is multiplied by the specified Ratio in order to calculate the initial reach outflow estimated value. The ratio is only used if the observed flow is between the minimum and maximum values or if the minimum or maximum values are left blank. It is required for the ratio to be greater than zero and it may be greater than one. However, using a ratio greater than one should generally only be done if the selected gage is located a significant distance downstream of the reach and notable contributing drainage area exists between the reach and the selected gage; this should be done only after careful analysis.

The Estimated Value is calculated from the observed flow gage using the ratio and substituting the default value according to the minimum and maximum values. At first, the estimated value is also copied as the final value. The Final Value can be edited to override the estimated value. Editing the final value may be necessary if current flow conditions diverge from the assumptions used to develop the ratio. The final value for each reach shown in the table will be reset to the estimated value when the Reset button is pressed.