For the Outflow Curve Routing Method, regardless of which storage method option is selected, the routing is always performed using only the storage-discharge curve, because HEC-HMS does not use triplet curves. After the routing is complete using the storage-discharge curve, the program will compute the elevation and surface area for each time step, depending on the selected storage method.

Interpolation is used when the Elevation-Storage-Discharge or Elevation-Area-Discharge storage methods are used. This means that it is not necessary for the storage-discharge and elevation-storage curves used in the Elevation-Storage-Discharge method to contain matching independent variables. The two curves do not need to have the same storage values in each curve, or even have the same number of rows. At compute time, the two curves selected by the user are combined into a single routing table with three rows: elevation, storage, and discharge. The table is initially configured using the curve selected by the user as the Primary curve. The remaining column is interpolated from the curve not selected as the primary curve. Finally the storage routing is completed from the combined table using the storage and outflow columns, and then elevation and area is calculated from the computed storages where possible. A similar procedure is also used with the Elevation-Area-Discharge storage method. Elevation is post-processed output.

The approach for calculating the outflow is basically the same as using Modified Puls routing with a single step (maximum attenuation) and assumes a level pool for the whole element. For the determination of routing, the Storage-Discharge relationship is not used directly. Instead a Storage Indication Table (which is also used for Modified Puls routing) is used:

$\begin{array}{l}\displaystyle SI_i= \frac{Stor_i}{∆t}+ \frac{1}{2} Q_i^{out}\end{array}$

$\begin{array}{l}\displaystyle SI_{i+1}= SI_i- Q_i^{out} + \frac{1}{2} (Q_i^{in}+Q_i^{out})\end{array}$

Storage Indication Table

SI	Qout

Applicability and Limitations of the Outflow Curve Method

The Outflow Curve Routing Method of modeling a reservoir is appropriate for simulating performance of any configuration of outlets. However, the model assumes that outflow is inlet-controlled. That is, the outflow is a function of the upstream water-surface elevation. If the configuration of the reservoir and outlet works is such that the outflow is controlled by a backwater effect (perhaps due to a downstream confluence), then the outflow curve method should not be used. Instead, an unsteady-flow network model must be used to properly represent the complex relationship of storage, pond outflow, and downstream conditions. Further, if the reservoir is gated, and the gate operation is not uniquely a function of storage, then a reservoir system simulation model, such as HEC-ResSim (USACE, 2013), should be used.